DO THE TON

Blood Sweat Tears and Grease => Engines => Topic started by: Sonreir on Jul 31, 2012, 20:44:31

Title: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jul 31, 2012, 20:44:31
OK... admittedly this is a very broad topic and I wasn't quite sure where to put it, so I stuck it in the Engines section because it's my intention to make this more of a technical type of article and also because I'm going to be talking mainly about engines.

I was reading a blog post from a friend of mine and it really struck a chord with me.  It seems that the café racer genre has really taken off these past few years, but a lot of the newcomers seem to be missing some of the basics.  They're immediately drawn to the looks of the café racer, but are perhaps not understanding the function (or even that the form follows the function).  Now don't get me wrong... I'm not hating on the guys.  Fresh faces are what will keep this knowledge alive and well in the years to come, but only if the knowledge is taught and understood.

Please allow me to repost the blog before we go too much further:
Quote from: kopcicle
What is a "Cafe Racer"?
Make it go ...
We all know that there are gains to be had just in the fine details without resorting to pistons , compression , valves , cams , extensive porting , carburetor and exhaust . Well , no , wait . Within reason and budget that is the idea !
Make it stop ...
Disc upgrade or pursue a lost art in tuning up drum brakes .
Make it turn ...
Make what you have the best it can be or bin it and adapt a modern front end entire.
Make it look ...
Like it's more at home carving corners than sitting outside the favorite pub .
Take pride ...
In the fact that each sub assembly is the very best that you can do with the tools , talent , time and budget available .Only then will the bike reflect that it is more than the sum total of it's parts .
Teach ...
What you learn . Without this not only the lesson but the spirit of the lesson dies with you .
Enjoy ...
What you do , what you build , what you ride .

Our bikes are not only a form of personal expression but a loosely defined art form that stems from the enthusiasts of previous decades . Without growth and change we stagnate and die . In a world of posing and posturing for the benefit of who knows who what is the point of modifying a bike to be just like whatever unless it's meant to be a replica . Be original . Be different . Experiment . Let function be your guide and form will follow .

Our legacy to the next  generation of riders and builders is our collective and individual vision . Our passion for that something extra defines our enthusiasm . Our ability to communicate and teach how to learn is our obligation . The definition of a "cafe racer" isn't rooted in our collective or individual past it will be defined by what we choose to do in the future . I've never known a brighter future for the genre in all my years turning a wrench . I can't wait to see what happens next .

Now one other thing... there's a been more than a little drama on forums recently and I think one lesson we could all take away from it is that "you catch more flies with honey than you will with vinegar".  That is, maintain a positive attitude.  Instead of telling someone what they're doing wrong, tell them what they can do right.  TEACH them right and wrong so that they can identify it for themselves.  Be aware that sometimes there is more than one way to be right, but also be aware that there is almost always one BEST way.

With these things in mind, I'd like to get back to the main topic of this post.  "Make it go".  My own personal philosophy about café racers can be boiled down to two concepts.

1.)  Form follow function - It has to work well first, look good second.  Generally speaking, if it works well, it will look good.
2.)  Built not bought - It's easy to take this maxim to an extreme, so please use this in the more moderate sense.  It's your bike, you need to understand what's going on with it.  If something breaks or there's something you want to improve, try it yourself.  It may cost you some extra parts when you screw it up, but you've learned something in the process and knowledge is priceless.

Now that the preamble is over.  Lets get down to it.  This article is an attempt to pass on a bit of the knowledge I've gathered over the past few years.  Undoubtedly, there are guys here with a lot more experience and a lot more knowledge than I and I welcome them to please correct me where I'm wrong and chime in with additions where applicable.  I readily admit that most of my knowledge comes from reading and studying rather than doing and so keep that in mind.  As always, empirical data trumps rhetoric.  Something tested, measured, and replicated is something proven.  Something written is just words on a screen.  If you disagree with something I've said, please post why so we can all learn from it.

A café racer without performance enhancements is just a tractor with a body kit.  Do you really want to be one of those kids in a 1.6L Honda Civic with glowing lights under the body panels racing his gutless wonder from stoplight to stoplight?  If the answer is, "yes", you can probably stop reading now.  ;)

Seriously though, the engine is the heart and soul of your bike and I'm seeing fewer and fewer builds that attempt to improve this key component.  I'm not sure why this is, but I suspect that our culture has come to favor looks over performance or perhaps people are a little wary of cracking open something that has so many parts?  Cost is also a consideration, but if you can afford to drop $500 on fiberglass seats and tanks and another couple of hundred on paint and upholstery, a bit of money for the engine doesn't seem out of line, right?

Building an engine can most certainly be done in stages, but the most important thing to keep in mind is to take a holistic approach.  There are few things you can change that won't also have an effect on something else.  Understand the consequences (both good and bad) of each action before you take it.  Not all parts will work in all circumstances and the final goal of your engine build SHOULD be the deciding factor of which parts go into it.  Building a comfortable long distance cruiser versus building a café racer is more than just adjusting the seat and control locations.  The engine characteristics are the soul of a bike.

So what are engine characteristics and how does the design of the engine affect them?  Well... put simply, the characteristics of the engine can be categorized by the throttle response, revs, acceleration, torque and a host of even more subjective items.  The engine from a semi truck can put out more than 600 horsepower, but you're never going to find one in a sports car (size issues aside).  Sports car drivers want high revs.  The very successful Honda S2000 can redline at 9,000 RPM.  Much higher than most passenger vehicles.  That redline is necessary to create an appropriate feel and power for the vehicle's purpose and equal thought should be given to your own engine.

There is something intangible to engine characteristics, though.  How much of a grin is on your face after your bike pushes you through a tight corner?  Is your bike still egging you on for more throttle even when you're skirting the ton?  Or is your bike telling you it's had enough when you start pushing 80?  The engine on a café bike should pull strong through the mid range and get even better as the rpms climb.  The engine on a café bike should bounce off of the redline after a gear change and actually feel a bit sad that you didn't take it further.  The engine on a café bike wants nothing more than to rev itself to pieces.  It would love the opportunity to see how fast it can spin before parts start flying out.  Is your bike screaming for more or is it screaming "enough"?  Your engine should be willing and your bike should be braver than you are.  Remember, café racers started out at street legal race replicas, mimicking the race bikes of their time.  Would your bike be at home on a race track or did you just build another tractor with a body kit?

So... enough rhetoric already.

Time for some theory.  I'll get into more details in subsequent posts, but for the remainder of this post I'm going to talk about the four general ways in which engine performance can be improved.  Before that, though, lets talk about a few engine basics just to make sure we're all on the same page.

The Workings of a Four Cycle Internal Combustion Engine (ICE)
This may seem a bit basic to many of you, but I'm of the mind that a decent house needs a decent foundation and so I've included the info here.

An ICE is basically a self-powered air pump.  Fuel and air is drawn into each cylinder, compressed, ignited/combusted, and then expelled.  Repeat as frequently as possible.  Those four steps are as follows:
Intake Stroke - Piston starts at Top Dead Center (TDC) and descends toward the crankshaft centerline.  During the entirety of this stroke, the intake valve(s) remain open and the descending piston can be thought of in a similar manner to the plunger on a syringe, drawing fluid into it as it opens.
Compression Stroke - The piston ascends from Bottom Dead Center (BDC) back toward the head as the intake valve begins to close.  The fuel/air mixture that was drawn into the cylinder during the intake stroke is now squished into a smaller and smaller volume.
Power Stroke - The compressed mixture is ignited with a spark from the plug and the ignition releases a great quantity of heat.  This heat is what causes the gases in the cylinder to expand and produce an increase in pressure.  This pressure pushes the piston back toward BDC.  Partway down, the exhaust valve opens and begins bleeding excess pressure out the exhaust headers.
Exhaust Stroke - The piston passes BDC and heads back toward TDC.  Going back to the syringe metaphor, this is the plunger being depressed and expelling all of the fluid back out of the syringe.  During the entirety of this stroke, the exhaust valve remains open.  The intake valve will open during this stroke as well.

An astute reader will have noticed that the piston descends and ascends twice for an entire period of the four cycles.  This means the crankshaft is rotating 720° for each complete period.

How efficiently each of these four strokes can be accomplished is what determines the performance of your engine.  These performance modifications will always go to serve one (or more) of four goals.  These four goals are the same four goals for everyone, everywhere, and your modifications need to answer to these objectives.

The Four Goals of Engine Performance
1.) Increase Displacement - All things being equal, a bigger engine will outperform a smaller one.
2.) Increase Revs - Horsepower is a unit derived from torque.  Torque is what is measured, horsepower is what is calculated.  HP = (T x RPM) / 5252.  If you can keep torque the same and increase your revs, you've just "created" horsepower.
3.) Parasitic Losses - The power generated by your engine goes into a lot more than just turning your rear wheel.  It takes a lot of energy to spin metal as fast as your bike does and that's even without having to contend with friction of all the components necessary to make it happen.  Reduce this friction and inertial losses and your engine will spin faster, sooner, and more of that power will make it to the ground.
4.) Brake Mean Effective Pressure (BMEP) - This is the average pressure within the cylinder, generated by the power stroke.  More BMEP directly translates to more torque, which, of course, means more horsepower.

I'm right around my mental limit of typing for the day, so I'll go into more detail on each of the above four items in a later posts.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Dragonfish on Jul 31, 2012, 21:18:29
Fantastic thread! Being new to bikes I really appreciate all the effort this is going to take. Subscribed.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: SeekingZero on Jul 31, 2012, 21:23:53
Nice opening post and thread!  I must admit I am guilty of building a tractor with a body kit, but my reasoning is sound and once my wife is done with it, it'll be time to tear it open.  I'll be following with great interest.  ;D
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: kopcicle on Aug 01, 2012, 05:36:45
Damn man , did I strike a chord or what ? I'm in this one for the long haul ...

~kop
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Aug 01, 2012, 16:36:46
Power Goal #1 - Increase Displacement

As mentioned previously, there are four main methods to employ in the quest for more power.  One of the simpler methods is to increase displacement.  As with my previous post, I'll start with simple and build from there.

Displacement is the volume of the area occupied by the pistons (at any time) within the cylinders.  If you were to measure all of the area occupied by a piston as it moves from BDC to TDC (called the "sweep" or "swept volume" of the piston), this would be the displacement.  As the calculation of a cylinder is fairly easy, calculations of displacement are also easy.

The formula for calculating displacement is to first divide the bore (diameter of the cylinder, though diameter of the piston can also used for this calculation) in half.  This will give you the radius.  The radius is then squared and multiplied by PI (3.1514).  The final answer is then multiplied by the stroke (the difference, in distance, between TDC and BDC).  You'll now have your displacement in whatever units you used during the calculation process.  For most of us, this will be cubic millimeters and so you may wish to divide by 1000 to convert to cubic centimeters.  Divide again by 16.387 if you want cubic inches.  Finally, multiply by the number of cylinders in your engine for a final answer.

For example, this is the displacement calculation for my own CJ360.  Bore is 69mm (non-standard) and stroke is 50.6mm.

     R = 69mm / 2 = 34.5mm
     R˛ = 34.5mm * 34.5mm = 1190.25mm
     A₁ = 1190.25mm * 3.1415 = 3739.17mm˛
     A₂ = 3739.17mm˛ * 50.6mm = 189202.00mmł = 189.2cc
     Final Displacement = 189.2cc * 2 = 378.4cc

This equation can also be "simplified" to one line in the following manner:

     Displacement = PI/4 * bore˛ * stroke * #cylinders

The displacement of your engine directly affects how much fuel and air can enter the cylinder and so more displacement will almost always translate into more torque and more power.  All things being equal, a big engine is more powerful than a small engine.

Not all displacement is created equal, however.  Many engine designs opt for a longer stroke or a larger bore.  Engines with a stroke longer than the bore diameter are called, "undersquare".  Engines with a bore diameter wider than the stroke is long are called, "oversquare".  If an engine has the same stroke and bore (or are within 5% of one another) then the engine is "square".  Generally speaking, motorcycle engines follow an oversquare design, though the more displacement an engine has the more likely it will be approaching undersquare.  All HDs of the modern age run undersquare engines and almost all sport bikes will run oversquare.

So lets cut to the chase.  Are oversquare engines better than undersquare engines?  Well that's kind like asking are apples better than oranges.  It'll depend on who you ask and the purposes of the build.  Generally speaking, undersquare engines hit peak torque sooner.  This gives them a feel of very strong acceleration, but it also tapers off quickly.  Oversquare engines take a little while to get up to pace but then will pull harder through the top end.


Undersquare Traits
An oversquare and an undersquare engine will have very different characteristics for the same displacement.  Because undersquare engines have a longer stroke, this means their pistons are moving faster than the oversquare engine for a given RPM.  Longer distance over the same time means more speed.  Engine components can only handle so many forces and as speeds double, forces quadruple. The increase in speed of the pistons directly translates into a need to reduce the engine RPMs before the breaking point of the bottom end is reached.

Piston speed is generally measure using "Mean Piston Speed".  This is usually listed in feet per minute or meters per second.  Mean Piston Speed is precisely the reason why the Triton was born.  The Triumph engine of the day was a better alternative to the Norton because of its ability to rev.  The maximum MPS for an engine is usually right around the 4000ft/min or 20m/s mark.  Some engines can go as high as 4900ft/min or 25ms but unless you've made modifications, I don't recommend it.  To calculate mean piston speed (in ft/min) multiply the stroke times two times rpm and then divide by 60.  For my own CJ360 it becomes:

     2 * 50.6 * 11,000 / 60 = 18,533mm/sec or 18.53 meters per second at 11,000 RPM

If I were to rev to a mean piston speed of 20m/s, then I'd be hitting nearly 12,000 RPM.  Probably doable for short periods of time, but I don't think I'd want to live up there.  For argument's sake, lets say I increased the stroke out to 60mm.  20m/s of MPS now occurs at 10,000 RPM.  I've had to drop my hypothetical redline by 2,000 RPM to accommodate the new stroke.

Furthermore, cylinder filling can become an issue at higher RPMs because the increase in displacement of the cylinder (but not it's diameter) prevents the use of bigger valves.  Next, the increased travel of the pistons creates more friction.  A majority of the friction in your engine comes from the piston rings against the cylinder walls and increasing the distance the pistons needs to travel increases parasitic losses.  Finally, the longer crankshaft arms (or pin offset) necessary to create a longer stroke also causes an increase in sidewall pressures of the cylinder and on the piston skirts.  This is illustrated below this paragraph.
(https://sphotos-b.xx.fbcdn.net/hphotos-prn1/524227_10151140977660159_235702596_n.jpg)

Lets assume this simple image is a side view of your crankshaft with the center of the crankshaft being the point, "C".  The black circle is the path which the big ends of your connecting rod follow during the rotation of the crankshaft and the blue line, "c", represents the conn rod, itself.  The graphic here represents the most extreme scenario; when the piston is halfway between TDC and BDC and the angle between the cylinder centerline ("b") and the conn rod are the greatest.  Let's assume that crankshaft is rotating clockwise and we're halfway through the power stroke.  The natural tendency will be for the expanding gases to push straight down onto the piston which will then push down through the conn rod and to the crankshaft.  The third law of motion tells us that the crankshaft and conn rod must be pushing back through the piston as well.  As this force is not applied directly upward at 0°, then some of the force is directed to the left (in the graphic).  In real world terms, the piston is forced against the side of the cylinder wall and how hard it is pushed against the side is related to the angle A, which is determined by the length of the conn rod, c, and the distance from the crankshaft centerline, a.

But, it's not all doom and gloom on the undersquare front.  Undersquare engines do provide some very distinct advantages.  By increasing the length of the crankshaft arms or by offsetting the pins, we create a greater mechanical advantage.  This mechanical advantage is easily thought of as the handle on a wrench.  It's much easier to turn a bolt with a long-handled wrench than with a short one.  The same holds true for a piston applying its forces onto the crankshaft; things turn easier with long handles.  So with a longer stroke, you're not only getting the torque advantage of more fuel and air, you're also a getting a mechanical advantage.

A further advantage of undersquare engines is the surface area of the cylinder, combustion chamber, and piston that is exposed to the ignition of the mixture.  As an engine gets more and more undersquare, the area exposed to the initial ignition shrinks in relation to the displacement.  This creates thermal efficiencies within combustion that directly translate into a greater BMEP for equal displacement engines.  Because of this greater thermal efficiency, higher compression can be employed without the need to use higher octane fuels (which is also beneficial because higher octane fuels burn more slowly).

Also, the longer stroke of the piston aids in port velocities.  Generally speaking, the greater the port velocities, the better the volumetric efficiencies (we'll cover that in a later post).

Finally, the flame front within an undersquare engine travels faster and the rate of increase in combustion chamber volume as the piston descends more closely matches the natural characteristics of the expansion of gases created by the combustion of gasoline.  This leads to smoother operations and another increase in BMEP.

Oversquare Traits
As you'd expect, the opposite of an undersquare engine is an oversquare engine and so many of their traits are opposite as well.  With a shorter stroke you are not only able to rev an engine higher to get more power, you're actually required to do to.  Oversquare engines will produce their peak torque at a higher RPM than that of its undersquare cousin.  The gives the engine a feeling of wanting to run.  Taken to an extreme, however, many oversquare engines will be feel high strung.

With a larger diameters than an oversquare engine, an undersquare engine can pack in larger valves or even more numerous valves.  This has the effect of increasing engine breathing at higher RPMs, but it does lower intake velocities and lower RPMs.  This is a significant reason why undersquare and oversquare engines achieve peak torque at different spots in the RPM band.  Also, the shorter stroke makes those higher RPM forces more tolerable for the engine components and results in lower frictional losses as well.

On the downside, oversquare engines will have a greater area exposed to the flame front and so will generally run hotter while getting less torque per cubic centimeter.  These engines need to be revved to get their full potential because they rely on speed for power rather than force.

Oversquare engines are most common in applications that require higher levels of power at the cost of efficiencies.  Race cars (yes, even NASCAR) use oversquare engines.  Semi trucks, marine diesels, trains, Toyata Prius, etc, all use undersquare engines when efficient operations are more of a concern.

Square Traits
Square engines, obviously, sit between the two extremes.  A well designed square engine can have the best of both worlds while a poorly designed square engine will have the worst.  Much of this will have to do the selection of peripheral components.

Which to Choose
Well... it's not that simple.  Even after laying out the traits of each motor, you can't just pick one and run with it.  If your starting platform is an 883 EVO engine, you're going to have a hell of a time reaching oversquare.  Likewise, you may encounter some difficulties if you want to stroke out your Ducati 1198.

An increase in bore will require larger diameter pistons.  Unless you spend some dough, these pistons will almost definitely be heavier and this has the adverse effect of adding more tension onto the conn rods and crank.  More tension means lower RPMs before parts start trying to occupy the same space at the same time.  A larger bore will also require the use of a specially made head gasket.  You can't have edges of the gasket poking into the combustion chamber or it won't last very long.  Copper is a common material for custom head gaskets but it's a bitch to seal properly (especially with iron-sleeved aluminum chambers)  and is NEVER as easy as it looks, although I'm not sure it ever looks easy.  Going very overlarge on the bore may require your combustion chambers to be remachined or even your entire head to be replaced.  In some cases, increase in compression or timing will be required to ensure the flame front continues to propagate at a reasonable speed.

An increase in stroke is accomplished with a new crankshaft and/or pin offsets between the crank and the conn rod.  This has the effect of causing the piston to rise higher and drop lower at TDC and BDC, effectively creating a larger circle in which the crankshaft spins.  Obviously, there needs to be room within the crankcase for this new length, but there also needs to be room for the pistons.  Because the pistons descend lower, the skirts are more likely to make contact with the cases or the crank, itself.  On the top end, the piston now might be making contact with the head and so that issue will need to be dealt with as well.  Pistons with the wrist pins situated closer to the ring lands (this lowers the piston at both TDC and BDC) and longer cylinders with higher decks are some examples of solutions, though the latter is usually reserved for big spenders or auto enthusiasts.  It's also not uncommon to use shorter rods in combination with a stroke increase.  This will help to reduce side-loading and usually reduces the weight and hence the forces at play.

So... which to choose?  That is going to depend on many factors.  Generally speaking, stroking an engine is going to give you more torque than an equal increase in displacement coming from bore alone, but the investment in time and money will be a lot greater.  HD engines LOVE a good stroking (don't we all?) and their popularity means there are a lot of aftermarket options available to pursue that avenue.  Aftermarket options for other engines may be more limited and so you'll have to undertake a lot of work yourself or follow the path of those that have come before.

My advice is to shoot for a modest increase in bore and then call it good, unless you're running with an HD engine.  Any increase in displacement is going to be a good thing and so, for this topic, the focus should be more on what's cost effective for your build rather than what provides the exact traits desired.  That said, if anyone feels like stroking out a CB350, please let me know.  That's a thread I want to be following.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: byrdo444 on Aug 01, 2012, 18:42:28
keep it coming! and thank you in advance. I wont get to my engine for a long time but when I'm ready to tackle the beast I will be back here taking notes! SUBBED
Title: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: rock2d2 on Aug 02, 2012, 02:16:24
Wow. Just wow. Not only have you given me good advice in the recent past but now you continue to put your $$ where your mouth is and provide this community with great knowledge.

It's a slippery slope when people just assume someone else (a newbie like me) has a ready grasp of knowledge required and then judges them for not completing a build as in depth or technically as others think they should. From someone (me) who has, not a build thread for their bike but instead a "bolt on" thread I cannot tell you how important all this information is for some.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: legendary_rider on Aug 02, 2012, 11:40:01
Right on. Thanks. Based on this thread I did it right when I dropped a bunch of money into the engine before anything else but still lack the knowledge to bring it all together. Performance before looks though. lol.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: swan on Aug 02, 2012, 18:37:40
Sonreir and Kopcicle you are right, cafe racers are about performance and handling, not fashion. Unfortunately, this primary characteristic is lost on many people recently drawn to cafe racers because apparently they are suddenly "cool" or the next 'big thing".   Cafe racers have been around since the 1930's and have never gone away. The recent popularity will eventually wane as people realize they actually need to maintain their new fashion accessory. 

While I encourage anyone to get on two wheels of any style, but I get really fucking annoyed reading member introductions and cafe build threads prattling on about looks and plans cosmetic modifications ( flat black, root beer powder coating, pinstriping, hole drilling etc) while ignoring basic improvements to the motor, crank balancing, fuel intake, braking, suspension etc. I will take a fast, well handling ugly or stock bike over a beautiful paint job and chrome any day.

We were all noobs once and although I have been riding motorcycles for more than 30 years and building cafe racers for more than 10 years, I strive to learn new build information and skills everyday. DTT and other forums have made information readily available and am thankful for them and I enjoy posting and sharing information if it helps another rider. Tuning information and instruction has never been easier to find and there is no excuse to not use it. With no formal training and being completely self-taught, I have built more than a dozen cafe racers including a highly modified and race tuned Triton. If I can do it, so can anyone else.

"Built not bought" though I appreciate the thought, is a slippery slope. Not everyone has a full shop, lathe, mills, welding equipment, CNC, casting equipment etc so some parts need to be bought. Even with access to a complete shop would you make you own frame? Cast your own engine casings, mill your con rods, design and turn your own hubs, or roll and punch your own rims?? Some things need to bought, but I encourage everyone to DIY as much as possible.

For me, the feeling of pushing a bike I built myself to the top end of its speed, cornering and handling and coming out alive is better than any compliment or praise you may get for how it looks parked in the street.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Aug 02, 2012, 19:39:05
The key for me is making the most of what you have. Not all of us have the resources to mine iron ore,forge con rods etc or even to weld or do simple machining.  We have different experience and knowledge but we can all aspire to do the best job we can on our projects.

Fancy paint is nice, but running without fenders or a fork brace is just silly.  Flow tested heads with big valves and cams that look like bricks on sticks look great in a thread but are rarely what we need.  What we do need are frames that are straight, wheels that run true and brakes that don't drag, levers that pull smoothly and cables the right length, motors that are clean and well adjusted and electrical connections that are clean.

The list goes on, but it really is about maximizing what we already have. Make it work properly.  get it so the levers are the right angle for your wrists and the bars and pegs are comfortable.  If we don't take care of the basics, all we have a shiny turd. Sorry to be so blunt, but it's true.

Who needs a shiny bike with all that cafe Racer style if it is hard to operate and doesn't do anything well? no one with any self esteem. It's all in the details and if you get those right, the bike will be fun to ride and will look good too.

Sure , we all like positive strokes and affirmation, but they don't last long if my bike is harder to ride than stock and is slower and handles badly at anything above in town speeds.  That's not a cafe racer - it's a fashion statement.

Bike can be a ton of fun to ride, and to work on and first and foremost they have to work well and be safe.

That's why so many posts exhort newbies to get it running first and learn to ride it and have fun riding. "building" is a poor substitute for riding IMHO especially for people with only 1 bike and limited experience and cash.

That's just my opinion.  Your mileage may vary.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: stroker crazy on Aug 02, 2012, 21:43:03
A great thread; sometimes we need to step back to focus on the full picture!

I will be reading this primer on the café racer with great interest.  Are there any plans for strokers to get a look-in?

Crazy


(http://blg.mypicturetown.com/cache/62PPgnDHjmSAqViLKu1prRbKuLPN-2VBW%25c3i0HKxkzOmN6j6%26abv-udH_.SW31C/item.JPG?)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Aug 03, 2012, 02:47:45
I don't disagree with Sonrier's treatise in general, but I'd add that the way to improve performance is first and foremost to reduce lost energy.  That's free performance and it's there to be taken.  Second is to improve volumetric efficiency and means making it breathe easier.  It can include cams and porting, but that's not always necessary or desirable - it all depends on your goals.

You can't beat cubes was an old saying that holds true to this day, a bigger engine will usually make more power than a smaller one, but the costs increase even faster.

let's take some examples.  I have say a CB360 and I decide I want more power and less weight. How much more power do I want and what is the path to that power?  Do I need bigger pistons custom made to my spec to go into new carbide coated aluminum sleeves and a lightened crank and Titanium rods etc.  And at the end of the day if I created the ultimate CB360 how much power would it make and would I have been better off just buying a more modern bike for less cash?

For me, the starting point was always to reduce weight because that's like free power - as long as I don't take that to extremes and have things break.

Then I can make sure that the wheels spin freely and brakes don't drag and there's more free performance. After that I can carefully fit thinner wheels for less rolling resistance or fatter tires for more side grip and I end up with stock sized tires in a soft sticky compound for the best of both worlds. 

Then I upgrade suspension at both ends to improve handling so it goes around bends faster or at least more securely and safely.

At that point I can think about modifying pistons, lightening cranks, porting the head, increasing compression and after that I'll think about how much more I want to spend of this sweet riding motorcycle. I'm an engine guy at heart but most of my performance improvement comes from attention to detail and a logical approach and critical thinking.

In the race world, as in the custom world, most modified bikes are slower than stock until the rider starts to put the pieces together and  gets the details right.  How many threads are about jetting and oil leaks and ignition timing and how many are about cast versus forged pistons or ways to reduce pumping losses? Make the most of what you have with the resources you have available. It's an optimization exercise and not a maximization trip.

Motorcycles are systems and the components don't exist on their own. Everything in life is a balance, and soit is with bikes.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: ronnie on Aug 03, 2012, 03:15:54
Great information, keep it going Sonrier..
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: 808shadow on Aug 03, 2012, 03:49:34
Awesome post. Were actually learning about auto cycles right now in my thermodynamics class and I find it fascinating. Even more so now that I've seen a literal application of these theories and math now in the forum. Reading to reality makes all the difference similar to what was said in the original post. I was wondering if someone could calculate out the increase of a small improvement such as a velocity stack. I would love to see how much of a difference they make mathematically if someone is able?  Thanks for the posts and keep em coming!

Sent from my PC36100 using Tapatalk 2
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: stroker crazy on Aug 03, 2012, 04:43:24
808shadow - if you're interested in some of the mathematics for intake bellmouth design I can PM  to you the pdf file of a research paper by Gordon P. Blair and W. Melvin Cahoon,

Crazy
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Aug 03, 2012, 11:36:48
Just a slight caveat to my post about displacement.  I tend to get caught up quite a bit in the minutiae of things and perhaps didn't make my conclusion strong enough in that post.

A friend read over what I wrote and had this to say:
Quote
The longstroke and shortstroke stuff is largely irrevelent in the modern era.  A CX 500 has a very short stroke and produces good power up and down in the rev range.  Rod length/stroke ratio is a very important determiner of power production and output.

And I do agree.  There's usually not much of a bad way to go about getting more displacement.  For you newer guys, any way you can get more CCs without adversely affecting your engine, that's what you want to do.  While not all displacement is created equal, stroke and bore are not leagues apart, either.

As for rod length:stroke, that is something I neglected to mention too much about.  A topic for a future post, perhaps...

Also, I totally agree with what swan and teazer have added.  Especially the portion about getting it running and learning your bike before you tear into it.  You MUST, MUST, MUST, have a baseline for your modifications.  If you modify a bike you've never ridden or don't know well, how are you to know whether your changes are improvements or detriments?

Finally, I also agree with teazer about tackling losses and volumetric efficiency, first.  VE, especially, is a topic that will take pages to cover and so I'm leaving it for a bit later.  The order in which I choose to address these topics has more to do with getting the "simple" stuff done first rather than implying that this is the order in which things should be done.  Trimming weight and, to a lesser extent, increases in displacement can be done with a minimum of investment.  When you start talking about significant gains in volumetric efficiency is when the money really starts to matter.  A future post, to be sure.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Aug 03, 2012, 17:27:23
808, I don't have the theoretical data but this empirical data was published by David Vizard and is posted on Michael Moore's Eurospares site

http://www.eurospares.com/graphics/engine/Vizard/

Scroll down until you get to VizardVstack001a.tif and download that image.


You will see that taper helps but the key to good flow is a rolled entry to allow air to enter smoothly from all around the stack.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Aug 03, 2012, 21:12:59
Power Goal #2 - Increased Revs

To first understand why revving an engine produces more power, you must understand that there is a difference between Force and Power.  Force is something that causes an object to undergo a change in direction or movement.  Power is how quickly that force is applied.  Zero force applied over any period of time is always zero power and infinite force applied over any period of time (except zero) is always infinite power.

Since I'm going to mainly talk about horsepower, I'll stick (mostly) with imperial units for this one.  This means we'll be dealing with foot-pounds for our unit of torque and horsepower for our unit of power.  For true correctness, this force unit is actually called a pound-foot, but the two terms are generally interchangeable, so I'll use with ft/lb as it's a bit more common.  A foot-pound is defined as the amount of force, measured at a central pivot point, generated by applying one pound of force at a distance of one foot from that central pivot point.  Because ft/lb is dependent on both direction and magnitude, this makes it a vector unit.  It can also be thought of as a force that "twists" something.  Because torque is a function of both the initial force (one pound) and the initial distance (one foot), it can be increased by either utilizing a larger distance or a larger initial force.  Doubling the distance or doubling the initial force will also double the torque.

Hopefully, that gives a basic idea about torque, because you need to understand torque to understand power.  Power is how much force is applied over time.  The definition of mechanical horsepower (the kind we equate with engines) is 33,000 ft/lb per minute.

If you recall from my first post, horsepower is derived from torque using the following equation:
     HP = (T * rpm) / 5252.

So how does that relate back to 33,000 ft/lb/min?  Well, all that has to do with our inclusion of Revolutions (it's the 'r' part of rpm) into the HP equation.  One revolution is equal to 2pi radians in the math world.  Dividing 33,000 by 2pi gets us 5252 and this is the reason for the use of that constant number in the equation.

Enough math for the moment; the important thing to remember is that without torque, you have no power.  Applying your torque faster gets you more power.  Furthermore, unless you're applying your torque with an appropriate speed, you also have no power.  To get power from an engine it must not only produce force, but apply that force quickly.  While the human body can produce great amounts of torque (especially with mechanical advantages), our inability to apply that torque quickly is what prevents bicycles from breaking the ton.

So... nitty-gritty time.  How does one go about revving an engine faster?  Well, it's not quite as easy as changing gears later in the rev range, though this is a basic gist of it.  There are several main factors in preventing your engine from operating well at high RPMs and each of these issues must be dealt with in order to create an engine lives up to its potential as it approaches five figures.

Mechanical Considerations
Though most everything on an engine can be qualified as a mechanical consideration, for this section I mean mechanical failure considerations.  If you hold the throttle open on your bike, in neutral, you will probably destroy your engine because of a mechanical failure.  I say probably, because some engines will fail past that point at which they stop making power and they don't make enough power to get to the immediate point of failure (I'll get to that in a second).  On race bikes, over revving an engine is a common cause of failure.  When you live on the edge, it's easy to fall off.

When a four stroke engine suffers a mechanical failure it almost always occurs at the end of the exhaust stroke / beginning of the intake stroke.  Why this occurs is quite simple and can be reduced to a few key points:

1.)  Almost all metals (including those used in the construction of your conn rods, crank, etc) have a higher strength rating when comparing compressive (pushing) to tensile (pulling) forces.
2.)  The tensile forces on the crank, piston, and conn rods are the greatest at the end of the exhaust stroke for two reasons:  First, the piston is the furthest from the crankshaft centerline at TDC than it is at any other point in the rotation and if rotational speed is held constant, the most force is exerted at the furthest point from the center of rotation.  Second, at the end of the compression stroke (the other time the piston is at TDC), there is less stress because the compression of the fuel/air mix combined with the increasing pressure of the ignition of the mixture reduces the tensile loading of the components and so they are less likely to fail.  In short, the compressed/ignited mixture is acting like a pillow for the piston and helping to cushion it.
3.)  For top end failures (due to valve float, etc), the exhaust stroke remains the most common because the speed at which the piston chases the exhaust valve is greater than the speed at which the intake valve chases the piston.  During the compression stroke, the exhaust valve is closed, and so the end of the exhaust stroke remains the critical time.  If you float a valve, it's going to be the exhaust valve that eats it first.

So how do you prevent mechanical failures?  Think of two words: Stronger and Lighter.  Chromoly steel is the weapon of choice for crankshafts, connecting rods, and rocker arms.  Titanium makes for excellent valves, though stainless steel is also a good option.  For pistons, forged aluminum is generally the best option, though hypereutectic aluminum will usually work for most applications where weight is more important than strength.

Remember, as speeds double, forces quadruple.  Pistons weighing in at 200 grams will produce 10 joules of energy at 10 m/s but 40 joules at 20 m/s.  Dropping the weight of your pistons by five grams will save you a joule based on mean piston speeds (it'll actually save you more on peak stresses, though).  That may not sound like a lot, but on a Honda 360, that's an extra 150 RPM you gain on the redline before a potential bottom end failure.  Again, 150 RPM may not sound like a lot, but lets say you can hold a steady 15ft/lb of torque from 11,850 RPM to 12,000 RPM.  For the only 150 RPMs, you've gained almost half of a horsepower.  Tacking on a thousand RPM instead of just 150 would net you closer to three horsepower, or an 8% increase (assuming that torque holds steady, which it won't).

When it comes to building a maximum effort engine, the little things don't mean a lot.  They mean EVERYTHING.  A gram here, a gram there frees up a joule here or a joule there, which adds a few more revs to your redline, which adds a few more ponies to your bike.

Pumping Losses and Parasitic Forces
Most people (it seems to me, at least) tend to think only of the mechanical failure portion of chasing RPMs, an equal consideration would be pumping losses and frictional losses.  While not sending rods through your crankcase is important, it's also useful to realize that the work your engine must do increases exponentially with the speed at which it operates.

For instance, as the rotational speed of the crankshaft increases, this causes more side-loading on the pistons and the friction between pistons/rings against the cylinder walls increases because of this.  Furthermore, it requires more energy to keep your pistons in motion because they are not only traveling faster through the cylinder, they're stopping and accelerating more often.  This change in direction, called reciprocation, requires more and more energy as rotational speeds increase.  Rocker arms, too, reciprocate and energy is required to move them.

Engines built to withstand high RPMs must also be able to deal with a phenomenon called, "valve float".  Valve float occurs when the valves springs are unable to provide enough force to close the valves in the time required.  To combat valve float, stronger springs are used in the valve train.  Unfortunately, strong springs means the engine has to use more force in opening the springs than previously required.  Force over time is power and so by strengthening the springs for more revs, you now require more power to open them.

Pumping losses are a big concern as well.  At high speeds, up to 20% of the power your engine creates goes directly into overcoming pumping losses (both above and below piston, though losses above the piston tend to accumulate more quickly in the very high RPM range as compared to below piston losses).  And that's even before you factor in friction, decreases in BMEP, or other concerns.

I won't spend too much time on pumping losses as it will be covered on more detail once I start attacking the BMEP section, but I will at least give a definition and a few examples.

Because ICEs are basically air/heat pumps, they all suffer from pumping losses of some sort.  And since air is a fluid, frictional fluid laws apply (Newton's third law of motion, again).  Think of this in the same way you would think of the the wind while you're riding.  There is a bigger difference in wind resistance between 60mph and 90mph than there is between 30mph and 60mph.

Air has inertia (and hence, momentum) and so accelerating it faster to fill a cylinder in less time requires more energy.  As your piston is descending and the intake valve remains open, a low pressure zone is created within the cylinder.  This low pressure is not only pulling air into the cylinder, it's also pulling up on the piston as it's trying to descend.  Likewise, as your piston starts the compression stroke, the mixture within the cylinder must be compressed at a more rapid rate.  The relationship between force and acceleration means that as greater acceleration is required, the greater the force must be.  These losses occur on the exhaust stroke, too, but in reverse.  The spent exhaust gases will initially rush to the lower pressure areas of the headers, but as the pressures within the cylinder and the exhaust system begin to equalize, the force of the piston is required to help expel the gases from the cylinder.  As with other fluid dynamics, the force required from the piston to expel the exhaust gases increases as engine speed increases.

Combatting above piston pumping losses can be achieved in a number of ways, but on older engines our usual plan is the same as increasing BMEP, which I will address in a future post.  It's also important to note that pumping losses are a byproduct of all ICEs.  You will never fully eliminate them.

Below piston pumping losses must be considered as well (especially with two strokes, because the crank cases are not open to the atmosphere).  No matter how well your cylinder is sealed, there will always be some blow by.  Some of the exhaust gases make it past the piston rings and into the crankcase.  This represents an increase in pressure within the crankcase and it is usually vented out through the crank case breather.  Though usually not a major concern for 180° twins or inline four-cylinder bikes, below piston pumping losses start becoming a real problem for other configurations.  This is because the crankcase volume varies depending on the angle of the crankshaft.  For 360° twins, both pistons are at TDC or BDC at the same time.  At TDC, the crankcase volume is the greatest and at BDC it is the smallest.  As the pistons approach TDC, a lower crankcase pressure requires more force to overcome it.  Likewise, as the pistons approach BDC, higher crankcase pressures require more force to overcome it.  Regardless of the engine configuration, however, the variations between the cylinder pressure and the crankcase pressure mean that some level of force is being exerted on the pistons as they travel between BDC and TDC.  Below piston pumping losses would only disappear if the crankcase pressure and the cylinder pressures were the same, always, which isn't possible.

For limiting below piston pumping losses, your best friend is going to be decent crankcase ventilation.

Combustion Speeds
Finally, there is one more major consideration that prevents high RPMs from being achievable, and that is combustion speed.  The gasoline being burned must create pressure at a rate which exceeds the increase in volume of the cylinder as the piston descends.  The reason for this is (again) inertia.  The momentum of the crank will want to keep the pistons reciprocating long after the the engine stops receiving fuel, air, and spark.  In a high RPM situation, the reciprocating nature of the pistons "steals" some of the energy from the combustion event because the piston was going back down whether or not that mixture was ignited.  It's the difference in trying to punch someone standing still or punch something that's running away from you.  You're going to get a much better hit on the guy who's standing there waiting for it, and the same is true for your pistons.  If they're running away from the flame, you're not making power.

There are some tried-and-true methods for increasing the combustion speed of your mixture, however.  One common way is ignition advance.  By lighting the mixture sooner, you can actually begin building pressure while the piston is still accelerating toward TDC.  By the time it reaches TDC, you also have a ton (literally) of PSI built up and so it pushes the piston back down with greater force.

The next, is good atomization of the fuel and air mixture.  Smaller droplets of fuel burn faster than bigger droplets and so they release their energy faster.  This occurs naturally as your piston speeds increase, which is why additional ignition advance past 3,000 or 4,000 usually isn't necessary.  Using the correct jets in your carbs, decent squish bands in your combustion chambers, and tumble/swirl from your valves also helps (more on these things in the BMEP section).

Also, increased compression will help combustion speeds.  Due to Brownian motion, an increase of density in the air molecules hammers the fuel droplets into smaller portions and smaller portions means faster burn times.  Increased compression also causes a spike in heat which helps to vaporize the fuel right before ignition.

Finally, it should be noted that low octane fuels burn faster than high octane.  It's a common misconception that octane is some measurement of power.  It's not.  Octane is technically a molecule, but it's commonly used as a word to describe a fuel's resistance to ignition.  That's right; higher octane fuels don't catch fire as well as low octane fuels.  Honda's famous RC166 bike required the use of 87 octane fuel in order to reach its redline of 18,500 RPM.  For more "streetable" motors, it's better to opt with higher compression and higher octane fuel as the increase in torque in the lower RPMs is usually more desirable than a few extra turns of the crankshaft at the redline.

Conclusion
In closing, chasing RPMs is actually a losing game (in the long run).  You have almost everything working against you.  As your RPMs increase, the cost in doing it, while still producing torque, is exponential.  It's OK to add a few extra revs once you've tackled some of the other performance areas, but merely adding components for the sole purpose of allowing more revs is not good value for money (though it can be done).  One thing I didn't mention in detail is that it's very possible to rev an engine past the point of being useful.  As more and more energy is required, that's less and less energy available toward moving you forward.  To add insult to injury, the redline of an engine is usually pretty far past peak torque and so while your power may be increasing (slowly), your torque is quickly dropping off.  The faster your torque drops, the slower your horsepower rises.  A large enough drop in torque and your horsepower begins to fall as well.  Building a high revving motor usually requires a peak torque to occur fairly late in the RPM band.  This may build winning race bikes, but they won't be much use on the street.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: legendary_rider on Aug 04, 2012, 11:42:55
 :o I didn't like hearing that last bit. lol. Looks like I'll be building another motor. lol. Thanks for all the input Matt.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Aug 04, 2012, 13:23:23
To Matt's last point, a full race motor is a complete PIA on the street.  Nothing happens until it's revved hard and getting away from the lights is fun for the first time and gets more miserable as time goes on until the clutch fails from abuse and then it's game over.

Even for the track, full race cams are rarely the way to go except at Daytona or RA. What you need is torque and you need it at reasonable revs. The two ways to get that at low revs are cubes and compression. Big ports, valves or carbs all hurt low to mid range pwoer and torque.

Thnink mild and think efficient. 
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: veloracermike on Aug 04, 2012, 15:52:10
I don't disagree with Sonrier's treatise in general, but I'd add that the way to improve performance is first and foremost to reduce lost energy.  That's free performance and it's there to be taken.  Second is to improve volumetric efficiency and means making it breathe easier.  It can include cams and porting, but that's not always necessary or desirable - it all depends on your goals.

You can't beat cubes was an old saying that holds true to this day, a bigger engine will usually make more power than a smaller one, but the costs increase even faster.

let's take some examples.  I have say a CB360 and I decide I want more power and less weight. How much more power do I want and what is the path to that power?  Do I need bigger pistons custom made to my spec to go into new carbide coated aluminum sleeves and a lightened crank and Titanium rods etc.  And at the end of the day if I created the ultimate CB360 how much power would it make and would I have been better off just buying a more modern bike for less cash?

For me, the starting point was always to reduce weight because that's like free power - as long as I don't take that to extremes and have things break.

Then I can make sure that the wheels spin freely and brakes don't drag and there's more free performance. After that I can carefully fit thinner wheels for less rolling resistance or fatter tires for more side grip and I end up with stock sized tires in a soft sticky compound for the best of both worlds. 

Then I upgrade suspension at both ends to improve handling so it goes around bends faster or at least more securely and safely.

At that point I can think about modifying pistons, lightening cranks, porting the head, increasing compression and after that I'll think about how much more I want to spend of this sweet riding motorcycle. I'm an engine guy at heart but most of my performance improvement comes from attention to detail and a logical approach and critical thinking.

In the race world, as in the custom world, most modified bikes are slower than stock until the rider starts to put the pieces together and  gets the details right.  How many threads are about jetting and oil leaks and ignition timing and how many are about cast versus forged pistons or ways to reduce pumping losses? Make the most of what you have with the resources you have available. It's an optimization exercise and not a maximization trip.

Motorcycles are systems and the components don't exist on their own. Everything in life is a balance, and soit is with bikes.

Bingo.  First lose the excess weight.  In doing so you will most likely be making other performance gains, a more free flowing exhaust system is going weigh less than the stock exhaust.   An after market exhaust is going to lead to induction changes as well.   Strip the frame of excess.  Lighter battery, smaller of if you are inclined no signals.  Ditch the steel wheels and go with al shouldered wheel. Lighter wheels will spin up faster and decelerate faster.  Lighter bike same power equals faster bike.  Lighter bike turns and stops better.   

Little things that will improve engine performance. Proper jetting and sync along with clean carbs will make the bike run better.  Proper valve adjustment and spark also will improve performance. Before building a bigger motor do these things. 

As for the builder v buyer argument I put my bike together.  I didn't paint it. I designed the paint scheme and helped the painter lay it out.  I cant' weld so I had to have certain things done, but they were done to my design.   I don't have a machine shop so some things that I put on my bike were made by people who do, like my rearsets.  I did all my own custom linkage though.   I've done about 90% of the wrenching on my bike.  I'm a builder?  Well not in the sense of say a Lossa but I also didn't hand my bike over to guy like Lossa and say "I wan't one of them style cafe bikes" either. 
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: legendary_rider on Aug 05, 2012, 13:06:15
To Matt's last point, a full race motor is a complete PIA on the street.  Nothing happens until it's revved hard and getting away from the lights is fun for the first time and gets more miserable as time goes on until the clutch fails from abuse and then it's game over.

Even for the track, full race cams are rarely the way to go except at Daytona or RA. What you need is torque and you need it at reasonable revs. The two ways to get that at low revs are cubes and compression. Big ports, valves or carbs all hurt low to mid range pwoer and torque.

Thnink mild and think efficient.

I built my bike to "do the ton" and then some. I wasn't thinking of putting around on the street at the time. It's meant to fly. I started thinking purdy mid point in the build. It'll be good for back country roads or on a race track.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Aug 08, 2012, 14:50:33
Power Goal #3 - Decreasing Parasitic Losses

The concept behind this goal is very simple, but the means used to achieve it can be exceeding complex and nearly without limit.  I'll cover some of the more common methods as well as the reasons behind them.  Freeing up power through a reduction in parasitic losses is one of the more elegant approaches to the problem of building power and is one of the few areas where gains tend to be exponential as engine speed increases.  Though this is often a long and difficult path, the rewards are many.  Decreased fuel consumption in combination with better acceleration and improved engine response can't be found just anywhere.

First up, it's very important to understand that all mass has inertia and inertia is what causes momentum.  Inertia is an object's resistance to changes in movement.  This includes an object's resistance to acceleration AND deceleration.  The more mass you have, the more inertia you have.  Quite simply, something that weighs a lot is hard to move and hard to stop and this relationship is inversely proportionate.  For every doubling in mass, you halve the acceleration.  Or, if you wish to maintain the same acceleration while doubling the mass, you must double the force.

Know, also, that this applies to EVERY movement on and within your bike.  The weight of the bike and rider both have an effect on acceleration.  If you are able to cut the weight of both yourself and your bike in half, you have just doubled your acceleration.

A bit of a corollary, here, but don't confuse acceleration and top speed.  You won't hit the ton by shedding weight, but your 0-60 times will be a lot better.  In order to hit the ton, you need power and/or streamlining.  It's important to know that acceleration is a function of power VS weight whereas top speed is a function of power VS friction (of the air).  If you want to be faster, you need more power and less drag.  If you want to be quicker, you need more power and less weight.

Anyway... back to the main point.  Every part on your bike moves, takes energy to do it.  The wheels need energy to rotate.  The piston rings need energy to overcome the friction of rubbing on the cylinder walls.  The cam and rockers need energy to be able to compress the valve springs and actuate the valves.  Your transmission needs energy to spin the gears and your chain needs energy to bend each and every single link as the link rounds your sprockets.  The vibration in your handlebars, when you blip the throttle, takes energy.

Now, you may notice from a few examples above, that not all parasitic losses have to do with inertia and so you need to understand how these different losses occur in order to be able to combat them.  As teazer mentioned earlier, this is an excellent starting point for a lot of engine builds and is THE starting point for professional race builders.  Your engine must be capable of getting as much power to the ground as possible, otherwise you're just throwing good money after bad.  You're not going to invest money in a company that is inefficient, why would you invest money in an engine that is inefficient?

Blueprinting
Blueprinting is the process by which most race engines begin their life.  While the concept is relatively simple, the process is tedious, difficult, and expensive.  Most of us don't have the money, skills, and/or equipment to be able to tackle a full engine blueprint, but it's something nice to consider or, at least, be aware of.

The blueprinting process involves disassembly of most or all of the engine components.  In an ideal world, brand new components are used for this process, often unfinished from the factory.  All components are checked for clearance specifications and then adjusted, if necessary.  Reciprocating and rotating components are also checked for balance.  It's important to note that the blueprinting process doesn't usually involve taking an engine's running specifications out to a different measure, but rather making use of the factory specs and just decreasing the tolerances.

For instance, the top ring end gap on a CB360 should be between .15 and .35 millimeters when it comes from the factory.  If I were building a drag racing bike from a 360 motor I may request that the ring end gap be between .150 and .185 millimeters.  A race bike designed to cover 300 road racing miles may specify a ring end gap of .250 and .350 millimeters.  In both cases, the specification falls within the manufacturer's allowances, but the precision is increased and a bias is given depending on the purpose of the engine.  For a drag bike that sees short bursts of power and frequent rebuilds with only few miles, a smaller gap will help with cylinder sealing and provide a bit more power.  For a road race bike that has to compete at high RPMs over a longer distance, the increased gap allows for more thermal expansion without a significant increase in friction on the cylinder walls.

Frictional Losses
Friction is the resistance faced when the surface of one object rubs up against the surface of another.  These two surfaces can be made up of anything that has mass.  So a bike traveling along the road encounters friction from the air as the air contacts all of the surfaces of the bike.  There is also friction from the tires contacting the road surface.  There is even friction from your metal parts rubbing up against oiled surfaces.

Inside your engine, two thirds of the friction will come from the piston assemblies, with the biggest majority of that being the rubbing of the piston rings against the cylinder walls.  The amount of friction from the rings is great enough to the point where many racing engines often run with only with a single ring, in order to keep friction to a minimum.  Unfortunately, this is not an option for most of us.  A single ring will not only reduce compression (forcing you to make it up elsewhere), but it also requires the use of aluminum cylinders with special coatings.  Single-ringed pistons also require frequent rebuilds; something else that most of us don't want to have to do.

There are modifications that can be done to further reduce the friction from the piston assembly, however.  One of the more common methods is to reduce the length of the piston skirts.  This is a great option because it also reduces the weight of the pistons.  Two birds with one stone.  In order to be able to remove metal from the pistons skirts, tight tolerances are needed.  The piston skirts give a mechanical advantage to the cylinder walls when it comes to keeping your piston from rocking back and forth.  You must use tighter tolerances to prevent this rocking motion if the skirts are to be shortened.  It is definitely possible to go too short on the skirts.  The ideal skirt length on any given bike will be different, based not only on the make and model, but the tolerances which are being employed.  Consult the professionals before making any changes.

In addition to skirt changes, is it not terribly uncommon to slightly offset the wrist pin of the piston to reduce side loading on the thrust side.  As I mentioned the previous post on displacement, the pistons are being pushed up against the side of the cylinder walls by the force of the crankshaft resisting the downward motion of the pistons during the power stroke.  By placing the wrist pin further away from the thrust side, it helps to reduce the mechanical advantage of the conn rod against the cylinder wall.  This directly translates into a savings in friction as the two surfaces are no longer being pushed together with the same force. 

Finally, further friction can be saved in the piston assembly by shortening the stroke.  Less distance for the rings and skirts to move against the cylinders means less friction.  This option requires careful consideration, however, as it will decrease displacement and compression, both.  It may also lead to an increased likelihood of detonation.  To help illustrate (side-loading, especially), I jacked this picture off the Internet.  Pay special attention to the upwards pointing arrow coming through the conn rod.  This is the force that is pushing your piston against the cylinder wall.

(https://sphotos-b.xx.fbcdn.net/hphotos-ash4/484528_10151154924775159_621825552_n.jpg)

The next largest source of friction within the engine is the valve train.  Most of this friction comes from the followers on the cam, but a good deal also comes from the valves and valve guides.  Reduction of friction between the cam and the follower usually isn't a primary goal because this is one of the very few areas in your engine where friction actually decreases as RPMs increase.  The inertia of the rocker arms and valves tends to work in our favor with this component.  As the speed of the camshaft increases, the rocker arms' resistance to movement means they don't press quite as hard against the camshaft.

The most common solution to the valve friction problem is to ensure everything is well within specifications.  Too little clearance and the increased interference causes friction.  Too much clearance and the valve stem will wobble within the guide and this causes friction, too.  It is also quite common to make materials changes as well.  Bronze offers less friction than aluminum or iron and so it is common for use in valve guides and bushings.  Most aftermarket valves will come with some sort of coating that aids in heat dissipation and/or friction.

Tackling friction in other areas of the engine starts to take some imagination.  Oil viscosity is one prime example.  Using a lower viscosity oil will usually result in less frictional loss because the oil isn't quite as thick.  However, at high RPM operations, a lower viscosity oil may not provide enough lubrication and friction will increase (not to mention wear on engine components).  Replacement of journals with bearings (often of the roller variety) will reduce friction in other areas as well.  Perhaps going to a dry sump and dry clutch are an option for your motor?  This will enable the use synthetic oils, which are generally slipperier.  It also keeps friction down because engine parts don't need to be drug through an oil bath.

When dealing with friction, the goal is to reduce contact as much as possible in as many places as possible.  Where contact is necessary, ensure the surfaces areas are well lubricated with quality oils.

Losses due to Inertia
Inertial losses aren't really losses, per se, but inertia does have an effect on your engine and so I'll discuss it, but briefly.  First off all, let me clarify what I mean by a "loss".  Yes, it takes energy to accelerate your pistons to TDC, stop them, and the reverse their direction.  But this isn't a loss in energy, it's merely changing where the energy is stored.

For instance, as your piston reaches the end of the exhaust stoke and approaches TDC, it must slow down and as it leaves TDC and approaches 72°, it accelerates.  "Ah ha, that takes energy to accelerate the piston and energy to decelerate the piston!", I can hear many of you say.  You are correct.  But your assumption on where that energy is coming from may need some revision.  As the piston is decelerating, it pulls on the crank and that causes the crank to accelerate and store the energy from the decelerating piston.  Next, as the piston passes TDC and begins to accelerate, the energy (for the intake stroke, at least) is coming from the, now decelerating, crankshaft.  The energy to move the pistons up and down isn't lost, it's merely transferred.  Now bear in mind that this energy transfer is not without losses, but they are minor.  The majority of losses during this process come back to our old friend friction.

This same concept applies to your valve train.  Yes, the valve springs take a lot of energy to compress.  But that energy is largely returned to the system as the springs decompress and apply pressure back to the camshaft through the rocker arms.

The biggest concern about inertia is acceleration.  While much of the energy that goes into creating inertia within your engine is reclaimed every or every other rotation, there still must be the initial expenditure of energy.  The energy has to go in before it can come out.  This creates a direction relationship between the inertia in your engine and the rate at which it accelerates.

The more mass and inertia your engine has, the more energy that is required to accelerate it.  But also, more energy is required to decelerate it.  Many production engines that have have areas at which great initial investment of energy is required will make use of heavier flywheels in order to preserve inertia.  Diesel engines, with their compression ratios approaching 20:1, will often make use of heavier flywheels.  This allows for smoother engine operations because the energy stored within the flywheel helps overcome the resistance of the mixture to compression and so keeps the engine turning at a more uniform rate throughout its rotation.

Generally speaking, the more inertia within your engine, the harder it is to start and the slower it will be to accelerate or decelerate.  Furthermore, the increased weight of the components will generally cause an increase in friction in all connecting assemblies.  One possible benefit to increased inertia is a lower idling speed, but this usually translates into a lower redline as well.

In a bike with a sport pedigree, the goal should be to lower inertia as much as possible.  In any moving part, your goal should be lighter without sacrificing strength.  The energy required to get these parts moving is energy that would otherwise go toward accelerating your bike.  On decel, an engine with lower inertia can also make better use of engine braking and provided your rubber holds, your bike will stop better as well.

For those of you that have been following crazypj's 360 build(s), you can see this in effect in the modifications he's made to his rotor and gears.  This is done to reduce inertia.
(http://i91.photobucket.com/albums/k315/1crazypj/Honda%20CB360/CB360modifiedgeneratorrotor.jpg) (http://i91.photobucket.com/albums/k315/1crazypj/Honda%20CB360/CB360primarygear1.jpg)

Also, think outside the box (engine) for combating inertia.  The chances are, anything that moves on your bike got the energy from your engine.  Chain, wheel hubs, wheels, rubber.  All these things have inertia.

Engine Balance
The one final area of parasitic losses I will address is engine balance.  Mainly, this deals with the balance of the crankshaft and that's the area of most concern.  Other rotating components will benefit from being balanced, as well, but all to a lesser degree than the crank and piston assemblies.

So why is it important to ensure your is balanced?  Well, primarily this has to do with wear.  If an engine is out of balance it will wear more rapidly and many of the components will be placed under greater friction and greater stress.  As a more minor problem, the vibrations of an unbalanced engine can be pretty damn annoying.

There are two types of crankshaft balance.  The first, and most important, type is called the primary balance.  Primary balance is pretty simple to understand as it is basically just ensuring that the counterweights on the crankshaft properly balance out the weight of the pistons and conn rods.  You want to ensure that the center of mass for rotation along the crankshaft is as central to the crankshaft as possible.  It is isn't always necessary to change the primary balance of the crankshaft when you change pistons or conn rods, but it certainly is desirable.  This change in balance usually comes in two forms.

In order to "overbalance" a crankshaft, or add counterweight, holes are drilled into the existing counterweights and tungsten plugs are then inserted into the holes.  To "underbalance", or remove counterweight, holes are drilled and then left empty.  It is usually much easier and cheaper to remove weight from the counterweights on a crankshaft and so many aftermarket crank options (where they exist) will be intentionally overbalanced from their maker.

Just about any crankshaft in any configuration can achieve a perfect or near-perfect primary balance, at a given RPM.

Secondary balance has to do with how the assembly balances when rotating under load.  This starts to take into account the kinetic energy of the pistons (which increases as the rotational speeds increase), sideways motions of the counterweights, and any changes in balance due to offset crank pins (in the case of some stroked engines) that would cause the pistons to operate outside of a normal sine wave-like pattern (called "sinusoidal").

The primary means of secondary balancing are the phase of the pistons along the crank (it is very common to rephase 360° twins such as the XS650) and the use of balancing shafts.  These shafts rotate at twice the speed of the crankshaft and work to negate the harmonics of the secondary forces.

As mentioned earlier, nearly any engine can be balanced for primary forces, but it can be very difficult to balance secondary forces.  Rephasing is never a complete solution and at higher speeds, the balancing shafts may need balancing shafts of their own (this is not actually done, to the best of my knowledge, but that is what would be required to remedy the situation).  The configuration of the engine will have a lot to do with how it is to balanced.  Opposed cylinders like you see on many BMWs are naturally balanced for secondary forces and have no need of balancing shafts or rephasing.

This next bit is purely academic, but the best configuration for a balanced engine is a flat eight design (or any number of opposed cylinders evenly divisible by eight).  This is because each bank of pistons is opposed by the piston opposite and the outside pistons of one bank are opposed by the inside pistons on the same bank.  This cancels all major secondary forces.

Conclusion
So... now that you know the idea behind parasitic forces, the solutions should be fairly straight forward.

The primary means of reducing parasitic forces is the reduction in weight of all components.  Anywhere you can shed weight without affecting performance, do it.  Unless you are well aware of the consequences of doing so, a reduction in strength of these components is a bad idea.  Ensure all bearings are in good working order, all clearances are within spec, and your oil is changed regularly.  Keep your chain lubed and your your wheel bearings greased.  Don't over-tighten anything.

For balancing, my advice is to leave this to the pros.  It takes special equipment and special know-how to not make things worse.  The addition or reduction of weight in a crankshaft is a precision operation.  Getting it wrong by just an ounce can add a couple of hundred of pounds in unbalanced forces as crank speeds approach redline.  Rephasing can be undertaken by the garage mechanic, but requires special tools and a custom camshaft.  A daunting task for the first time, but a rewarding experience when done correctly.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Aug 08, 2012, 18:04:59
Single rings are great on race bikes and useless on teh street.  The only comparison data I saw from an actual test suggests that power above 9000 is better with a single compression ring and power below that is markedly better with twin compression rings.

On all of our 4 strike race bikes and all street bikes that means twin rings and on things like an RS125 racer that never drops below 10,000 single ring is marginally better and that's how they come.

Ring gaps make surprising little difference, but they are worth keeping an eye on. More relevant are ring designs and thickness. Thick rings flutter at speed and thinner rings can reach higher engine speeds before they flutter. It's a function of piston acceleration rather than velocity though.

Biggest source of drag is between the piston and barrel which is why pistons are not round, by\ut are oval shaped - smaller side to side than front to back. On a race motor where every little helps, it can sometimes be useful to increase piston side clearance, but it's not much of an issue on the street.

Pumping losses are an issue though and as revs rise they get worse.  On some of our race motors, we open up the space under the piston to allow the gas displaced by a falling piston to move into an adjacent chamber.  If you look at late model GSXRs there are huge holes between each adjacent cylinders to allow the gas to move around and gives them access to a larger space for lower pumping losses.  Doesn't work on 2 strokes though ;-)

without spending cubic dollars, the best way to build a motor si to make sure every part moves smoothly with minimal resistance.  Assemble one part at a time and heck it.  If it's stiff, find out why and fix it. Sometimes a part is tweaked.  Other times it's a tight bearing or a bearing cocked slightly on a shaft.  Its' the same for wheels.  If they don't spin easily find out why.

Are drum bakes rubbing or disks dragging or warped.  If so fix them.

This stuff isn't rocket science, it's about attention to detail, one little part at a time.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: snmavridis on Aug 09, 2012, 00:13:22
I've only read the first post and i'm in. THANK YOU FOR THIS THREAD. THIS IS EXACTLY WHAT I WANT TO SEE!!! coming from a family with a history for racing, i HATE "tractors with bodykits" and those civic douchebags. the engine is what i want to work on most, but i have no idea how to get started with a motorcycle engine, and i have no idea how to work on a motorcycle tranny. hopefully this thread will boil everything down. and if thats the case, you guys will be seeing a new engine build thread from  me in the coming months!!! thanks so much for this thread! this is the kind of contribution that keeps the racing spirit alive!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: juan@crqcycles on Aug 19, 2012, 01:45:16
Very interesting thread! I`m having a hard time towards performance with my 125 cc honda. I know it is not a bike that is meant to be fast, but thats what I have. I already made the thing as light as I could without dropping functionality or security; but I don't know what could I do or should do in the engine department.
Any (well thought and explained) ideas?
Thanks!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Aug 19, 2012, 16:47:48
Suggest you head over to Powroll and see what they have to offer.  There are also a ton of parts for Laifan etc Chinese clones and in Japan for small single motors. 

http://tboltusa.com/store/tb-crf100-xr100-120cc-kit-race-cam-p-222.html

http://powroll.com/P_HONDA_VINTAGE_100-125.htm for example
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: kopcicle on Aug 19, 2012, 18:35:26
Powroll , heh , heh , heh , He said Powroll ....

Okay kidding aside they are some of my favorite people to deal with . Now here is the detail . Powroll will give you dollar for dollar a larger , more reliable power increase across the rpm range than I can by way of bore and porting alone . Not that porting for the new parts won't make it better but what you get from them is surprising area under the curve .

This is a great place to experiment with porting , intake and exhaust design , larger valves , combustion chamber redesign ,,, the list goes on . If you choose not to go with the Powroll Stroker then you can acquire several top ends and swap them out within minutes . Using copper gaskets the cost is just time and sealant .

When I was in school a few decades ago I was constantly blowing one of these up . I was changing whole engines on a lunch break or top ends on a 15 in the afternoon . I tried maybe 20 exhausts and a bunch of combustion chamber designs . All I can say for the experience was it might have been slow next to all the xs400's , cb350's , rd250,s but it sure was loud .

I'll do some digging and try to find some of the notes from that prehistoric period .

~kop
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: kopcicle on Aug 19, 2012, 19:44:50
http://www.dotheton.com/forum/index.php?topic=37216.0 (http://www.dotheton.com/forum/index.php?topic=37216.0)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Aug 21, 2012, 16:11:37
Power Goal #4 - Brake Mean Effective Pressure

OK... I've saved the best for last.  To me, this is the most interesting portion of building power, but it's also one of the most complicated.  I'll be dedicating more than one post to this topic, because there is a lot to cover, here.  The majority of engine upgrades that take place, do so in order to effect cylinder pressures.  Because so many different types of upgrades exist, it's too much to cover in one post.  To that end, this post will be largely dealing with more theory, but following posts will start to talk a lot more about application.

So far, the other three topic we've covered have been relatively simple, but here's a quick recap just to make sure the point made it across:

So... what is Brake Mean Effective Pressure?  It's the average pressure within the cylinder during one cycle of the engine.  Like power, BMEP is a calculated value.  However, unlike torque or power, this number is unaffected by the displacement of the engine and so it can be a useful metric in calculating the relative ability of an engine to do work.  This means it is entire possible for a CB160 to put out a higher BMEP value than a CBX.  It's a useful number because it illustrates how well any given engine is working.  A higher number means the engine is producing more torque for its size than another engine.  BMEP is the real yard stick by which engines are measured.

To get BMEP, simply multiply the peak horsepower of an engine (in Kilowatts, as measured at the crank) by 1200 and then divide that total by the product of the engine's displacement (in liters) by the RPM at which peak horsepower is achieved.

For a 1973 Honda CB350 the formula looks like this:

BMEP = (26.85 * 1200) / (.325 * 10500) = 9.44 bar

A fairly respectable number.  Most naturally aspirated, four stroke, gasoline engines will run between 8.5 and 10 bar.  Anything above this is impressive.  Turbo engines usually run in the 14 bar range.

Before we go too much further, though, it's important to have an understanding of how cylinder pressures are created.  Contrary to popular belief, the ignition of the fuel/air mix does not create an explosion, nor does it create additional gases that fill up the cylinder.  What burning gasoline does is to create heat.  This heat is the sole contributor to cylinder pressures.  If you were to cool down the exhaust gases to room temperature, they would take up only slightly more volume than the air that went into the cylinder before ignition.

The reason for this can be explained using the Ideal Gas Law.  The IGL states that all gases will behave in a very similar fashion given the same conditions.  The part with which we are concerned is the increase in volume of a gas due to the influence of heat.  Basically speaking, all gases expand as they heat up.  The more they heat up, the more they expand.  The ratio of expansion can be calculated in a fairly simple matter.  First, you must know the starting temperature of the gas.  Let's assume 100°F.  Next, you must know the ending temperature of the gas and for this we will assume 1600°F.

We will need to convert these two temperatures to the Rankine scale and so our values now become 559.67 and 2059.67, respectively.  By dividing our upper number by our lower number we know have the expansion rate for the gas.  In this scenario, we have about 3.7.  Finally, multiply this number by 1.07 to account for the conversion of the liquid fuel into a gas and our final value is 3.94.

How about if we lower our starting temperature to 80°F and raise our ending temperature to 1700°F?  We get a final expansion ratio of 4.28.  That's almost a 9% increase in pressure over our original number.  By applying this idea to intake and combustion chambers we can quickly see how reducing intake temperatures along with increasing combustion temperatures can easily generate more pressure and more torque within your engine.

They are many different ways to increase BMEP, but my initial focus will be on two topics (which I consider to be the primary methods).

Compression Ratios
An increase in compression ratio is one of the very best things that can be done with an engine.  It provides an increase in torque throughout the RPM band and increases fuel efficiency as well.  I consider an increase in compression to almost be mandatory for any person looking to trick out their engine.  Furthermore, it is usually very simple to accomplish, at least for modest increases.

To understand why high compression ratios are desirable, it's important to know what they do within your engine.  The primary function of an increase in compression is to increase thermal efficiency.  Thermal efficiency is the measure by which your engine converts heat energy into mechanical energy.  The better this efficiency, the more power you're getting from burning gasoline.  Most of this efficiency comes from what is known as the expansion ratio.  Generally speaking, the higher your compression ratio, the higher your expansion ratio.  This happens because with higher compression ratios, the increase in volume occurs faster as the piston descends.  By the time the piston nears BDC, the pressures within the cylinder will be less than they would otherwise be with a lower expansion ratio.  This means that when the exhaust valve opens and starts bleeding out the excess pressure, there is less excess.  The engine has done a better job of reclaiming that heat-generated pressure into mechanical energy.

Let's illustrate this using two hypothetical cylinders.  Both cylinders have the same displacement, but they differ in their compression ratios.  Cylinder one is running high compression at a 12:1 ratio.  Cylinder two is running low compression, at 6:1.  Assuming a displacement of 200cc per cylinder and peak cylinder pressure of 1,000 PSI.  For cylinder one, we have a combustion chamber volume of 15ccs and cylinder two will have a combustion chamber volume of 36ccs.  Following through, we can see that as the piston descends, the pressure in cylinder one drops more quickly and so more of the energy is being reclaimed, rather than expelled out of the exhaust.  This is clearly illustrated by the spreadsheet listed below:
(https://sphotos-a.xx.fbcdn.net/hphotos-snc6/255571_10151186511770159_979283662_n.jpg)

The other benefit (but also detriment, as described in a bit) is the increase in temperature brought about by the increase in compression.  Going back to our calculations on volume and how it relates to temperature, a greater total difference in temperature creates a greater pressure.  That relates to compression because compressing a gas adds heat into the gas and this will create greater temperatures on the top end of our measurements.  Please see the attached YouTube video as an example.  In this quick video, a piece of tissue paper is placed into a test tube and then a rubber plunger is quickly depressed.  The resulting compression creates enough heat to ignite the tissue paper.  In your cylinder, this increased heat prior to ignition will result in a higher final combustion temperature.

http://www.youtube.com/watch?v=ADeYiOYGGYY

While high temperatures are what net you power, they also need to be kept in check.  Too much temperature prior to ignition will result in detonation.  This is when the energy in your fuel is released instantaneously (or near enough) as kinetic energy in the form of a shockwave, rather than as an increase in pressure due to temperatures.  Because the energy is released so suddenly, this results huge strain on your engine components and parts do not last long when exposed to this environment.  In maximum effort engines, it's not uncommon for any form of detonation to destroy the entire engine with little or no warning.

To keep detonation at bay, higher octane fuels are generally used.  With combustion chamber design and racing fuel being used, compression ratios of 17:1 are not unheard of.  For the street, 11:1 is quite respectable.

Generally speaking, a full point of compression will net you around a three or four percent increase in torque across the entire RPM band.  The best increases come when compression is already low.  For instance, going from 8:1 to 9:1 will be better than going from 10:1 to 11:1.

In order to increase the compression on your motor, there are several options.  The best option (in my opinion), though also the most expensive, is to go with replacement pistons.  The crown on the piston can be raised to take up more volume in the combustion chamber.  Though a domed piston can slow down the flame front, you also avoid many of the other problems associated with other methods.  The second best option would be to add more metal to the combustion chamber.  Technically, this is generally preferable to domed pistons, but this is a precise and expensive operation; not something generally done by the casual enthusiast.

The cheapest method, that is available to almost all bike owners, is to run without a base gasket and use case sealant (such as Threebond), instead.  This can often add a full point of compression.  This will result in a slight retardation of the cam timing, but this is not usually an issue for most engines.  Always double-check valve timing and clearances prior to running the engine when reducing the distance between the pistons and the head.

Shaving metal from the head and/or cylinder jugs has the same basic effect as running without the base gasket and is often desirable because it's also an excellent time to smooth out the sealing surfaces of your engine.

Volumetric Efficiency
The other great contributor to BMEP is volumetric efficiency, or VE.  This represents the percentage of fuel/air mix that occupies the volume of the cylinder at BDC.  For instance, if you have 100% VE then your cylinder is fully filled with fresh fuel/air mix.  80% VE may indicate that some exhaust gases remain within the cylinder or that the cylinder didn't have a chance to fully fill before the intake valve closes (or even a combination of those factors).  Peak VE of an engine usually corresponds to peak torque.

The goal of all engines built for power is 100%+ VE, under wide open throttle.  Unfortunately, maximizing VE is a difficult task and as engine conditions change, so does VE.

One of the major contributors to affecting VE over the range of an engine's operation is valve timing and valve lift.  As most of you know, valve timing is controlled largely by the camshaft.  As the lobes of the camshaft move the rocker arms, valves are opened and closed in relation to the crankshaft.  When the valves open, how far they open, and when they close is a major contributor to VE because of our old friend inertia.

At slower engine speeds, the inertia of the air flow in and out of the engine is less and so maximum VE is achieved with a later opening of the valves along with an earlier closing.  As the engine speeds increase, valves should be opened earlier and closed later.  The reason for this is two fold.  First, as the crankshaft spins faster, there is less time for intake mixture to enter the cylinder and less time for exhaust gases to leave it.  By opening valves earlier and closing them later, we can allow more time for gases to flow.  Furthermore, the increase in gas inertia, provided by the increase in velocity of the gases within the intake and exhaust, allows for a "stuffing" and "scavenging" effect.

On the intake side, a faster moving gas will compress itself as it enters a closed area.  So as the intake mixture passes through the intake port and begins to fill the cylinder, the fast moving gases coming in from behind will help to push the gases in front and this will create a higher pressure than could be achieved were the intake gases moving more slowly.  The same holds true for the exhaust system, only we're relying on the low pressure wake as the faster exhaust gases leave the cylinder.

However, holding the valves open longer (or open wider) is only beneficial when the gases are moving quickly.  Generally speaking, these gases are only moving quickly when the engine is spinning quickly and so tuning an engine for maximum VE late in the RPM band will result in very poor VE lower in the RPM band.  This occurs because holding these valves open longer allows unburned fuel/air mix to be pushed back out of the intake or to spill into the exhaust without ever being burned.  This process is called reversion and is not a desirable trait.

The amount of time the valves spend open is known as the duration and is generally listed in degrees.  This number represents the number of degrees, over two full rotations (720°) that a valve is held open.  Cylinder filling is also affect by lift, which is how far a valve is opened before it begins to close.  Both lift and duration affect VE and some engines respond better to more lift whereas others will do better with duration.  Generally speaking, both increased lift and duration will result in better VE later in the RPM band, but one of those two values will provide better VE over a longer RPM range, which is definitely desirable.  Remember, our goal is maximum VE at WOT, not just maximum VE at maximum RPM.

Aside from valve timing, the other major consideration for VE is air flow in general.  There is an ideal air speed (for both intake and exhaust) that your engine configuration will enjoy.  This air speed can be adjusted through a number of factors such as the size of your valves, the length and diameter of the intake, and the configuration of the exhaust system.  Other engine modifications will change the ideal air speed.  You can adjust this air speed through a number of methods, but this must be done intelligently and with a goal in mind.  The velocity of the intake and exhaust gases have a very real effect on your engine's performance and making more power is never as simple as cutting of the muffler and slapping on pod filters.

With almost all VE modifications, they will be a trade off.  Increasing your VE toward the top of the RPM band will almost always decrease it toward the bottom, though keeping VE high throughout the RPM range is the ideal scenario.  The trade offs are not always equal, either.  You may gain 10% more power at redline, but lose 30% power at idle.  For maximum effort engines designed for land speed records and oval track racing, it is usually desirable to stack the volumetric efficiency very high in the RPM band.  For road racers and "souped up" street bikes, peak VE should come in about half way (or a little over half way) in the RPM range for a good compromise.

For increases in VE, common modifications include aftermarket camshafts, valves, tuned intakes and exhaust systems, and porting work.  Not all of these are done as a matter of course and the goals for your bike and engine should dictate which are chosen, if any.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: miken5678 on Aug 21, 2012, 17:15:15
interesting read.

in regards to the ve area it is interesting and a great benefit these days to have the variable valve timing setups out there that throw out the limitations of either low end or top end tuning.  Honda is using this on their motorcycles, wonder if vanos will ever make its way over to the bmw line.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: scm on Aug 21, 2012, 17:42:23
Quote
...  Cylinder one is running high compression at a 12:1 ratio.  Cylinder two is running low compression, at 6:1.  Assuming a displacement of 200cc per cylinder and peak cylinder pressure of 1,000 PSI.  For cylinder one, we have a combustion chamber volume of 15ccs and cylinder two will have a combustion chamber volume of 36ccs.  Following through, we can see that as the piston descends, the pressure in cylinder one drops more quickly and so more of the energy is being reclaimed, rather than expelled out of the exhaust.  This is clearly illustrated by the spreadsheet listed below: ...

Sorry, according to your chart, pressure i.e. force applied to piston one (high comp.) is less/equal
than the one applied to piston two. For given stroke resp. travel of piston, less force creates less
work. So if operated at the same speed, cylinder one will generate less power.  ;)



Nice essays anyway!

Best regards
Sven
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Aug 21, 2012, 18:53:05
Sorry, according to your chart, pressure i.e. force applied to piston one (high comp.) is less/equal
than the one applied to piston two. For given stroke resp. travel of piston, less force creates less
work. So if operated at the same speed, cylinder one will generate less power.  ;)

Nice essays anyway!

Best regards
Sven

It's not so much about how much pressure is applied within the cylinder, it's how much is reclaimed as mechanical energy.  In order to for torque to be generated the pressure must decrease.  While you're viewing it as less pressure within the cylinder (and this is true) you can/should also view it as more energy transferred through the pistons into the crankshaft.  Because energy is never lost, a decrease in pressure/force inside the combustion chamber (which, for all intents and purposes is closed system) must result in a equal transfer of energy to the crankshaft.  Conservation of energy and all that...

For cylinder number one, the decrease is more sudden and more energy is reclaimed at the beginning of the power stroke.  For cylinder number two, the decrease takes longer and ultimately results in less reclaimed energy.  Cylinder two also suffers further in a real-world scenario because the total pressure in the cylinder would be higher during the opening of the exhaust valve, allowing for less total expansion than cylinder one; again.

Examining each cylinder as a function of it's PSI per cc we can see cylinder one starts with 66.66 PSI per CC and finishes at BDC at 4.65 PSI per CC.  Cylinder two is 27.77 and 4.24 PSI per CC, respectively.  This gives us an expansion ratio 14.34 for cylinder one and 6.55 for cylinder two.  This difference in expansion ratios is what causes the pressure to be more efficiently reclaimed by cylinder number one.  Experimentation and use of the Atkinson cycle is currently in use in many hybrid vehicles in an attempt to increase expansion ratios over what would "normally" be allowed for a given compression ratio.  A lot of people think, "compression", when it comes to efficiency, but they should really be thinking, "expansion".
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Aug 21, 2012, 20:53:05
That doesn't sound right, but maybe I missed something.
 
A lower rise in pressure in one cylinder equates to less force applied to the piston and less work done. Expansion comes from combustion and higher compression means higher peak pressure applied over a longer time.  Combustion pressures and speed of pressure rise at particular engine speed are a function of many factors including fuel, combustion chamber space, swirl, tumble, fuel droplet size, and so on. 

I like that you want to share what you are learning - that's really good.  I find that the best way to pass on knowledge is to make it highly relevant and simple to understand.  Even then, I sometimes get carried away, but KISS is a great way to approach this stuff.

The issues are complicated enough when it's broken down into chunks.  Maybe the way to approach this thesis is to think in terms of what can people change and then explain it in those terms. Guys like David Vizard and A Graham Bell are excellent examples of authors that come to mind.  They break it down into very simple explanation of the underlying theory and then a short treatise on how that might be useful in a practical example.

Those two authors tend to write about an engine they worked on so we can see the practical effect of the changes.  Or maybe split the thread into two.  This one on "How to" and maybe another thread with all the underlying theory and latest trends in F1 and motoGP which are hard to find and fascinating.

Kevin Cameron is another great author on all things mechanical.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Aug 21, 2012, 21:09:29
Matt, That chart doesn't look right.  A lower compression cylinder cannot have a higher pressure inside the cylinder at TDC.  All other things being equal - when they never are - the higher CR side will have much higher starting pressure and it will remain higher until the port opened by which time >90% of the work has been done.

The problem is that peak pressure occurs at around 15 degrees ATDC and would be much higher in the higher comp cylinder.

Cylinder pressure of 1000psi is in the right ball park - 65-70bar with high compression and BMEP around 10bar which is fairly high for a well developed 2 valve motor.

:-) 
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Aug 29, 2012, 14:01:26
Sorry... I wasn't meaning to illustrate a real world scenario with my chart.  I was hoping to demonstrate cylinder pressure decay as a result of compression/expansion ratios.  Faster pressure decay means faster transfer of energy from heat energy into mechanical energy.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Aug 29, 2012, 16:09:03
Applications - Camshaft
OK... lets talk a little less about theory and a little more about application.  We won't be going completely away from the academics, but we will be talking a little bit more about specific parts and changes and how they affect the running of your engine.

I want to talk about camshafts because this is probably the first performance modification made after an increase in displacement and compression has already been completed.  Hopefully I explained displacement and compression in a satisfactory manner in my earlier posts but just to sum it up again:

Displacement - Bigger engine = more power.  No brainer.
Compression - More compression = more power.  Increase in compression causes an increase in heat.  This heat is what makes more power, but too much of an increase in heat and your engine explodes.

The nice thing about displacement and compression is that they benefit your engine no matter the speed at which it operates.  This is often the reason they are among the first modifications done.  Most of the other modifications you make to your engine will be a trade-off of some sort and the camshaft is no exception.

To understand why selecting a camshaft is a compromise, we need to look at it in more detail.  To do that, here are a few definitions:
Lift - This is how far the valves are held open
Duration - This is how long the valves are held open
Intake Velocity - This is how fast the air, coming into the engine, is traveling

Also important for understanding these concepts is a basic knowledge of fluid dynamics (yes, air is a fluid).  Understand that air has mass (and therefore inertia) and that air is compressible.  Now regardless of engine speed, our goal is to have the cylinder fully filled with fresh air and fuel, when the throttle is fully open.  If you recall from my previous post, this is called Volumetric Efficiency (VE) and we always want 100% or better, though this is rarely possible.

For our initial example, lets examine an engine turning very slowly.  Lets say 1 RPM for argument's sake.  In order to achieve our maximum VE, the intake valve should open very near TDC and then close very near BDC.  This will give us a duration of 180° because the crankshaft has turned 180° during the time in which the intake valve was open.  The reason this level of duration is effective is because the air is able to enter through the intake valve at a rate which matches the descent of the piston.  It would take the piston a full thirty seconds to descend from TDC to BDC and so it is highly unlikely that the pressure inside the cylinder would differ from atmospheric pressure at any point.

When we speed things up, however, the situation changes.  If we increase the engine speed to 6000 RPM, the piston now descends from TDC to BDC in .01 seconds.  That allows precious little time for the air to pass through a relatively small intake opening and into the cylinder.  In order to maximize that time, it is beneficial to open the intake sooner and then close it later.  Not only does this allow for more time in cylinder filling, it is assumed that an increase in engine speed also translates into an increase in intake velocity.  Faster moving air is able to pack itself into the cylinder in less time and so this helps as well, provided we help it.

Opening the intake earlier allows air within the intake to start entering the cylinder earlier and this builds momentum within the intake tract.  By holding the intake valve open longer, we utilize this momentum to compress the mixture further.  Fast moving air will more readily act against the pressure created by the rising piston.  So even though the cylinder may be at 80% of it's total volume due to the rising piston, the fast-moving intake charge is still filling it.  If we were to maintain opening the intake valve at TDC and closing the valve at BDC, we would be losing quite a bit of VE because it takes time to build momentum in the intake and the piston may already be halfway down the cylinder by the time this occurs.  The cylinder would then still be at less than atmospheric pressure when the intake valve closes at BDC.  On the other side of the coin, if we were to close the intake valve well past BDC in our first, slower, example, then the piston has already pushed much of the fresh intake mixture back out of the intake port (this is called reversion).  An ideal opening and closing time exists for a given intake velocity.  Conversely, changing the intake velocity will change the ideal opening and closing times of the valves.

For the exhaust valve, the concepts are similar, but there are some differences due to the desire for the gases to flow out of the cylinder rather than into it.  Furthermore, the gases are usually much higher pressure than atmospheric and so that changes the dynamic as well.  Just like the intake, a performance cam will usually opt to open the exhaust valve sooner and close it later.  Also, just like the intake, this creates a situation where the engine produces less torque at lower RPMs.  By opening the exhaust valve earlier, we are in effect bleeding off the still-expanding gases from the cylinder before they've fully acted upon the piston.  This quite literally sends power out the exhaust port, but there is a good reason for doing so.  Again, the purpose is to build momentum in the gases.  Having a higher cylinder pressure when the exhaust valve opens creates a faster flow of gas, sooner.  The faster the gases move, the higher their momentum.  This leaves a low-pressure zone in their wake and helps to scavenge exhaust gases from the cylinder prior to the intake stroke.  In some extreme cases, it can help alleviate pumping loses, because the low pressure actually pulls up on the ascending piston.  If the intake valve opens while the exhaust valve is still open (this is called overlap) this low-pressure can also help to start the intake mixture moving into the cylinder.

If it wasn't clear by now, the lobes of the camshaft are what control the valves.  The lobes (in conjunction with the rocker arms) are what determine the amount of lift and the duration.  Lift and duration are roughly interchangeable.  That is, increasing the lift and decreasing the duration will result in roughly the same VE.  Usually, both lift and duration are increasing on a performance camshaft.  Certain engine configurations will favor lift over duration and vice versa.  A couple of articles I've read seem to be placing more emphasis on lift for high-revving motorcycle engines, but your mileage may vary.  As a corollary, increasing lift will place more stress upon the valve train and increase duration will decrease that stress.  Furthermore, increased lift will almost always require upgraded valve springs, which increase stress even further.

As I mentioned in a previous post, peak VE is usually where peak torque occurs.  By moving peak torque further up in the RPM range, we increase the maximum horsepower of the engine.  By moving the torque further down in the RPM range, we decrease the total horsepower of the engine.  Keeping the torque low in the RPM band isn't such a bad thing, though.  Bikes with low-range torque are easier to ride and respond well to throttle over a larger RPM band.  When it comes time to chase ponies, we usually opt for more power, later in the RPM range.

It would be nice to have the best of both worlds; a low duration cam for low RPMs and a high duration cam for high RPM operations, but unfortunately, most of us will need to choose a cam with set duration.  It will only work best within a narrow RPM range of around 1000-1500 RPMs.  Whether that best range is from 3000-4000 RPM or from 6500-8000 RPM is going to be dependent upon our desires for the bike and this is precisely what we need to keep in mind when making our selection.

It's a common rookie mistake to choose the lumpiest cam with the biggest numbers.  This is almost never the most desirable option.  Yes, it will theoretically create the most horsepower, but you do so at a greatly reduced level of torque early in the RPM range.  This means that you need to slip the clutch like a madman just to get going from a stop and your engine needs to operate in the top third to a quarter of the RPM band.  For a Honda 350, this means keeping your revs about 7000 RPM pretty much all of the time.  If the revs drop below that point, then power drops off VERY quickly.  Choosing too big of a cam is called overcamming an engine and it's surprising easy to do.  Unless your plan is to run the salt flats or race an oval track, stay away from cams that are too big.  For short courses with more turns, cams with less duration are beneficial because they provide more torque down low.  A quick question to ask yourself, "Do I prefer accelerating through the twisties or running flat out?".  If your answer is the former, keep to a milder cam.  If speed and power are your ultimate goal (and you have a course where you can utilize those things) a hotter cam will usually be more beneficial.  One more caveat to big cams:  It is entirely possible to move your peak torque so high up the RPM band that you never hit it.  Your cam never reaches its potential and so you've actually robber yourself of power.  This can occur either by moving the peak VE past your redline or by choosing a cam that is so lumpy that your engine doesn't have enough torque to continue to rev up to the ideal range of operations in the higher gears.

So what is too big?  That will depend on your engine.  A duration of 270° is a hell of a lot for some engines or it might be a mild performance upgrade for others.  If your budget allows, I suggest trying new cams one at a time.  Go for small increases to begin with and decide whether or not it's giving you what you desire.  Going for the biggest cam right out of the box is a sure path to disappointment.  Getting the right cam on your bike is going to give you one huge grin, however.

For my own 360, I've opted for a cam that provides .040" additional lift and an extra 30° duration (absolute lift and duration of .341" and 251°, respectively).  It's a little slow off the line and hill starts are not fun at all, but if I keep the revs above 4000, it screams.  The gear ratios are pretty good on a 360 as well, so this helps and is also something to consider with your own build.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Sep 01, 2012, 03:09:10
To expand on that last post, the question is how do you know how much is too much? The answer lies in gas velocity through the ports.  Big ports, big valves or a long duration cam with too much lift all result in lower gas velocity than smaller lift, smaller ports and shorter duration.

At lower engine speeds we need small ports to generate high enough gas velocity to effectively fill or empty the cylinder.  As engine speed rises, the gas velocity also rises until it hits a certain speed at which it stalls.  that speed is approximately 0.55mach at which point a pressure wave is generated that restricts gas flow.

On our stock motor, that may never be reached but an increase in capacity automatically means we are drawing in more gas (or expelling it) per revolution and if ports and valves and cam remain the same, the motor will reach that critical gas velocity at lower revs and that's one reason that big motors don't like to rev.

Motors that came from the factory over cammed or with ports that are too large for the stock motor may well rev as hard when capacity is increased.  Motors with marginal ports or valves will not rev as well if capacity s increased.

Another thing that happens as revs rise is that there is less time to fill or empty the cylinder.  at 10,000 revs, there is only half as much time as at 5,000 revs, so we have to hold the valve open for longer to allow more time for the gases to get to where they are going.

So a motor that is designed to rev high will typically have larger ports and longer duration cams than one designed to lope along at more modest revs.

The trick is balancing all those different factors.  When modifying a motor, determine how much gas flow is required for the target HP at target revs and then have the head gas flow tested to see if it flows enough. Next, match the cam to the motor's flow rates.  For example many Honda's need a lot of lift but no much duration to get the job done.  Others respond well to extra timing.

Cb160/175/200 for example works best with a short duration mild cam.  On the dyno a fully developed motor will lose power in the midrange and make no extra power at the top end with more lift or longer duration. On the track, fastest lap times backed that dyno finding up - mild street and track cam is faster than the race cam.  We speculate that with a different cam drive and crank design, the motor would benefit from more cam, but not with conventional tuning techniques.

That's why many engines just work better with smaller carbs. We have run CB160/175 race moors with different cams, heads, ports, carbs and stock 20mm carbs are OK up to 10,000 on a 204cc CB160 and a 181cc CL175 motor runs really nicely on s road & track cam with 26mm Super Hawk carbs up to 11,250. And our 240cc CL175 runs.... well never mind how it runs....  It's almost stock so nothing to see here.  Move along. :-)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Sep 12, 2012, 16:51:35
Applications - Compression

As I mentioned earlier, compression is one of those areas in which you can't really go wrong.  More compression increases the efficiency of your engine and provides a boost in torque, and hence power, throughout the entire RPM band.  For this post, unlike my previous post on compression, I'm going to talk a little bit more about the application and a bit of a "how to" rather than stick mostly with the theory.

The ways in which compression help are many.  The primary reason is that an increase in compression increases combustion temperatures.  Because your engine is basically a heat pump, more heat means more power.  Another important aspect is an increase in expansion ratio.  In order for your engine to be able to reclaim heat energy as mechanical energy, you must have a favorable expansion ratio.  Finally, increasing compression ratio also tends to increase combustion speed.  This increase in combustion speed allows your engine to hit higher peak pressures, sooner, and so reduces the amount of energy that is bled into the walls of the combustion chamber (more on this detail in a future post).  A good rule of thumb is that each full point of compression ratio increase will yield between a 3% and 4% increase in torque.  Initial gains will prove to be better than gains higher in the scale.  So this means going from 8:1 to 9:1 will be better than going from 11:1 to 12:1.

Let's examine a hypothetical engine configuration.  For my examples, we'll assume these engine characteristics:

Bore - 67mm
Stroke - 50.6mm
Total Gasket Thickness (Both Head and Base) - 2mm
Combustion Chamber Volume - 26cc
Piston Dome Volume - 10cc
Piston Deck Clearance - 0.5mm (Most pistons do not come all the way up in the cylinder sleeves.  This is the amount left over at TDC).

These numbers get us a static compression ratio of 8.2:1.  In the following topics, I'll be "massaging" these numbers to illustrate how changes we could make represent a real increase in compression ratio.

There are several ways in which higher compression is usually achieved and a number of significant things to keep in mind when chasing bigger numbers.  I'll deal with these five sub-topics in order.

Compression Method #1 - Shortening the Stack
This is probably the easiest method of chasing a modest compression increase.  The idea, here, is to reduce the length of the cylinders (or head) while maintaining the length of the stroke.  There are a number of approaches to implementing this method and it's possible to use all or just one of them.

The first, and probably easiest, option is to run thinner gaskets.  For most engines, it's possible to forgo a base gasket entirely, and simply use a sealant such as Threebond.  A slightly thinner gasket can have a much greater effect than one would think.  Going back to our example, lets say we skip the base gasket entirely and our total gasket thickness is reduced from 2mm down to 1mm.  Our static compression ratio is increased from 8.2:1 to 9.4:1.  Not bad at all.

The second option is to have some metal milled off of the cylinders and/or head.  This process is commonly known as "decking the head".  It's slightly more expensive, but it is slightly more efficient and comes with some other benefits as well.  The basic concept is that the same goal is accomplished, but you get a few extras thrown in for free.  For example, removing metal from the head will reduce the combustion chamber volume, but it also give you an opportunity to have the head surface reconditioned, providing a better sealing surface.  Likewise, removing some metal from the top of the cylinder jugs can produce the same effect, but it also has the added benefit of reducing the piston deck clearance.  Assuming we take a combination of the two options and remove .020" from both the cylinder jugs and the head, we end up with a static compression ratio of 9.4:1.  This is the same as our first example, but we've also managed to clean up the sealing surfaces.

This is a bit of a corollary, but with both this example and the previous, the piston was effectively raised up within the cylinder.  This tends to increase the turbulence within the cylinder during the compression stroke and helps to keep the fuel evenly distributed within fuel/air mix.  It also helps to keep the fuel droplets smaller, leading to faster combustion.  These things are HIGHLY desirable in a "built" motor and I'll talk a bit more about this in a future post.

Milling metal from just the head or using a thinner head gasket (as opposed to base gasket) will not usually produce this effect and so those two options can be considered less desirable in some builds.

Compression Method #2 - High Compression Pistons
The next most common option for increasing compression is to replace the stock pistons with an aftermarket set.  Almost all aftermarket pistons will have at least a modest increase in compression ratio.  The most common way in which pistons increase the compression ratio is through an increase in the dome height.  Basically speaking, the piston now takes up more room with the combustion chamber.  It's not uncommon for some piston domes to be quite large.  In the following pics, you can see a stock CB450 piston as compared with a high compression piston for the same engine.
(http://img.auctiva.com/imgdata/8/9/7/6/2/7/webimg/585768899_tp.jpg) (http://lh3.ggpht.com/_1D_up_JQCqc/TG2wYYMs9MI/AAAAAAAACdI/YwvBBR1wBPc/s912/DSCN6922.JPG)

Going back to our example, using a piston with an increased dome volume of 4cc will result in an increase in compression ratio to 9.6:1.  Replacing pistons in order to increase compression is quite good because it also allows you an opportunity to increase the bore diameter and get yourself a nice boost in displacement at the same time.  It's uncommon to see high compression pistons that are not also larger in diameter than the OEM part and when it does occur it's usually due to displacement restrictions in racing classes.

With pistons, it's also possible to increase compression by lowering with wrist pin location.  More often than not, you'll usually find that pistons with a wrist pin location change opt for higher pins, though.  This almost always indicates that these pistons are for use in engines with increased stroke.  The reason for this is because increasing the stroke also increases compression unless something is done.  An increase in stroke worth chasing will almost always result in too much compression with a stock configuration and so part of increasing the stroke length is finding a way to then decrease compression.

Compression Method #3 - Reducing Combustion Chamber Size
The final way to increase compression is to decrease the size of the combustion chamber through the addition of metal.  This is expensive and has more ways it can go wrong than it can go right, and so I don't recommend it to just anybody.  There are usually some pretty good gains gains to be made here (though not strictly in compression), but it's not an easy undertaking and most folks, myself included, omit it unless they're chasing a specific goal.  Furthermore, its usually very difficult to make significant changes to the combustion chamber unless you have special pistons, anyway, and so my recommendation would be to stick with options #1 and #2 unless you know a good deal about (or don't mind learning the expensive way) what constitutes good chamber design for your application.

Simply because I provided a mathematical example for the other sections, I will do so here, as well.  Furthermore, because changes in combustion chamber volume usually involve using differently shaped piston domes, I'll alter both of those values for this example.  Assume you're able to fill in the sides of the combustion chamber and change your chamber shape to a rough oval, instead of the hemisphere seen on a lot of modern bikes.  This will possibly translate to a new combustion chamber volume of 18cc.  The domes on the pistons will need to be altered as well, and so they're going to be reduced from 12cc down to 8cc (higher, but thinner).  Our static compression ratio is now sitting at a healthy 10.5:1.

Caveat #1 - Clearances
First and foremost, when you reduce the area available in the combustion chamber (which is precisely what increasing compression ratio does), you increase the chance of clearance issues.  Many parts are now closer together than they previously were.  Depending on the route you've taken, some areas will be more likely than others, but all should be checked.  The most commonly affected areas are the clearances between the piston and each of the valves and between the piston and the head.  It's also a good idea to check and make sure your gaskets aren't overhanging into the combustion chamber as well.  This is possible if you've gone with aftermarket gasket options and/or a larger bore.

The common methods for checking these clearances is with modeling clay.  Before assembling the engine, spray the interior of the combustion chamber and the top of the pistons with WD40 and lay down a thin layer of clay.  If your gaskets are the compressible type, you'll want to have several of these on hand, you're about to go through at least one of them.

After getting the clay into place, reassemble the engine and torque the head down to spec, set valve timing and clearances... all that jazz.  Now, SLOWLY rotate the engine in the direction of its normal operation.  DO NOT force it if you feel it binding.  It's quite possible to bend steel components with a lot less force than you'd think would be necessary.  After completing two full rotations of the engine (or if you felt something bind), remove the head from the engine and inspect the clay.

It should end up looking something like this:
(https://sphotos-b.xx.fbcdn.net/hphotos-snc6/226959_10150244540310159_1735127_n.jpg)

This pic was taken during the reassembly of my own engine, for the first time.  At the time, all the clearances checked out OK.  You can just see through the clay on the left side of the piston and this was due to a gasket overhang.  On the right side, you can see the clay was broken, but I suspected this was due to the clay folding over when it was pulled back up by the exhaust valve.  I repeated the check to verify that this was the case.  Unlike in the above pic, it's also a very good idea to put some clay on the sides of the pistons to check the clearance between piston and head.

Now, providing everything looks good, it's time to actually measure.  It's possible to measure the clay if you have a steady and gentle hand, but you can also buy special wax strips (can't remember what they're called) from most auto parts stores.  Simply put the wax down in the same way as you've done with the clay.  Assemble the engine, turn it a few times, and then disassemble again.  Time to break out the micrometer.  Though each engine is different in the tolerances it will allow, the clearance between head and piston shouldn't be less than .020".  The clearance between intake valve and piston should exceed .040" and the clearance between the piston and the exhaust valve should be at least .080".  Tighter clearances are possible, but I don't recommend it unless you've done this a few times already.  When clearances get tight here, they're a lot less forgiving elsewhere, too.  Adjust your tappets without enough spacing and you're just trashed your pistons.  Brilliant.  Skip a tooth the cam gear and now you need new valves.  Sweet.

Caveat #2 - Detonation and Preignition
One of the effects of additional compression is additional heat.  This heat is not only what provides the increase in power, but it can also cause two other issues.  These issues are detonation and preignition.  Both of these problems will destroy a motor in short order (especially preignition) and so neither are acceptable.

Detonation is the spontaneous combustion of the remaining fuel/air mix after the normal combustion process is nearing completion.  This is caused through the heat and pressure initiated during the combustion process and as both heat and pressure rise, it will get to a point that the molecules within the mix are pounding into one another so violently that they ignite, themselves.  Death by detonation usually results in broken rings or ring lands.

Preignition differs from detonation in that it's not so much as a spontaneous combustion of the mixture.  Preignition is a begin to the combustion prior to ignition from the spark plug.  Preignition usually occurs when a part or parts of the combustion chamber heat up too much.  This can be anything from an excess of carbon deposits (not usually an issue on a freshly assembled engine), damage to the exhaust valve, or an overheated spark plug.  What happens in this case is that whatever causes the preignition has heated up to a point where it actually starts the combustion of the fuel/air mix before the spark plug fires.  This causes cylinder pressures to rise too early (sometimes when the piston is still approaching TDC) and so peak cylinder pressures occur too early in the cycle.  This causes greatly increased stress on engine components and will usually kill an engine a lot earlier than detonation will.  Engine failure due to preignition will almost always result from holes in a piston.

Though there are many ways to combat detonation and preignition, those will be saved for a later post.  For now, just be aware they can be potential problems and the most common method of dealing with these issues is to use high octane fuel.  Consider premium gas to be the only acceptable fuel for a high-compression engine.  Better to push the bike home than fill it with regular.  Also, it's important to note that high compression engines are MUCH LESS tolerant to running lean than the stock factory offering.  Most stock engines will run all day long on a lean mix, but a high compression engine at WOT will blow up right in your face as soon as the float bowls start to get even a little shallow.

Caveat #3 - Combustion Speeds and Timing
This particular issue can be hit or miss depending on how you've achieved your increase in compression, but it's unlikely you'll be able to avoid these effects all together.

First up, it should be known that increasing compression increases the combustion speed of the mixture within the cylinder.  This is generally considered a good thing.  Furthermore, increasing piston dome volume will generally reduce the combustion speed (especially at low RPM).  In theory, this results in a need to change the ignition timing of your engine.  In practice, no change may be necessary or you may not have the data available to make a change that is beneficial.

So what is it we're chasing when we change the ignition timing?  We're changing the point at which Peak Cylinder Pressures (PCPs) are attained.  In four strokes, this PCP should occur at 14° ATDC.  If ignition comes on too early, then PCP occurs too close to TDC.  This places engine components under undue stress and robs the engine of power.  If ignition comes on too late then less power is generated because the expansion of the gases, due to heat, are no longer allowed to follow the sinusoidal pattern allowed by a healthy expansion ratio.  In plain English, the volume of the cylinder (due to the descending piston) is expanding more than is desirable when compared to the expansion of the combustion gases and so pressures never build to what would be otherwise possible.

So what does it come down to, in practice?  Running high-compression pistons will likely lead to a slight advancement of the ignition timing in the lower RPM ranges.  Without increasing the ignition advance, the bike may stutter when blipping the throttle, despite having an appropriate fuel mixture ratio.  Instead of initial timing at 14° BTDC, you may find that 18° provides the throttle response you're after.

As for the increased combustion speed, this is something that many folks just choose to accept and deal with.  In reality, the increased combustion speed that is achieved with a couple of points of compression is not something that needs to be dialed out.  The perfectionist or a person chasing a maximum effort engine will likely pursue some dyno time at this point.  Ideal total timing can be achieved through the measuring of exhaust gas temperatures, and in some cases, cylinder pressures.  For the garage builder and home enthusiast, slightly advanced timing usually results only in a concern for the increased heat and this is usually handled through increased octane or manipulation of the thermostat (in liquid cooled operations).

Now aside from ignition timing, valve timing may also be affected by a compression change.  Shortening the stack in any way (thinner gaskets, decked head, etc) will retard the cam timing.  This causes all of the valve events to happen later.  At the very least, this causes a drop in peak horsepower as all of the valve events rob your engine of a bit of volumetric efficiency.  At lower RPMs, there may be a slight increase in torque.  In more serious situations, this can cause clearance issues as the exhaust valve is now closing later in the rotational cycle and so it is being held open too long while the piston is approaching TDC.

You'll not likely need to correct this issue unless you've taken more than .020" from the total height of the stack (or if you're making other modifications to the engine).  In order to fix this problem, you're going to need a degree wheel, timing information about your camshaft, and a few other basic machinists tools.  The idea is to assemble everything according to spec and then use the degree wheel to determine how far our the valve timing is.  You then either slot the existing cam gear or buy an aftermarket cam gear in order to correct the timing.  By mounting the cam gear at a different rotation than what is called for by the OEM part, your valve timing stays where it is meant to be.

General Considerations and Extra Info
Not much rhyme or reason to this last part, but just a few extra thoughts on compression.

Conclusion
When it comes down to it, high compression is probably one of the defining characteristics of a "built" motor.  In moderate cases, it can be completed by folks who know very little about engines and I recommend this alteration to anyone who plans on taking their engine apart.  In its simplest form, just replacing some gaskets with thinner material is all that is necessary in order to achieve a bit of a compression increase.  Yes, it can get complicated fast, but doesn't everything?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Sep 12, 2012, 17:36:08
One other thing to keep in mind about domed pistons...  There is the definite possibility of having "too much of a good thing".

Looking back to the domed piston pic midway through my previous post, you can see how big of a lump that thing is.  Weight considerations aside, what may not be immediately apparent is how that can affect the filling and purging of the cylinder (though the former is definitely more of a concern than the latter).

With large domes, you're not only slowing down the propagation of the flame front (and the entire combustion event, for that matter), you're also disrupting the air flow into and within your cylinder.  At low lift (near TDC), the piston will almost completely block the air flow coming into cylinder and these low lift events can be crucial.  Filling your cylinder with fresh fuel/air mix is a race and many races can come down to the kind of start you achieve.  What you've gained in compression, through high doming, you can easily lose in volumetric efficiency (because of slowed intake velocities) and reduced swirl (though the last item isn't applicable to engines with only a single intake valve and usually isn't application to vintage engines as a whole).

The ideal piston shape for a performance engine is one that is relatively flat, but as most of our engines have hemispherical chambers, domed pistons are a necessary evil.  It's usually OK to chase a bit of compression through domed pistons, but don't go too crazy.  A big lumpy piston may look cool, but you have to take many things into consideration.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: damarble on Sep 12, 2012, 17:43:55
It will take a while to get through all this but so far it's a fantastic thread. Lots of good info. Thanks Sonreir.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Bert Jan on Sep 12, 2012, 21:49:26
Dial K for knowledge. Very much appreciated. Printed this and it's now safe for ever :)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: 72texas350 on Sep 13, 2012, 04:36:28
Awesome thread! Thank you!!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: buzznichols on Sep 13, 2012, 16:57:17
" The weight of the bike and rider both have an effect on acceleration.  If you are able to cut the weight of both yourself and your bike in half, you have just doubled your acceleration."

Heavy sigh.

Anybody else haunted by the knowledge that the real "low hanging fruit" of performance is your own body mass?  Really, I sometimes think that I might get better performance out of my bike if I saved the money I blow on parts and devote it to paying a babysitter so I could work out more regularly.

Resultant negative body image issues notwithstanding, thanks to Matt for an excellent survey course on maximizing bike performance
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: kopcicle on Sep 13, 2012, 21:19:19
bench press the kids

jus sayin ...

~kop
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DreadRock on Sep 13, 2012, 23:02:13
Now thats a thread .. Thanks Matt,teazer,kop for the knowledge !!!!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: veloracermike on Sep 14, 2012, 12:34:15
" The weight of the bike and rider both have an effect on acceleration.  If you are able to cut the weight of both yourself and your bike in half, you have just doubled your acceleration."

Heavy sigh.

Anybody else haunted by the knowledge that the real "low hanging fruit" of performance is your own body mass?  Really, I sometimes think that I might get better performance out of my bike if I saved the money I blow on parts and devote it to paying a babysitter so I could work out more regularly.

Resultant negative body image issues notwithstanding, thanks to Matt for an excellent survey course on maximizing bike performance

As a bicycle racer this is a fight I wage every day.  Currently at the end of the racing season I'm at 145lb with about 8% body fat.  For my A races I get to around 142 and 7%..at 50 I'm pretty happy with those numbers.  I've cut a lot fat from the bike as well (both the one I power the one with Honda in it)  Next big expenditure will be aluminum wheels.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Captmilk76 on Sep 14, 2012, 14:06:40
Diet+MY Fatass=Faster bike. It's a great way to get me to lose a couple pounds. I recently lost 40lbs and I've hit the ton 4 times since. I want to drop another 25.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: 50gary on Sep 14, 2012, 15:20:42
I race and ride bicycles as well, my job is athletic trainer.  So I'm in total agreement on the body weight issue.  Lower your own weight and you'll be faster and braking will be more effective as well, plus it's cheap!  I'm 170# and 5% to 6% body fat, the recommended for a healthy male is 15% to 18% sadly most are not.  For the record the recommended for healthy female is 18% to 22%  Not preaching just saying for the info. 
  Cheers, 50gary
 
Title: Re: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: bjonesin on Sep 14, 2012, 15:27:12
As a bicycle racer this is a fight I wage every day.  Currently at the end of the racing season I'm at 145lb with about 8% body fat.  For my A races I get to around 142 and 7%..at 50 I'm pretty happy with those numbers.  I've cut a lot fat from the bike as well (both the one I power the one with Honda in it)  Next big expenditure will be aluminum wheels.
having received a jacket from veloracermike, i can attest to his low body fat. My love handles had a hard time conforming to his skinny ass.

Sent from my DROIDX using Tapatalk 2
Title: Re: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: veloracermike on Sep 14, 2012, 15:36:55
having received a jacket from veloracermike, i can attest to his low body fat. My love handles had a hard time conforming to his skinny ass.

Sent from my DROIDX using Tapatalk 2

Well the downside is finding jackets that fit. I'm 5'10" and smalls fit me through the chest but are tight in the shoulders and generally the sleeves are too short. A medium is super loose in the chest and waist.  The only jacket I own that really fits is my Hein Gericke vintage leather jacket. 

But to the topic on hand. Power to weight is key especially for little bikes like mine.  Getting the bike weight down, and getting the rotational weight off the wheels makes a huge difference.  Not just on acceleration but decel as well.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Sep 14, 2012, 16:38:29
But to the topic on hand. Power to weight is key especially for little bikes like mine.  Getting the bike weight down, and getting the rotational weight off the wheels makes a huge difference.  Not just on acceleration but decel as well.

Definitely.  My mom's husband races BSA Bantams and on a couple of tracks, he strips the fairings off for extra weight savings.  The fairings only help if you get going fast.  Acceleration is power-to-weight ratios.  Top speed is power-to-drag ratios.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: damarble on Sep 17, 2012, 13:56:15
That's something I am not willing to do, lose weight to go faster. If I need to go faster I'd rather get a bigger bike. I'm 6ft ~200lb and like having a sturdy build. None of it is bulgy where it shouldn't be. If anything I should hit the gym harder and gain even more.
Title: Re: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: AgentX on Sep 17, 2012, 14:05:11
Well the downside is finding jackets that fit. I'm 5'10" and smalls fit me through the chest but are tight in the shoulders and generally the sleeves are too short. A medium is super loose in the chest and waist.  The only jacket I own that really fits is my Hein Gericke vintage leather jacket. 

But to the topic on hand. Power to weight is key especially for little bikes like mine.  Getting the bike weight down, and getting the rotational weight off the wheels makes a huge difference.  Not just on acceleration but decel as well.

Similar jacket problems--have you tried Vanson's classic stuff?  Generally fits me well.  Love my Model A.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Sep 17, 2012, 14:29:16
Think of it this way, 7 pounds is more or less equivalent to 1hp. That's cheap power until you get to titanium fasteners and then you are into the wrong end of the cost -benefit curve.  Weight savings on wheels also help the bike to turn faster.

For small bikes, weight saving is a must to improve power to weight and acceleration.

As speed goes up, drag goes up faster and that's where aerodynamics start to play a huge part in going faster. 
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Sep 27, 2012, 20:33:07
Applications - Intake Tract

OK.  Up next I'd like to ramble on a bit about intake tracts.  By "tract" I mean everything before the cylinder (including the valves, valve seats, ports, carbs, and filters).

As usual, I'll break it into (hopefully) digestible sections and as we're onto applications I'll cover some of the common modifications and how/when they should be undertaken.  As always, please feel free to add more into this thread and/or ask questions.  It was never my intention for this to be a book, but rather a more participatory kind of thing...

Also, as usual, a little bit of info up front...  As you may have noticed by now, I'm addressing these topics in the rough order in which I would apply them to my own build.  I'm not saying its the only way, but it is my way.  I would not begin making changes to the intake prior to increases in compression or displacement.  Nor would I make changes before I've decided upon a camshaft.  What you can infer from this statement is that slapping on pod filters before making any other engine changes may not be the best way to go about freeing up some extra ponies.

There are several reasons for completing other engine modifications prior to changing the intake and rather than address them all right now, I'll talk a bit about it as I go through each subsection of this post.  The primary reason, however, is that most changes you make to the insides of an engine are going to change the way it breathes and even the way it wants to breathe.  If you change your intake first, it's fairly unlikely that you'll get it "right".  That said, there are likely to be some changes that can be done as a matter of course, but they're not numerous.

Intake Length - Wave Tuning
The first thing to consider during the modifications of your intake is the total length.  This is because the total length of your intake has a performance implication.  It's not a big implication (only a couple of percentage points), but it all adds up so you might as well do it right.  The reason for deciding upon a length for your intake is because of something called Wave Tuning.  This is also known as Pulse Tuning or sometimes, Ram Effect (not to be confused with Ram Air systems).

The theory behind wave tuning has to do with the harmonics of the intake.  Like any other "instrument", changing the length changes the tone.  Certain tones will correspond to an increase in power at certain RPMs.  The ideal length of the intake will depend largely upon the duration of your camshaft and so the total length of the intake is something that should be calculated after you know your intake valve duration.  Furthermore, you will need have a decent idea of which RPM range upon which you wish to focus.  The ram effect only works best over a fairly narrow RPM range and so you need to choose carefully.  You'll want to select a point in your RPM band where you wish to increase your power and common tuning theory insists that you stack as many of your benefits as possible.  This usually corresponds to the halfway point between when your cam starts to come on and your redline.  This is likely to fall somewhere around 2/3 and 3/4 of your redline.  If your redline is at 10,000 RPM, then you're tuning for 6500 RPM or 7500 RPM.  Actual mileage may vary.

Now the reason this works has to do with waves or pulses (as the name suggests) that propagate up and down your intake tract.  Each time the intake valve closes, it sends a shock wave back upstream along the intake.  When that shock wave reaches the beginning of your intake (the air filters, usually) it collides with the relatively slow-moving air of the atmosphere and reflect back again toward the intake port.  If this reflection comes back and contacts the intake valve just as it opens, you get a small increase in power due to the shock wave helping to push the air into the cylinder.  This shock wave helps to overcome the inertia of the air and quickly speeds it up and into the cylinder.  In practice, this shock wave will help even if the valve is already open, or just closing.  The greatest benefit, however, will be just after opening when the air in the intake is starting to speed up.

So... how do we calculate it?  It's actually not that hard.  You do need to know the intake duration of your camshaft, though.  For my example calculations, I'll stick with 221° which is the stock duration on a Honda 360 camshaft.  Also needed is the desired RPMs of operations and I'll go with 8000 RPM as an arbitrary example.

First up, we need to calculate how long our intake valves remain closed over one complete cycle of the engine.  As you probably recall, a complete cycle is two rotations on a four stroke engine and that's the same as 720°.  So the time the valves spend open is the 221° (as determined by the camshaft) and so finding the time the valves are closed is just simple subtraction.  720° - 221° = 499°.

Well, that's only part of what we need.  In order to get an actual time value from that data we also need to know the speed at which the engine is rotating.  We've already decided upon 8000 RPM and so we need to work out how many rotations per second we're seeing.  8000RPM / 60 = 133.33 RPS.  Now, we need to convert to degrees per second by multiplying by 360°.  This gives us 48,000° of rotation every second.  Finally, to get the amount of time the intake valves remain closed, we divide our 499° value by our 48,000° value.  The degrees cancel out and we're left with .01034 seconds.

"Great, now what?", I can hear you say.  Well... what we've done is to calculate the time that elapses between the intake valve resting into the seat and then coming off it again, or the times it remains closed before opening once more.  As I mentioned earlier, our final goal is to find a length and so now that we have a time component for our calculation, we are able to find the distance component once we know the speed.

The speed of the propagating waves is roughly the speed of sound (Remember?  Tone?  Harmonics?).  I usually use 90% of the speed of sound to help account for twists and turns in the intake as well as turbulent air which will also slow down the speed of sound in this environment.  The speed of sound is 340.3 meters per second and 90% of that comes in at 306.3m/s.  So now that we have speed and time, we can figure out distance.  Simply multiply the speed by the time and we arrive at distance covered.  306.3m/s * .01034s = 3.18m.  Also, bear in mind that because the wave is reflected at the beginning of the intake, we only need to use half of this value due the wave actually covering the distance of our intake twice.  Our final value comes in at 1.59 meters.

Those of you even slightly familiar with the metric system will probably have noticed that the intake length I'm suggesting is pretty damn long.  5.21 feet.  This isn't really possible in the real world and so, instead, we take advantage of the fact that the wave, upon hitting a (still) closed intake valve will reflect again back upstream.  It will actually continue to reflect back and forth five or six times (in reality, it will reflect many, many, times but as it loses energy during each reflection, it becomes useless for our purposes past the fifth or sixth reflection) and this is something we can make use of.

It is pretty much unheard of to try to capture the first or even second reflection.  Not only would it be difficult because the intake would be so long and cumbersome, the added weight and engineering of such a design would likely negate any real benefits. NASCAR intakes are usually tuned for third wave, to give you an idea of what's done in the racing world.

So lets scale it down a bit and shoot for the 4th wave.  We can calculate this out simply by dividing our original calculated length by the number of the wave which we are trying to capture.  1.59 meters divided by 4 gives us a value 0.3975 meters, or 397.5mm.  A lot more manageable.  Especially considering that this length includes the length of the ports, the intake manifold, and the venturi of the carb.  The length of our Honda 360's intake port, intake manifold, and stock CV carb venturi comes in at 214mm.  This means, we want a further intake length of 183.5mm (7.2 inches) added onto the end of the carbs in order for our intake to be properly tuned.

Changing the duration of the camshaft or the desired wave will obviously affect these calculations.  If you were to be running a cam with 251° duration instead of 221°, then the length has just decreased to 160mm because the valves are closed for less time.  Likewise, trying to capture 5th (instead of 4th) wave on our original setup gives us a length 104mm.  Finally, increasing the RPM target will shorten the intake and decreasing the RPM target will lengthen it.

Should your intake length be fixed, for some reason, you can at least calculate out where your small ram effect gains will occur.

Porting and Polishing
Porting and polishing is a phrase that gets thrown around a lot and it's something I don't think a lot of folks really understand.  Many newcomers to performance enhancement believe it is something that every engine undergoes during its transformation from a modest lump of metal into a fire breathing, kitten eater.  Unfortunately, this is mostly incorrect.

As mentioned several times previously, decent intake velocity is a key component in getting the most out of your engine.  As the ports are opened up, the intake velocity slows down.  Remember, the engine can only utilize so much volume of air and opening the ports past a certain size will not increase performance.  The reason for this is that the intake stroke actually consists of three separate stages.

The first stage of the intake stroke is overlap.  This is when the intake valves first begin to open and the exhaust valves are still open and getting closer to closing.  In a well tuned system (at the ideal RPM), the exhaust gases leaving the cylinder will have a low pressure zone behind them that help to pull the fresh intake charge into the cylinder.

The second stage of the intake stroke follows the more generally accepted definition.  The piston descends, creating a low pressure area into which the fresh intake charge rushes.  Nature abhors a vacuum.  Big ports help with this portion.

The third stage acts like a mini compression stroke as the actual compression stroke begins.  This is where big ports may actually hurt performance.  As the piston begins to ascend, pressure builds within the cylinder because the volume is decreasing.  This occurs while the intake valve is still open.  With smaller ports, the intake charge moves faster and is more difficult to stop because the air has inertia.  If the ports are too large, then the air moves slowly and is stopped more easily.  The formula for kinetic energy is E = ˝mv˛.  As you can see, increasing the velocity gives you more bang for the buck than increasing the mass.  So while big ports are useful for initial filling, they're much less so when it comes time to push that last little bit of air into the cylinder.  This problem is even more noticeable as engine revs increase because the time allowed to fill each cylinder during the intake stroke decreases dramatically.  As engine revs increase, intake velocity quickly becomes the primary means by which volumetric efficiency is achieved.  It is entirely possible to size the ports such that your engine cannot reach the RPM at which that size will be most useful.

Running your head on a flow bench can tell you a lot of information about this, but it won't tell you everything.  If you have the opportunity, it's worthwhile to stuff as much clay as you can onto the floors of the intake ports and see how little the CFMs actually change.  This is all dead space indicating that ports are oversized.  If the addition of clay significantly reduces the CFM, then it may be time to consider enlarging the ports.

For 90% of us, that day is unlikely to come.  Most engines will require significant modification before the size of the ports becomes a restriction to making power and most ports come oversized from the factory.  And even then, it's likely that a change in port shape may be more beneficial than simply enlarging the ports.  Most stock ports are roughly circular, but this is the not the ideal shape for air flow into the cylinder.  What you're really after is a 'D' shaped port with the flat portion of the D as the port floor.  By allowing the air to spread out evenly along a flat floor, you prevent it from "bunching up" as it turns the radius to go down past the valves and into the cylinder.

What can (and should) be done, however, is to clean up the ports.  Casting marks should be removed.  Areas around the valve guides and where the head meets the valve seats are likely to have excess metal that disrupts the air flow.  The following pic is my own intake port halfway through the clean up process.  I've ground off a lot of the casting marks, but you can still see some that are left to go.  Also note the casting seam on the port floor (top of the pic, head is upside down) that still needs to come off.
(https://sphotos-b.xx.fbcdn.net/hphotos-ash3/581206_10150933625025159_2041578381_n.jpg)

As for a polish?  Skip it unless you're running with fuel injection.  Polishing your intake ports encourages a boundary layer of air to form along the port walls.  This will cause your fuel to more easily drop out of the charge mixture and this fuel will begin to pool.  It then gets sucked into the cylinders at uneven intervals making your engine performance less than smooth.  After you're done removing the casting marks, finish with a rough grit sand paper.  I use 220 though I know some folks will go as high as 400 grit.  I don't recommend any higher than that.  Definitely stay away from the valve seats during this whole process.  If you nick a valve seat with a Dremel or sand it with the paper, it's not likely to seal well and a machinist may need to reseat your valves.

Valve Seats and Intake Valves
There are a few options when it comes to valves, as well.  You can often go oversized on valves.  Again, enlarging anything will likely result in slower intake velocity.  Pursue with caution.  Additionally, there are some modifications you can make to the valves themselves.  One desirable trait in valves is a narrowing of the valve stem when it begins to meet with the valve head.  This promotes smoother (read: better) air flow.  Additionally, valves can be backcut to encourage better flow at low lift.  This is especially important for getting the air into your cylinder, faster, as the valve begins to open.  Titanium valves are nice if you can afford them, but stainless steel is a common performance material as well.  These next pic illustrates stem taper (stock valve compared with aftermarket).  Taper on a valve is something most machinists won't do because it's pretty darn easy to destroy a valve, even when being careful.  You may need to purchase new valves in order to get this feature.
(https://sphotos-a.xx.fbcdn.net/hphotos-ash3/536806_10150790679025159_1031556551_n.jpg)

Additionally, backcutting the valves is something that can be done as a matter of course anytime you happen to have the engine open and the valves out.  A 30° backcut helps to start flowing air sooner, as the valve begins to open.  That can be seen in the following pic as the small metal strip located above the normal 45° valve sealing surface.
(https://sphotos-b.xx.fbcdn.net/hphotos-ash3/579447_10150799180625159_279022705_n.jpg)

The valve seats, themselves, should also undergo treatment.  A three angle grind is a standard performance modification and five angle is even better.  More angles leads to smoother transitions of moving air and will aid in cylinder filling.  Running flat out, good backcutting and a three angle job is usually good for an extra percentage point or two of VE.

Some valve makers also offer a swirl polish that is meant to help more evenly distribute the mixture around the cylinder, but I'm afraid I don't have too much info on this other than its existence.  Maybe someone else can offer up their opinions and experience?

For multivalved engines after 1985 (or so), it's likely that your valve design will also include what is known as "tumble".  The goal of this is also to ensure more even cylinder filling (not just high VE, but good distribution of fresh mixture).  By getting fresh mixture into as many corners of the combustion chamber as possible, we help to keep combustion chamber temperature under control and this permits higher compression and/or timing advance; both of which usually lead to more power.  I've not heard of anyone incorporating tumble into a head not already designed for it, but maybe you have a machinist with the knowledge and desire to make it happen?

Carburetor Selection
One of the topics I wanted to talk about here was carburetor selection.  It seems all too common for folks to believe that by slapping on a new set of carbs they've somehow created horsepower.  I hate to disappoint, but the engineers at your bike's birth place didn't select your carbs on a whim.  Possible that the cost control guys came in and told them to choose something else, but I digress...

The point I'm trying to make is that your stock carbs, if working correctly and adjusted correctly, are probably fine for your bike, even after a few engine changes.  If anything, they may even be too big. Luckily, there is a way to find out if your carbs are sized correctly.

Before I get to that, there are a couple of other reasons to consider a carb change.  Not all carbs are created equal.  In addition to metering the fuel, the carbs should restrict the air as little as possible (at WOT, of course), be easy to tune (this is especially important for built engines as you will be messing with the carbs a lot), and do a decent job of atomizing or emulsifying the fuel.  This last item is important, but maybe not as much as you'd think.  Poor atomization of the fuel by the carbs can often be made up by good chamber design, quench area, tumble/swirl from the valve design, and even increased compression.  If the carbs can do a good job of atomizing the fuel, then that's a big plus, but not a deal breaker if they can't.

Anyway... back to sizing.  There are two basic ways to determine proper carb sizing.  The first method is through calculation and the second method is through experimentation.

I'll cover experimentation first, because the concept is fairly simple.  The airflow through carburetors is usually measured at an industry-standard 3" of Mercury for single barrel carbs.  Multibarrel carbs and carbs manufactured for automobile use may be rated at 1.5" of Mercury, so pay attention.  For those unfamiliar with the concept of inches of Mercury, it's a measurement for determining negative pressures (usually called vacuum, though the terminology of "vacuum" isn't technically correct).  This measurement derives from the inches of Mercury that are "pushed" up a low pressure tube by the relatively higher pressure of the atmosphere.  The lower the pressure in the tube, the higher the number of inches.

For experimentation purposes, this means that your carbs, when sized well for your engine, should be pulling 3" of Mercury at WOT and redline.  The experiment for this is deceptively simple.  Hook up a vacuum gauge to your carbs and go for a spin.  Take the bike up to redline at WOT and check the vacuum.  It should read between 3" and 3.5" of Mercury.  If the reading is lower (1.5", for instance) then your carbs are too big.  Time to downsize.  If the reading is higher, then you need bigger carbs.  If you're even a little bit out of this 3"-3.5" range, it would be wise to consider a change.  Ensure, however, that the restriction is actually from your carbs.  Your air filter may be causing some of this reading.  This can be verified by reading the number of inches of Mercury by plugging your vacuum gauge into the intake (before the carbs) and repeating the test.

The calculations for determining carb sizes are also pretty simple, but a bit lengthy.  First, you need your engine's displacement in cubic inches.  If you already know this value in cubic centimeters then just multiply this value by .061.  A stock Honda 360 (356cc) is 21.7 cubic inches.  Next, multiply this value by half of your redline value.  Assuming a 10,000 RPM redline, we'll use 5,000.  So 21.7 * 5000 = 108,500 cubic inches per minute.  Conversion to cubic feet gives us 62.8 CFM of air is required by the engine at redline.  Next, divide this value by the number of carburetors in use by your engine.  Continuing with our example of the Honda 360, we divide by 2 and arrive at 31.4 CFM per carburetor.  Finally, we multiply by our estimated volumetric efficiency.  For a stock engine, this value is usually around .85 (85%).  Built engines push closer to .9 (90%) and full race engines get between .95 and 1.0 (95% and 100%).  I'll assume a stock engine and use the .85 value.  We now how a final flow rate of 31.4 * .85 = 26.7 CFM.

At this point, your best bet is to find a carb manufacturer that lists the CFM ratings for their carbs and pick the one that most closely matches your requirements.  For my own purposes, I usually round down.  For instance, lets say we want some new carbs for our 360 in the above example.  Ideally, we're looking for something rated at 27 CFM.  Supposing we find carbs rated at 26 CFM and 28 CFM, but not 27 CFM.  I'd select the set that are rated at 26 CFM.  In real world use, your bike isn't going to be seeing WOT very often, even if you ride like a madman.  The increased throttle response you get from a slightly smaller carb in the low and midrange will definitely outweigh the potential gains you get from a slightly oversized carb at WOT.  It is a common error to select overly large carbs with the idea that it somehow creates horsepower.  In theory, perhaps it will free up a few ponies at redline, but in reality you're going to be spending 99% of your riding at part throttle and/or out of redline.  Only a full blown race bike (on a relatively straight, high speed course) will see any benefit.

If your carb manufacturer does not provide a CFM, it's time to either bust out some math or hope you have access to a flow bench.  I'm assuming the former, so lets get down to it.  In order to calculate the area needed to flow 26.7 CFM, we must first know the air speed velocity through the carbs.  Unfortunately, this isn't something easy to know.  Ideally, this would be measured, but for this model we're going to assume 65 feet per second.  As yet another corollary, it's important to realize that the intake system for your engine is constantly decreasing in radius along its track.  The carbs will have a much larger radius than the ports and the valve seat area is even smaller (especially with the valves taking up some of that space).  It's not uncommon for port velocities to measure between 300 and 500 fps in a well setup engine, but you won't see numbers that high when measuring velocities at the carbs.

Anyway... back to the math.  Volumetric flow rate of a fluid (yes, air is still a fluid) through a round pipe uses the formula of v = Q / A where Q is the flow rate, v is the velocity, and A is the cross sectional (inside) area of the tube.  Because we know our flow rate (26.7 CFM) and we know our air velocity (65 fps) then all we have to do it convert the units and solve for area.

First, we need to ensure we're using the same units and so we need to convert our air velocity in to feet per minute instead of feet per second.  This gets us 3900 feet per minute (65 feet * 60 seconds per minute).  And so our equation now reads 3900 = 26.7 / A.  Algebra tells us we can swap the locations of the A and 3900 values in order to make this easier to solve (since we're tying to solve for A) and so now the equation reads A = 26.7 / 3900.  Completing the division gives us A = 0.00685 square feet.  Conversion over to metric (since carb bores are listed in metric) gives is a cross sectional area of 636.39 square millimeters.  Divide by PI (3.1415) to get the radius squares and we have 202.58 = r˛.  Next, take the square root of 202.58 and we have a radius of 14.23 millimeters.  Double it and we get a diameter of 28.5 millimeters.  The ideal sized carbs for our engine will be right around the 28mm mark.  VM28s would be perfect.

Finally, it's important to note that the air velocity through the carbs is directly related to the inches of Mercury being pulled.  Assuming a WOT condition at max RPMs, increased air speed will lead to decreased pressure.  Because it's pressure with which we're concerned, measuring this value is generally of more use than calculating out your carb sizes.  Furthermore, multivalved engines can usually maintain a lower vacuum signal across the carbs and still remain responsive.  Values as low at 2" of Mercury may be acceptable in those cases but if time and money permit, don't be afraid to experiment.

Intake Filters and Velocity Stacks
OK... we've made it this far.  It's time to take a look at the different options available for filtering the air coming into the carbs.  First up, there are some carburetors that require some level of restriction in order to work properly.  Removing the stock air box from these carbs can create issues that will need to be dealt with.

Mostly this applies to CV carbs where an uneven and turbulent air flow can lead to improper metering of fuel.  Usually this will manifest itself in the form of a rich midrange or, assuming you've adjusted that out through jetting changes, a lean top end.  There exist several solutions and these include, but are not limited to, lengthening the intake to smooth the incoming air flow, using stronger (or even stretching the stock) diaphragm springs.  In many cases it may be more desirable simply to leave the stock system in place.

But if you're building an engine for performance reasons, there are likely to be some gains to be had by freeing up the breathing prior to the carbs.  How do you know?  Well, it's time to bust out the vacuum gauge and go for a ride.  This time, we're going to want to plug the gauge into the intake prior to the carbs, instead of into the vacuum port on the carbs themselves.  You may need to tap into the intake system in order to make this happen.  Just like when testing vacuum on your carbs, you want to get the bike going and take it to redline at WOT.  We're aiming for zero inches of Mercury on this reading.  Anything more than that represents a loss in power due, not only, to the reduced air coming into your engine but also the extra work your engine is doing fighting lower pressures (this is an example of pumping losses).

Quite often a simple switch to a new filter or performance filter can resolve this issue with a minimum of headache and tuning.  Other times it may be required to remove the stock filtration system and opt for something different.  The usual course of action is put on pod filters, but I don't recommend this until after you've tested your intake system.  Replacing parts just because everyone else is doing or because it looks good is not often a path to success.

Aside from pod filters, there are also foam filters and other styles available.  Largely, they do basically the same thing.  They keep the air as clean as possible while restricting the intake as little as possible.

One of the better performance options to investigate is velocity stacks.  These don't usually come cheap, but most engines with a well chosen velocity stack will get a slight boost in performance (almost all at the very top end of the power band).  A well designed velocity stack can usually allow a carburetor to perform above its rated CFM by a couple of percentage points.  A useful feature if you're nearing the top of what your carb can provide but aren't ready to pick up a new set just yet.  More often than not, these items get added for looks though, so be aware that there are MANY velocity stacks on the market that will not provide much in the way of performance gains.

The way velocity stacks work is the same way the rest of your intake functions, it just starts the process earlier.  A full taper velocity stack (the only kind worth buying, in my opinion) will be wider at the bell mouth than it will be at the carb side.  This steady constriction increases velocity of the incoming air and promotes laminar flow prior to the throttle body.  By getting the air all moving in the same direction and same speed, the molecules spend less time bumping into each other and more time moving into the engine where they can do some good.  This smoothing of the air may also be beneficial, when put into use on CV carbs, where pod filters may not.  A velocity stack designed for performance (as aside for just looks) should be polished on the inside and the lip of the bell mouth should make a full 180° turn so that it's edge is pointing back toward the carbs.  This have been proven in several tests to promote the maximum air flow.

On the down side, most velocity stacks reduce or eliminate the ability to use an air filter.  Even if you don't go riding around in the dirt and mud, it is expected that you will need to re-ring your pistons every 5,000 miles or so.  Valve seats and/or valves may last you 10,000 or 15,000, if you're lucky.

Other Stuff
As mentioned in the BMEP post a few pages ago, cold air makes more power.  If possible, consider heat shielding between your intake and the rest of your engine.  Carbs should always be connected to the head via a manifold that does not conduct heat well.  Many plastics and rubber compounds fall under this category.  If a metal manifold must be used, consider gasket materials that are poor conductors of heat.  Keeping heat out of the intake can add significant power to your engine and the best way to do this is ensure you're pulling your air from a cool source and keeping the intake tracts insulated from engine heat as much as possible.

I also wanted to touch briefly on the topic of ram air tubes.  Skip these.  They do have some benefit at higher speeds, but they're difficult to install and have working correctly (ESPECIALLY on a carbed bike).

Finally, I've seen a few two-into-one or four-into-two intake manifolds floating around.  Skip these, too.  They actually hurt the performance of your engine.  What you may gain in having fewer carbs to mess with, you'll lose in tuning ability.  More often than not, these systems are put in place for looks alone and may lead to uneven running of the bike as the runners are rarely balanced.

Conclusion
In my opinion, the intake system of an engine is one of the places where many gains are to be had and perhaps it's because of that reason that so much misinformation exits.  It's funny how many people I meet that just swap out their air filters for some pods and think they've added five horse power...  Like any other section of engine building, this needs to be approached with a goal in mind, some tests you should expect to undertake, and a methodical process to follow.  If you're unsure how to proceed in your tests, find a quiet and straight section of road.  Run it at WOT and time yourself.  The clock is the great equalizer and it never lies.

You can make additional power in this area and it's not terribly difficult (especially cleaning up your own intake ports or having a machinist backcut your stock valves), but take your time and make sure you understand what you're doing and what you hope to accomplish.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Sep 28, 2012, 02:52:51
Nice write up but I was always taught the the intake length logic the other way round.  I do like that logic though and it probably has some effect as well.

Conventional wisdom is that as the intake valve opens, it sends a pressure wave back towards the carb.  That wave is negative ie it's low pressure.  As it reaches the open end at the bellmouth or carb entrance, it is reflected back with the opposite sign, so it goes back down the port as a positive wave.

We want that +ve wave to arrive back at the valve just before it closes, to create what is generally known as ram tuning.

As you pointed out, it's the 3rd or 4th wave that works out in the real world and what it demonstrates is that there are multiple waves in action at any time.  I don't have the latest version of Engine Expert, but MOTA shows the same effect in intakes in 2 strokes.  As the port opens you can see a pressure wave travel back and be reflected from the open end and you can see a series of other waves including the weaker wave generated as the port closes.  Thsoe waves continue to go back and forth many times before the subsequent intake event.

On intake ports, the key thing to keep in mind is smooth transitions.  Ans gas flows fastest around the outside of a bend and that's also the high pressure area.  The floor flows very little.  It's still important but less so than the roof.

The most important thing with intake ports is the shape from guide to valve seat.  AG Bell and Vizard have laid down guidelines for the different sizes relative to each other and those dictate vlave and intake port/carb sizes.

Car guys are all into CFM measurements and port volumes.  The alternate approach is gas velocity which gives a much clearer picture of what's happening. It's also a good way to size a carb and valves.  NX4 uses flow rates to predict power and revs but it doesn't track well to reality on small engines.  I used that to explore our CB160 race bikes intakes and the correlation between intake flow and HP was low.

We all have numbers that we like to see in terms of gas velocity and they are not always consistent. Numbers for sizing valve area in a 4 valve head don't work well for a 2 valve motor.  AOD (Army of Darkness) ran some interesting numbers for their big FZR400 racer some years ago that wok well on a 4 valve but not so good on a 2 valve, but the spreadsheet can be tweaked.

On most older Hondas, in our experience, intakes are often oversized for a stock or tuned motor and have certain parts of the port that may be way out of spec relative to others. In some cases they require larger valves to work with the port shape and in other cases, they need Devcon F to make part of the port smaller and more "right sized".

A great way to work a motor is to start off with a stock head and have it gas flowed.  They test flow in CFM at different valve lift and after you port it, check it again. In some cases the flow is the same after porting even with it all looking much better than stock.

For example a Honda 175 head flows all the air that motor could ever use up to some crazy revs it could never reach. The exhaust on the other hand can't get out of its own shadow and that's where the problem lies.  A CB160 is even worse.  The intakes are not so good but after porting are fine, but the exhaust can't be made to flow well enough without some innovative approaches.

If the exhaust flow is lower than your target relative to intake flow, the motor will remain constipated.  We can try to cram more air in, but if the exhaust doesn't flow well, then some burned gasses remain in the head and can't be cleared, so intake flow and power don't change much. That can be very disappointing.  What we want is the right amount of flow at the highest velocity we can get away with.

Tuning these old bikes is one part theory, one part measurement and two parts try it and see.

Thanks again Matt for taking the time to go through some of this stuff.  It turns out that it's way more complicated than could imagine and books and articles by Bell and Vizard are invaluable.  There's some great stuff by Ricardo and Welsake that laid the foundations and guys like Smoke and Cr Axtell and Jerry Branch added volumes to the knowledge we have access to today. 
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: AgentX on Sep 28, 2012, 06:11:13
I need to email you guys some beer...
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Sep 28, 2012, 12:07:36
Thanks.

I'd invite everyone to buy their own hardback copy of AG Bell's Performance Tuning book.  You will read it and write notes all over it and leave it with highlighter.

Then find any article by David Vizard and read them all.

Anything on head porting is worth reading even if it's on large block hemis.  The specifics do not apply, but it all adds to your education.  Matt is trying to squeeze the contents of many books into a few short posts to cover the basics to give people a sampler.  For example intake port design is a whole chapter in Bell's book and he doesn't cover everything that the world knows.

Exhaust port and exhaust design has come along a ways in teh last few years and is still very basic compared to a two stroke. But check out the exhaust on Stoner or Pedrosa's Honda MotoGP bike. They are way longer than we might expect or calculate so there must be a reason for that that isn't necessarily obvious.  They don't have to worry about part throttle low rpm use, so they have a different set of parameters to work within than the rest of us.

Performance Bikes magazine in the UK did a series of articles years ago about each aspect of motorcycle engine design.   In a previous life as Motorcycle Mechanics they had articles on all sorts of cafe racer builds and how to build one.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: longhorn717 on Oct 06, 2012, 15:41:51
Great info and must agree going faster is partly your bike and partly my fatass. Nobody would ever guess it and nurses triple check their scales but I weigh in at 270lbs. and am 6'. would love to lose 40-50lbs. and I'm sure my little cb400t would love me for it. as far as my bike goes its getting a complete workover. suspension, new rear two-up section and seat, new tank with knee indents, pods and tune or replace carbs(from what I hear they're some of the more difficult ones to jet properly, currently designing new exhaust with crossover on header and 2-1 collector/muffler under motor. going with a smaller headlight and may ditch the turn signals, clipons, and going to go through the engine soon and at the very least clean out the 32 years of grime and button it up with a new gasket kit. maybe clean up the head too.

As for the exhaust I just read a interesting article on why you DON'T want back pressure. but how do I keep it free flowing and not burn my exhaust valves? here's the link....   http://my.prostreetonline.com/forums/archive/index.php?t-1639.htm
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Oct 15, 2012, 20:12:25
Applications - Top End Repair and Health

Well, I was going to continue with my semi-logical order of performance upgrades, but I've had an interesting couple of weeks and I figure I'll diverge a little bit in order to share what's been going on in my shop.

A couple of weeks ago, on the way to work, my daily driver (1996 Geo Metro, 1.0L) started missing on one of the cylinders at idle.  Once I revved it up to 3,000 RPM or so, the cylinder would kick back in.  The car got me to work, but on the way back home, the engine started overheating as well.

I suspected a blown head gasket.  A loss of compression will cause the misfire and leaking coolant system will cause the overheating.  Both of these things occur when the head gasket fails on a liquid cooled engine.

Turns out, I was correct.  This is the oil I drained from the engine and when it's in this condition it is known as "milkshake".  If drained from a recently run engine, it will be very frothy, hence the name.  I was expecting about three quarts of oil, and ended up with a gallon of this stuff; another sure sign something is not right.

Funnily enough, I've never really taken apart a car engine before, but having some experience with bikes I figured I'd give it a shot.  $100 in parts sure beats $1000 for the parts and labor, I figure.

So a few hours later, I'm at this point (this pic was taken after cleaning up the sealing surface on the top of the block).
(https://scontent.xx.fbcdn.net/hphotos-xaf1/v/t1.0-9/564934_10151275229895159_254301028_n.jpg?oh=dcda41cae89d90fd242b3aa3804f7a0d&oe=56570A7D)

Right away, we can see that cylinder one was the problem.  The coolant, leaking into the cylinder during operation, has steam cleaned the piston and combustion chamber, resulting in the shiny surface you see here.

While everything was disassembled, it's also a great time for a general health check of the top end.  I recommend this to anyone looking to restore a vintage bike.  I'm pretty sure I mentioned it earlier, but it bears repeating:  An engine is only as good as its weakest link.  You can have trick components, a hot cam, and high compression pistons, but if your valves are leaking then it's wasted effort.  A stock engine in good health will almost always outperform a built engine that's limping along on one or more questionable parts.

The Acetone Test
Now that I had the engine apart, it was time to start checking things over.  The first test I usually perform on the head is what many folks call the acetone test.  The idea is fairly simple.  You pour some acetone into each of the ports, one at a time.  Sit the head up on its edge and add a couple of table spoons into the first port.  Let it sit for a few minutes and inspect the combustion chamber for leakage.  Any acetone that makes it through indicates a failure of the valve to seal and this must be corrected.  Here's a pic of a failed acetone test on GS450.  The liquid leaking past is clearly visible.
(http://i401.photobucket.com/albums/pp98/KirkN_photos/Suzuki%20GS700/Copyof009nogo.jpg)

It's important to note that other fluids may be used, but you're looking for something with very little surface tension.  Paint thinner, denatured alcohol, and gasoline are all viable fluids for this test.

Like the picture above, my Metro failed on all six valves.  Time for some work.

Inspecting the Valves
Now that we know that valve sealing is an issue with our engine, we can pull the valves and inspect what may be wrong.  Even if nothing is wrong and you pass the acetone test, it may still be a good idea to pull the valves.  This will let you check the valve stem diameters, stem-to-guide clearances, and also lets you replace the valve stem seals.  These next sets of pics are the intake valves, and the exhaust valves, respectively.  All six valves were cleaned using a combination of Simple Green and a soft wire brush, prior to taking these photos.
(https://scontent.xx.fbcdn.net/hphotos-xfa1/v/t1.0-9/251257_10151275230475159_969100524_n.jpg?oh=38af1e985c4b1865d3c6302f1d827ebc&oe=5614E427) (https://scontent.xx.fbcdn.net/hphotos-xpa1/v/t1.0-9/46501_10151275230155159_1946990235_n.jpg?oh=105d6a7c1d56903644ba5ead24a4cacd&oe=5613BE56)

As you may have guess from the photos, the exhaust valves are all shot.  Each valve is clearly pitted along the sealing surface.  The intake valves are showing a little discoloration, but are in otherwise decent condition.  I reused them.

OK.  So we know that new exhaust valves were needed.  Time to inspect the valve seats within the head.
(https://scontent.xx.fbcdn.net/hphotos-xaf1/v/t1.0-9/564934_10151275229895159_254301028_n.jpg?oh=dcda41cae89d90fd242b3aa3804f7a0d&oe=56570A7D)

Again, the intake valve seats look to be in good condition (nice shiny surface, no pitting), but the exhaust valve seats have seen better days.  Though not immediately apparent in this pic, there was very little pitting, but they still weren't as good as I would like and carbon build up was a clear sign that we aren't getting as much metal-to-metal contact as is required.

Now... Sometimes compromises need to be struck.  In a perfect world, every job should be done perfectly, but this is not a perfect world.  We are limited by our time, our resources, and our own skill levels.  An argument can be made for not doing a job to perfection, but there is no excuse to not maximize the use of the time, resources, and skills we do have.  In this scenario, I was limited very much by resources ($$$), and so I chose not to have a machinist clean up these valves seats.  The price for a valve job usually runs about $20-$25 per valve, so I was expecting a bill of around $150 to have the valve seats cleaned up and that's something I could not afford at the moment.  A car that is running at a sub-optimal level is more useful to me than a car that is not running at all and so I decided to proceed without the valve job.  I do want to make it clear that is something that should have been done, but was not.  If this were to enter a mechanic's shop, they probably wouldn't guarantee the work without having the valve seats recut by a machinist. The overall health of my engine was affected by this decision, and I will get into that later.  But basically, know this:  Before you make a decision to stray from the beaten path, you should ask yourself one single question.  "What will happen if I do it this way, instead?"  If you can't answer that question, don't do it.  Decisions should be made from a base of knowledge rather than necessity.  In this case, I know that my approach may not fix the problem, but it wasn't likely to make it any worse, either, and so I choose to proceed.  Also, had this been on my motorcycle engine I would have done the job properly and not taken any chances, even if I had to wait a few months to get the money together.  My bike and my car serve different purposes in my life and so they get different treatments.

Clean up
Now that we know some work is needed, the next step in the process is to get everything cleaned up.  Even if no work is needed, it's still advised to undertake this step.  The only reason I list the acetone test first is that it helps to give you an idea of what problems may exist and it saves you at least one set of (dis)assembly routines.

Aside from pain involved in working with greasy and dirty components, sometimes it's just plain impossible to make the measurements needed with bits of leftover gasket, chunks of carbon, and blobs of grease all over everything, so it's time to make this thing shine.

Chances are, your head is made from aluminum and so you're going to want to follow some important procedures when cleaning.  First, don't expose the head to any heat levels that would be exceed normal operating temperatures.  No blow torches or crap like that.  For my own cleaning, hot water is about as hot as I get.  Second, try to avoid media blasting, if you can.  Soda is OK, but stuff like glass and shells can plug things up and coarser media like sand will usually cause damage.

The first step to cleaning is a soak in a citrus cleaner like Zep, Simple Green, or even Pine Sol.  For really caked on grease, you can use a plastic brush to work the cleaner into those areas.  The second step is a good scrubbing with soap and hot water.  I use a combination of another plastic brush, and occasionally a 3M scratch pad.  Any carbon deposits left after this stage can be cleaned with a carbon dissolver such as Zep Morado, but don't leave it in contact too long.  This stuff can hurt aluminum over time and it shouldn't be used on areas without carbon build up.  A wire brush may be necessary to fully remove the carbon deposits.  To remove any gaskets, use a razor blade, but scrape against the angle of the blade instead of into it.  This will avoid gouging the head and causing any scratches.  If there is coked oil at any point, kerosene or diesel can help get these off.  Finally, any left over gasket bits can be removed with some 400 grit paper and WD40.  Be very careful when sanding, however, as sealing surfaces are very intolerant of low spots.  Most specs allow for only .003" height differences across the entire sealing surface.  You may need to have the head resurfaced if you get too heavy handed at this point.  A final wash in soap and hot water followed by air drying should get you ready for the next steps.  3M Roloc pads in a die grinder can also be used for gasket removal, but use them sparingly.  Power tools get through aluminum in a hurry and what may save you time now, may cost you both time and money later.

The Valve Job
As I just mentioned, I chose not to pursue a valve job at this time, but I'm going to include the information here for future reference to those seeking to "do it right".

Many people, at this point, would hand the head over to a machinist for the necessary work, and that's OK.  I did the same with my own 360 build.  I'm going to cover how to do your own valve job, however.  You will need some special tools and this does take time, but it's also quite rewarding.  The pics for this section have been "borrowed" from around the 'net, so these represent what I would have done, rather than what actually happened.

First up, the tool set.  You will definitely need a valve cutter.  Neway is a company that makes these for a reasonable cost and no special power tools are needed for it's operation.  They usually cost between $250 and $400 for one of these things, so it's probably only worth buying if you plan to do your own valves at least half a dozen times (many vintage bikes will not have a three angle grind from the factory, so this tool can be used for performance and not just repair).  Make sure to buy the kit with the cutting heads of the appropriate size.  These kits are not one-size-fits-all, but they are one-size-fits most.  They look like this when they're new:
(https://scontent.xx.fbcdn.net/hphotos-xaf1/v/t1.0-9/185060_10151275324090159_1921929818_n.jpg?oh=b471c1a2cd79dff99a2daec1674e2ff6&oe=56107E1E)

The first step in the cutting process is to insert the pilot.  This acts as a guide for the cutter to ensure the cut is precise.  If you are planning to replace the valve stem guides, you'll definitely want to do that before doing any valve cutting.
(https://scontent.xx.fbcdn.net/hphotos-xaf1/v/t1.0-9/184997_10151275323895159_1002695148_n.jpg?oh=4b97cfd14702d363eba0048708994b58&oe=56123CF7)

Next, slip the 45° cutter in place over the pilot.
(https://scontent.xx.fbcdn.net/hphotos-xpf1/v/t1.0-9/58754_10151275323920159_1400597779_n.jpg?oh=82ed9fcb8ab0c18a2a4bcf17588f81f6&oe=56104C0B)

And finally, the cutting handle.
(https://scontent.xx.fbcdn.net/hphotos-xfa1/v/t1.0-9/522917_10151275324005159_1510182551_n.jpg?oh=6cc46dbb50bc982c280ff374142ac016&oe=5610C958)

After a few turns (only a little pressure is needed, better to cut too little than too much), some of the valve seat should be cut.

Repeat this process for the inside (60°) and then the outside (30°) angle and you should now have a nice, shiny, valve seat.
(https://scontent.xx.fbcdn.net/hphotos-xfp1/v/t1.0-9/47152_10151275323840159_617509487_n.jpg?oh=c3ee38feb2904d056423a964924c40a3&oe=5625086F)

At this point, you'll want to cover your nice work with some layout dye (aka machinist's dye aka Prussian Blue).
(https://scontent.xx.fbcdn.net/hphotos-xaf1/v/t1.0-9/190281_10151275324055159_1870825735_n.jpg?oh=4c20c050be3ea460d95c239a1ce9e283&oe=561E1813)

And then repeat your 45° cut.
(https://scontent.xx.fbcdn.net/hphotos-ash4/v/t1.0-9/2888_10151275634730159_1308741101_n.jpg?oh=a1118dd9fa6dbc6d26f4c5522c633020&oe=5621342B)

The thickness of the metal strip that is exposed should be around .040" for the intake valves and .060" for the exhaust valves.  This 45° cut serves two purposes.  First, this is the sealing surface.  When your cut is finished, all of the blue dye should be removed.  Any spots of dye that are left after the cut usually indicate pitting in the valve seat.  You're going to need to cut again, but slightly deeper.  The second purpose is that the thickness of the valve seat has to do with heat transfer.  Because fresh intake mixture is coming over the intake valves every time they open, they are cooled by this mixture.  On the flip side, it's all hot exhaust gases passing by the exhaust valves and so the seats need to be thicker to allow the heat to transfer from the (relatively) hotter exhaust valves to the cooler head.  If this seat is too thin, then you burn up your valves in short order.

Your 60° cut (the one further up the intake port) should be roughly twice the width of the 45° cut and the 30° should be about 75% of the width of your 45° cut.  If you've made your 45° cut too wide, if can be narrowed by recutting with the 30° cutter in order to make that width wider.  There are only so many cuts you can do in a head, though.  Cutting extra means the total lifespan of your head is reduced as most heads do not have replaceable seats.  Some guys are clever enough to be able to add more metal to this area, but don't count on simply finding someone who is capable of this type of work.  That said, it'll usually be cheaper just to get a new head, anyway.

At this point, you can reassemble things and place the valves, springs, retainers, etc, back into the head.  Repeat the acetone test and hopefully you will see zero leakage after around five minutes of waiting.

If leakage still occurs, it is important to know why.  Were you a bit haphazard in your approach?  Did all the measurements check out?  Did the 45° cut fully remove the dye from the valve seat?  If you're happy with the way things went during the cutting process, it may be necessary to lap the valves.

Valve Lapping
Many machinists will advise you to lap the valves as a matter of course.  For me, this seems unnecessary and I think this recommendation is one of those things that just gets handed down out of tradition rather than real need.  As a side note, if you're running with titanium valves, you don't want to lap them.  If the acetone test fails, then you need to recut the seats.  Titanium valves almost always include special coatings and lapping will remove them.

Lapping is a lot like precision sanding.  Lapping compounds are usually some sort of lubricant with different levels of grit mixed into it.  The idea is the rub the valves back and forth across the seats until both the valves and the seats are nice and smooth.  This is accomplished by adding a bit of lapping compound onto the valve head and pressing it into the seat (by hand).  A valve lapping tool is then used to spin the valve in place and the compound cuts into the valve and the valve seat.  A lapping tool is essentially a wooden stick with a suction cup on each end.  Don't buy a cheap one of these off of eBay as you want a good suction cup for this not to end up being a huge headache.  Auto parts stores usually stock the good ones.  Lapping should not be done by attaching things to the valve stem and spinning (such as I've seen suggest with power drills and such).  Also, lapping should be done prior to the replacement of the valve stem seals.  It's OK to lap with the old seals in place or removed, but not OK to use the new seals during this process.

For the lapping process, itself, just do what this video is showing.
http://www.youtube.com/watch?v=fhXsH12Rg6s

Repeat this process until the "sound" of the grit starts getting quieter.  Switch to the next finest grit until it gets quieter and so one and so forth.  Grits commonly used for valve lapping are 120 grit, followed by 220 grit, followed by 400 grit.  Some people even go out to 800 grit, but I think this is overkill.  Also, you may not need to start at 120 grit unless you skipped the cutting process, like I did.

As another side note, lapping compound is useful to have around the shop to clean up sealing surfaces on heads and jugs prior to assembly.  Spread a liberal amount over a sheet of plate glass and work your part in a figure eight pattern for ten minutes, rotate it 90°, and go for another ten.  This clears up microscratches and the like and is also a good way to correct any scratches you may have introduced to sealing surfaces during the removal of old gaskets.  A bit of diesel or kerosene can be added to the lapping compound to thin it out a bit.  Thinned out compound is called "slurry" and is available for purchase directly, but I just prefer to keep the compound around and thin it myself, if needed.

When you have finished the lapping process, the shiny valves and valve seats will have developed a satin gray stripe along the middle.  Lapped valve on the left, new valve on the right.
(https://scontent.xx.fbcdn.net/hphotos-xaf1/v/t1.0-9/487401_10151275716105159_2133448671_n.jpg?oh=4053113fbbde5e8a085eb779d886390d&oe=5620735B)

After running the valves in your engine for a few hundred miles, this gray satin finish will eventually be worn into the shiny finish like you see in the valve on the right.  The lapping process helps provide an initial seal, but like cross hatching on cylinder walls, it eventually wears away and creates an even better seal after doing so.  You should not count on the engine to do all the work in this process, though, the idea behind a good lapping job is to minimize the time this wearing-in process takes and doing it poorly can result in a poor final seal as well.

After this lapping process, you should have a seal good enough to pass the acetone test.  If not, you need to make a decision.  What the problem with the cutting or was it with the lapping?  More Prussian Blue can tell the tale.  Paint it around the valve seal and place the valve into position without twisting it.  Pulling the valve back out should result in a coating of dye all the way around the valve.  Any missed spots indicate a failure in the cutting process and you'll likely want to repeat it.  If you get dye all the way around the valve, you can try lapping it again.  A failure after another round of lapping will mean you'll want to recut, though.  Make sure your seat widths are within spec, as well.  Too narrow and you could have sealing problems.  Other things to check are bent valve stems or damaged valve guides.

Back to my Decision
As I mentioned earlier, we'd revisit my decision not to cut the valve seats.  After getting the new head gasket into place and reassembling the engine, I decided to run a compression check.  The spec on a fresh engine for the 1.0L Geo Metro is 195 PSI with the lower rebuild limit coming in at 165 PSI.  My first test netted me values of around 130 PSI.  After discovering some cam timing issues and retorquing the head (this should be done after covering a few miles so as to compress the gasket a bit further) I'm up to 150, but still below spec.  There are a couple of things which could be working against me.  First up, I mentioned that I was doing this work under tight financial constraints and so I choose not to have the valve work done.  One thing I hadn't yet mentioned is that I also skipped out on the head surfacing (which was probably needed as well, but would have been another $100).  Here's a pic of the sealing surface on the head and the pitting here is quite clear.
(https://scontent.xx.fbcdn.net/hphotos-xpf1/v/t1.0-9/403702_10151275247780159_555013930_n.jpg?oh=2038b05d9311a24ff3fa52239ebb0f74&oe=5658526F)

So... how does that affect my compression?  Well, it is not uncommon for head gasket kits (especially those in the auto industry) to come with slightly thicker-than-stock head gaskets.  It is assumed that resurfacing the head after 200,000 miles is something that will just be done and so the head gaskets come slightly thicker than they would, otherwise.  I don't know that this is for certain in my case as I did not measure either the new or the old head gasket.  This is simply a possibility.

Assuming the head gaskets had no difference in thickness, then I suspect the lapping I did on the valves either helped only a little, or not at all.  The car's power output does seem a bit low, but that could also be from me being used to riding on two wheels these past few months.  I have no data to which I can compare and I know only that it could be better than what it is right now.  With any luck, the valves will continue to seat in and my compression will slowly rise over the coming miles.  But having to choose between a car that doesn't run and one that doesn't run as well as it could, the choice was pretty clear.

Conclusion
OK... so your head is screwed up and you're not sure where to begin.  Start with some simple tests and some cleaning and go from there.  Remember guys, you're working with technology that has been around for a while now.  This isn't rocket science and it's not as hard as you'd think.  Take your time, make your measurements, implement your solutions.  An engine that runs always puts out more horsepower than an engine that doesn't, and so making sure you're up to spec should be job number one.  The reliability and performance of your engine will depend a lot on the time you take in diagnosing and correcting any problems you encounter.  Also, be aware, that when it comes to engines, new parts aren't always enough.  Some parts (even new ones) may need machining in order to fit, but not all machining needs to be done by a machinist.  There are a lot of hand tools available that will get the job done and you can learn a lot of new things along the way.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Oct 15, 2012, 23:55:24
Good run down there Matt.

We get all our valves and seats cut on a Serdi machine.  It's more accurate and doesn't leave striations on the seat- aka chatter.  With a Serdi cut it is neither desirable nor necessary to lap the valves.  We just pop them back in and they are perfect.  Whereas if I do them, they are not so perfect.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Oct 16, 2012, 00:16:49
Please correct me if I'm wrong, but with a Serdi you also get all three angles in one cutting?  That'd be nice.  :D
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: bradj on Oct 16, 2012, 09:30:38
I was a machinist for 8 years and take alot of what you posted for granet but its not. kool write up it will prove helpful to many im sure. O yea we had a vaccum guage to check valve seating it was pretty simple and very telling if there was a problem
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: scottyfunj on Oct 23, 2012, 13:39:41
Thanks for all the extremely valuable info.  This thread is becoming something of a motorhead's bible.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: mohabie on Oct 23, 2012, 16:31:14
Thanks for the valve job step by step... Though I will still pay someone to do it, I understand it better than before.

Thanks!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Corsair on Oct 23, 2012, 21:21:30
Matt,
Excellent post, and well written. Also, perfect timing, since I'm just getting into the engine work on my CB350f. Looking forward to more!

Sent from my Nexus S 4G using Tapatalk 2

Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Nov 07, 2012, 13:16:40
Applications - Exhaust Design
Time for a new post, I figure.  As you may have expected, one of the major areas still to cover is the exhaust system.  I've
saved this item until now because your design at this point is going to be very dependent upon earlier selections.  Furthermore,
there are several options you can take in order to change the overall characteristics of your engine (for good or bad) with only
a little regard as to your previous choices.  In short, the exhaust system is one of those areas where you can really make a
difference in the performance of your bike, especially in bikes as old as ours.  Even in newer machines, it is not uncommon to
see exhaust changes as the primary means of modification in engine performance.  For those of you guys riding smokers, the
exhaust design is, arguably, the single most performance consideration.

As with previous posts, the plan is to break this up into manageable chunks.  Each subsection will deal with a different
consideration and will hopefully tie it all together as we reach the end of this post.

Before we begin, there are a few things you'll need to know in order to make the best use of this information.  First, you need
to know your displacement as well as the number of cylinders on your bike.  If you don't know these first two things, please hand your keys over and go buy a Toyota.  ;D  Second, you need to know the exhaust timing on your selected cam.  Finally, like with most everything else, you need to have a general idea as to where you'd like your peak torque to occur.

Exhaust Valves and Ports
The first part of the exhaust system actually begins inside the head.  Like intake valves and ports, a three or five angle grind will provide benefit here.  But unlike intake valves, less emphasis is placed on things like tapered stems or backcutting.  The reason being is that the gases exiting the engine are under much higher pressure than the mixture when it enters.  This created a much greater pressure differential between the cylinder and exhaust system than that is between the cylinder and the intake system.  This greater differential (often called "delta P") means that the little details become less important and the bigger picture become more important.  In this case, the bigger details are exhaust valve and port sizes.  The traditional approach is that exhaust valves should be sized at approximately 80% the size of the intake valves.  Hemispherical heads with a sharper include angle will benefit from slightly larger exhaust valves due to the tight radius the exhaust ports must follow.  This design philosophy is clearly visible in the ratios seen in the Honda 350s and 360s (34mm intake valve, 28mm exhaust valve, 82.4% sizing ratio).

For exhaust ports, the diameter should be the same as the valve size (or only slightly smaller).  Usually they are smaller and so taking some metal out could be a good idea.  Pay special attention to the port ceiling as this is where most of the gains will occur.  Also, like intake ports, a 'D' shape with a flat floor will generally out perform purely circular ports.  Exhaust port walls should be as smooth as possible and can be sanded down as fine as 800 grit paper.

Finally, the transition to the header should occur with a sharp step.  The header will have a significantly larger diameter than your exhaust port and the change to this larger diameter should be sudden, not smooth.  This fast change helps to prevent reversion, especially at lower RPMs.  Reversion is a killer of torque and so you can expect a bit more power down low and in the mid range with this step in place (especially if you're running a hotter cam).  If, for some reason, this step doesn't exist in your system, an insert can usually correct the problem.  I know MikesXS sells them, and they're not too hard to make, either.  They look like this, and you can clearly see the sharp step created as you transition from the head to the header:
(http://www.mikesxs.net/parts/img600/07-0769.jpg)

Header Design
When it comes to exhausts, there are two types of tuning with which you should be familiar.  The first, called inertia tuning, which involves overall design of the exhaust and its length.  The second, known as acoustic tuning (very similar concept as pulse tuning for intakes), is very important for two strokes, but also has an effect on four stroke engines.  Both, however, are of some importance and so you should be familiar with the basics.  As with my previous posts, I'll stick mainly to the four stroke applications.

Also, the majority of the design in an exhaust system will go into selections on the header.  The gases in the exhaust system are the hottest just after they leave the cylinder and enter the header, so this is where many systems fall down.  Correct sizing and lengths can make several percentage points of difference in peak numbers.  Tweaking things here can also change the characteristics of the engine without having to open up the cases again, so if money isn't an object it's definitely worth exploring having several different header configurations.

Inertia Tuning
The concept behind inertia tuning is that the exhaust gases leave the cylinder in regular pulses and that these pulses tend to stick together as they travel through the exhaust system.  This means that each pulse is a discrete unit of high pressure traveling through pipe and due to inertia, these pulses leave a low pressure zone behind them as they travel.  The efficiency at which your cylinder is emptied of exhaust gases is directly related to the delta P between the cylinder and the header and so keeping the exhaust system in a low pressure state will translate into better volumetric efficiency and more torque.

The best design for inertia tuning has been shown to be a four-into-two-into-one (4:2:1) for best mid-range torque or four-into-one (4:1) for max power.  Obviously, twins only get the 2:1 option.

Finally, we need to decide on a total length for the exhaust system (we'll handle lengths for the headers and collectors and such in the next section).  For inertia tuning to work best, an exhaust pulse must be able to travel the entire length of the pipe before another pulse enters.  It's OK to have two pulses in the pipe at once, but zero is bad.  As a pulse leaves the end of the pipe, the low pressure zone in its wake will pull in gases from wherever possible.  We want this low pressure zone acting on our cylinder to pull out more exhaust gases and not acting on the atmosphere to pull fresh air into the end of the muffler.  Basically speaking, our exhaust needs to be long enough so that an exhaust pulse remains in the pipe up to the point at which another pulses is added.  At 10,000 RPM, a new pulse is entering the pipe every 0.012 seconds.  This means our pipe needs to be long enough for a pulse to travel its distance in .012 seconds (or more).  If the pulse can travel the distance of the pipe in less than .012 seconds, we need a longer pipe.

Exhaust gases will usually travel at between 200 and 300 feet per second.  Overly large exhaust pipe diameters will slow this speed (which isn't good) and pipe diameters that are too small will speed this up (also not good).  I prefer to use the 300fps number as it's a bit of a worst case scenario.  The actual speed of your exhaust gases can be calculated and I'll run through the math for that toward the end of this post.  Just remember that if you set things up correctly, your exhaust gas velocity should fall somewhere in or very near this range.

So... now we know how fast our gases are traveling, the next thing we need to do is decide upon the ideal RPM at which we want our exhaust to work.  Just like everything else, the total exhaust length should be tailored to the optimal RPM of your engine.  For this example, I'll use 8,000 RPM.  At 8,000 RPM, a four stroke engine is creating an exhaust pulse 4,000 times per minute.  That reduces down to 66.7 times per second. One divided by 66.7 gives us an exhaust gas pulse every .015 seconds.  We can then multiply our 300 feet per second value by .015 seconds to get our resulting length.  We end up at 4.5 feet.  If your desired RPM is higher, this length will reduce.  If your desired RPM is lower, the pipes with be longer.  When in doubt, err on the side of length.  Chopping pipes too short seems to be a common design decision these days (looks?) and hurts power up until the point where the engine revs fast enough to take advantage.  For pipes chopped up near the foot pegs, few bikes can even rev high enough to take advantage of this.  Even if your bike can rev up to 15,000 (in theory), short pipes are hurting power until you get to those speeds and so your redline (in practice) may be lower.  Looking at Honda's iconic RC166, we can see the pipes extend all the way to the back wheel, despite having a redline of 18,000 RPM.
(http://2.bp.blogspot.com/-q4QojyurkgU/TWJLfDcfSwI/AAAAAAAAGoE/voIE_1v8G24/s640/66rc166.jpg)

Bear in mind that this calculation method applies to a single pipe per cylinder.  For the more idealized approach of collected and merged header, total length will be shorter, again.  For merged headers, the idea is to use the amount of time elapsed between each pulse of all the cylinders rather than each pulse from each cylinder.  For bikes with more than two cylinders, use the Acoustic Tuning methods listed below.  For 360° twins (XS650, Brit bikes, etc), take the total length and divide by two.  180° twins like the vintage Hondas should multiply the total length by a factor of .75.

Acoustic Tuning
Acoustical tuning in the exhaust system works on the same theory as pulse tuning in the intake system.  When the exhaust gases explode from the cylinder and head through the exhaust system, a high pressure sound wave is generated.  A low pressure wave reflects back from the high pressure wave as it exits the pipe and the idea is to time this low pressure wave to come back to the exhaust valve at the right time.

Strangely enough, the total length isn't so much as calculated as it is derived.  By summing the lengths of the properly designed components, we end up at a more-or-less ideal total length.  The length of the headers sort of changes the slopes of the peak torque curve.  Longer headers will begin to generate more torque, sooner, but will cause a rapid drop off as RPMs increase.  Short headers will do the opposite; a rapid increase in torque with a slower drop off.

In order to accurately size the header pipes, you will need to know your cam timing as well as the displacement of each cylinder.  We'll get to the cam timing part in a minute, but we'll talk about displacement right now.  The displacement of each cylinder dictates how much exhaust gases are generated.  The amount of exhaust gas and the diameter of the header work together to dictate the speed of the exhaust gases.

Time for some math...  First up, we need to know how much exhaust gas is being generated by your cylinder (not cylinders, we're only interested in one of them right now) at your ideal RPM.  Falling back to my previous examples of my own CJ360, I'm aiming for an ideal engine speed of 8,000 RPM with a per cylinder displacement of 189cc.

The first thing to do is to decide on the inside diameter of the primary headers.  The diameter of the header pipes is what determines the exhaust gas velocities.  Smaller pipes increase the gas velocity and cause peak torque to occur earlier, while larger diameter pipes slow down gas velocities and cause peak torque to occur later.  I'll touch on the heavier portions of the math later in this post, but for now, just take it on faith that each cylinder is generating 866cc of exhaust gases every period (two rotations per period).  At 8,000 RPM, we're getting 4,000 pulses every minute and so this translates to 3,464,000cc of exhaust gas per minute.  This is almost exactly 122.33 cubic feet of gas per minute.

Mr. A.G. Bell has been kind enough to test things thoroughly for us, and so we know that peak torque, from the exhaust system, generally occurs at 250 feet per second.

Using our ideal gas velocity and the amount of gas being generated at our desired RPM, it is now possible to calculate the inside diameter of the header pipe which will meet these requirements.

The formula for calculating gas velocity through a duct is v = q/A.  v is the resulting velocity in feet per minute, q is the gas flow in cubic feet per minute, and A is the cross sectional area of the duct in question.  We know the velocity is 250 feet per second and so we convert to feet per minute and get 15,000.  Plug the values into the formula and we get 15000 = 101 / A.  Algebra allows us to reconfigure the equation to solve for A.  We now have A = 122.33 / 15000.  A equals 0.008155 square feet.  Convert back to inches and we have 1.17437.  Because we're working with a circular pipe, it's time to break out the geometry skills.  1.17437 in˛ = pi*r˛.  1.17437 divided by pi give us .3738 = r˛.  Take the square root of both sides and we now have r = .6114 inches.  Double it to get our diameter and we're at 1.223 inches.  The nearest sized standard pipe is 1.25 inches and so we'll make use of that.

Now that we have our pipe sized correctly, it's time to get the length of the header.  The calculation for the length is fairly simple.  Length, in inches, is equal to (((850 * (180 + EBBC)) / RPM) - 3.  EBBC is the number of degrees before BDC that the exhaust valve opens.  In my own engine, this number is 54° and so the calculation becomes (((850 * (180 + 54)) / RPM) - 3.  Unlike other RPM selections, the RPM value used in this calculation should either be the redline of the engine, or the midpoint between peak torque and redline.  I'll be using my redline of 11,000 RPM.  Working the math through gives us an ideal primary header length of 15.1 inches.  Should you ever arrive at a length of less than 15 inches, just go with 15.  You don't want these things too short.

Before we talk about the next aspect of exhaust design, I'll give you a slightly easier method of pipe diameter selection.  This method tends to result in a slightly larger diameter pipe, but it's a lot easier to follow (especially after I get to the "heavy" math section).  After calculating the primary length, you can solve the issue of diameter by D = sqrt((cc / (pL + 3) * 25) * 2.1.  Basically, take the square root of the cylinder displacement divided by the primary length plus 3, times 25.  Then take that result and multiply by a further 2.1.  Using the same numbers are my above calculations we end up with an inside diameter of 1.35 inches.  When it comes time to match up to a standard pipe size of 1.375, this is only one step up from my original choice of 1.25 inches.

Secondary Headers
OK... with that out of the way, we can calculate the length of the secondary header.  This part only applies if you're planning on running a 4:2:1 system (or basically any system that doesn't converge directly into one pipe, so this applies to 6:3:1 as well, but not 4:1 or 3:1 or even 6:1).  The "secondary" nomenclature applies to the section that falls between the primary headers and the tail pipe.  In four cylinder applications, this is the set of two, after the four, but before the one.

The length of the secondary header is calculated in very much the same way as the primary header, but with an important difference.  The total length of the primary plus the secondary should be the value from the formula, multiplied by two.  If the formula yields a value of less than 15 inches, the primary header should remain at fifteen inches and the secondary head should be shortened accordingly.  For instance, if our formula gives us a length of 13 inches, we get 26 inches for the total length of both the primary and secondary headers.  15 of those inches are reserved for the primary header and the secondary header (and collector) should take up the remaining 11.

When adding a secondary header into the system, it is important to recalculate the diameter of the pipes being used.  The secondary headers should be slightly larger in diameter.  To calculate the value, raise the original value to the power of two and then multiply by two.  Square that result and then multiply by .93.  Round to the nearest standard pipe size and you're good to go.  Going back to my original example of 1.223 inches we follow the math through like so:  sqrt((1.223 * 1.223) * 2) * .93 = 1.609.  Round to standard pipe sizes and the secondary headers should be 1.625.

Exhaust Tuning
One of the nice things about saving the exhaust system until last is that we can fine tune the engine to the desired levels without having to open it back up again.  Changing primary and secondary header sizes and lengths will move and reshape the torque curve to match your desired characteristics.

For instance, if you want to lower the RPM at which peak torque occurs, you can reduce the diameter of the primary header (because secondary header diameter is derived from primary header diameter, don't forget to resize the secondary as well).  A reduction in size of a single step will drop peak torque up to 1,000 RPMs lower.  An increase in header length will result in lowering the peak torque as well.  It's even possible to use unequal length headers to broaden the torque curve as one cylinder will hit peak torque before the other(s).  On a four cylinder engine, you may decide to go -2 inches on one header, keep the second the same, and then go to +2 and +4 on the other two cylinders, respectively.  This approach is clear to see on the CB400F pipes.  They look good because they work well.
(http://www.mynode.com/images/2wheel/cb400f/pipeart.jpg)

Collectors
One thing that hasn't yet been mentioned is collectors.  The collector is the section of pipe that joins the primary headers to the secondary headers and the secondary headers to the tail pipe.  The design of the collector can help to promote low-end torque.  The most common type is what's known as the merge collector.  The concept is that each of the header pipes smoothly transition into the collector rather then end abruptly.  This type of collector is best suited toward maximum horsepower, but does little to promote any boost in torque in the lower RPM ranges.  You'll also see these types of collectors in use for turbo manifolds.  In this particular example, a venturi has been added in order to help scavenge gases.  The venturi makes this design slightly more efficient from an inertial stand point.
(http://www.epi-eng.com/images/Engine/exhaust_technology_04.JPG)

Another type of collector is known as a baffle-type.  In this design, each header ends abruptly and is joined by a single cone to the tail pipe (or secondary header).  This design is simple to implement, but suffers for pure racing applications due to some turbulence as the exhaust gases merge. For street use, this is just fine.

Split interference collectors are kind of a halfway house between the two designs listed above.  They're what you usually see in stock collectors.  Each header will end abruptly, but instead of a simple cone for the collector, you get a smoother transition like you would see in a merge collector.

I'll talk a bit more about collector design in a future post, but for now, just be aware that there are different types and they don't all behave equally.  It's not life-or-death differences though, either.

Tailpipe
Selecting the length of the tailpipe is fairly easy.  Though I prefer to use the more complicated method I first listed a dozen paragraphs ago, you can always take the easy way out.  If you've designed a system with secondary headers, simply double the length of the existing system and you'll be at the correct length.  If only primary headers are being used, you'll want to quadruple the length.  Going back to our original example of 15.1 inches for the primary headers, our total exhaust length ends up at 60.4 inches, or just over five feet.  The length of the collector for the tailpipe should be included in this number.

The final diameter for the tailpipe is a fairly simple calculation as well.  First, double the displacement of a single cylinder.  Then divide that number by the primary length, plus three, and then multiplied by 25.  Take the square root and then multiply by two.  Now round to the nearest standard pipe size.  For our example of 189cc per cylinder and primary length of 15.1 inches, the equation should look like this sqrt((189 * 2) / ((15.1 + 3) * 25) * 2.  The result ends up at 1.83 inches which would round to 1.875 inches.

Mufflers
Unless you're making a full on race bike, you're going to want these.  All things being equal (and I mean equal in the performance sense) then open pipes will generally produce the greatest level of horsepower.  On the street, though, you're probably going to want some level of noise suppression.  The drone of open pipes gets pretty old after the first few times.  When selecting a muffler, you're going to want something that provides a decent level of noise control with the minimum amount of flow restriction.  Unfortunately, these values are rarely listed by the manufacturer.  If performance is more of a concern than money, try to stick to name brands with dyno-proven improvements over stock.  Also, the length of the muffler should count toward the overall length of your exhaust system.  Adjust things accordingly.

Tips and Tricks
There are a few little tricks you can pull to help things out even more.  One of those is called "stepped headers".  This is the same theory that is used to help two strokes out with exhaust design.  The concept is that as the header lengthens, you widen the pipe once or twice along it's length.  You only go up a step at a time and only need to do this once or, sometimes, twice.  The first step is usually located between 10 and 14 inches from the head.  Larger diameter header pipes will tend to favor the longer side of the 10-14 range while smaller diameters favor the shorter side.  If a second step is desired, this one is usually located just an inch or two before the first collector.

Also, like the intake tract, the exhaust tract doesn't much care for lazy welds or rough spots.  To keep the gases flowing, you want smooth surfaces with smooth transitions (steps, aside).  Welds, as much as possible, should be ground down on the inside, especially around the flanges and the head where gas velocities are the highest.

Next, the header pipes should exit from the head in a relatively straight manner without too much of a bend.

Finally, if your design necessitates headers that are of a smaller ID than your exhaust ports, try to have the headers flared to match the exhaust port diameter.  The ideal situation is one where the headers are of larger diameter than the ports, but if that ideal cannot be met, at least aim for the same diameter.

The Math
OK... I said I'd get to this part...  This is how I calculated out the 866cc of exhaust gases per cylinder, per period.  The reason I prefer this calculation over the simpler one is that it takes less for granted (or, at the very least, includes some extra variables which can be tweaked).

If you recall, the majority of the expansion of gases in a cylinder is due to the heat created by the combustion of the gasoline.  We can take advantage of this fact by using the ideal gas law to figure out how much expansion the gases have undergone.  All we need to know is the temperature of the mixture before the combustion event and the temperature of the gases after the combustion event.  For these two values, I am going to assume 72°F and 1800°F.  These numbers, of course, can be accurately measured with the correct equipment and I encourage that approach.  When these temperatures are converted to the Rankine scale, the quotient between the two becomes the factor of expansion.  For instance, 72°F is 531.67 Rankine and 1800°F is 2259.67 Rankine.  2259.67 / 531.67 = 4.25.  So the gases will expand 4.25 times due to heat, alone.

Though that number alone is probably a decent facsimile, lets get a bit more accurate and also account for the expansion in volume due to the combustion of gasoline (liquid to a gas).  For gasoline, we'll use C8H18 for its chemical representation and this gives us a molecular weight of 114.  Stoichiometry tells us that 12.5 moles of Oxygen will be required to combust every 114 grams of gasoline and will result in an output of 8CO2 + 9H2O.  Equal moles of gas are also equal in volume and so we know that the expansion in volume of the gasoline is going to come in at a factor of (8+9)/12.5 = 1.36.  However, the presence of Oxygen in the atmosphere is around 21% and so only 21% of our intake gas is affected by this calculation.  The final expansion in volume due to combustion can be calculated as (100 - 21) + (21*1.36) = 107.56%.

Going back to our rate of expansion due to temperatures, we can multiply that by 1.0756 to include the expansion due to combustion and multiply again by the displacement in our cylinder to get the final volume of the exhaust gases.  4.25 * 1.0756 * 189.58cc = 866.63cc.

Materials
One final thing before I finish this up.  Use stainless steel wherever possible.  A lot of the exhaust gas velocity is dependent upon the volume of the exhaust gases.  As we know, as the gases cool, the volume decrease.  This decreases pressure which decreases velocity.  In order to help keep the exhaust gases hot, stainless steel is the material of choice when building exhaust systems.  Other metals tend to be better conductors of heat and so stainless steel gets the nod in this application.

If, you find, during your sizing calculations that you regularly had to round up when choosing standard pipe size, the application of fiberglass header wrap can also net you a few extra ponies, especially toward the upper end of the RPM band.  If you've routinely had to round down on sizes, you may be better off skipping the header wrap (unless you're trying to shield something from the heat).

Conclusion
Despite the length of this post, I'm hoping that the details behind a decent exhaust system seem a bit easier to comprehend.  Like everything else, it's not black magic.  Selecting the correct lengths and sizes of your tubing is 90% of the battle.  Quality welds and smooth bends are the rest.

If you have access to a dyno, experiment with different lengths and sizes.  Pretty much everything listed above is just rule of thumb kind of stuff.  It'll get you into the ball park, but the home runs are only hit after empirical data is collected.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Nov 07, 2012, 15:33:26
Nice write up Matt.

Collectors are interesting.  In order for them to work the way they are described in all the books, is that they have to end in an abrupt change in section.  Merge collectors do the opposite.  They try to make it a smooth transition. A sharp change produces the wave effects that Matt described but flow is not smooth.  Merge collectors have much better flow but generate much weaker pulses.

Current pipe designs favor flow over pulses and F1 uses merge collectors.

When calculating tuned lengths you must remember that the waves travel at the speed of sound and gas travels much slower.  We use the speed of sound in those calculations and that is effected by density and temperature of the molecules in the pipe, so it changes.  And we use gas velocity when working with the pipe sizes.

Anti reversion headers are interesting in that the flow out is primarily on the roof of the port down to the center of the pipe.  Reversion typically is on the lower side where there is less outward flow. The perfect shape is a D shaped port with raised floor.  A simple step simply restricts flow at all speeds and typically hurts top end power.  On a street bike that isn't always a bad compromise.

Jim Fueling had a patent (4,206,600) in 1980 with that simple design that Matt shared with us.  I have an article here that shows HP gains across the range with a special exhaust using aspects of that design. It was pretty clever for the time.  Subsequent development by guys like David Vizard and AG Bell took that one stage further and came up with the D shaped port which is more common now.

Stepped headers attempt to achieve a similar effect and crudely mimic a 2 stroke tapered header.  You will also see tapered sections in some MotoGP pipes and some Akropovic and Yosh pipes.  The idea there is to create a long duration, low amplitude negative pressure wave to assist in evacuating the cylinder.

In general, any sharp change in section creates a pressure wave and disrupts flow. The trick is to harness one while minimizing the harmful effects of the other.

Stainless is nice, but hard to weld and costs more. Mild steel is cheap and easy to weld but rusts.  Zinc coated steel lasts longer but generates toxic gases when welded, so be careful.  I only use mild steel and a can of VHT paint.

See the way that collector above flares out after the outlet?  That allows the gas to flow correctly.  Parallel pipe there hinders flow there somewhat.  merge angles also have an effect on gas flow and wave generation. Lost of stuff to think about if you want every last 0.1hp out of your project.




Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Nov 09, 2012, 19:13:55
  I only use mild steel and a can of VHT paint.
 merge angles also have an effect on gas flow and wave generation. Lost of stuff to think about if you want every last 0.1hp out of your project.

I use mild steel, it's a lot cheaper.
Isn't the optimum angle 14 degrees? (included angle)
I remember reading about it on a NACA or NASA site years ago
 Last time I checked, SERDI 3 angle cutters were around $90.00 a pop, the tool to mount onto head (hand version) was around $1,500.00
Of course, it's cheap compared to industrial version ($2~$300,000, but it does 4~8 valve seats in one cut)
 BTW, like your math on the carb sizing for 360, I had hell of a time getting people to listen when I said 30mm instead of 32mm Mikuni's, I knew they didn't want the truth that 28mm was optimum on 350/360  ;D
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Corsair on Nov 09, 2012, 19:39:45
I am well and truly impressed! Very nice write up, clear and easy to understand. I knew a lot of what you wrote due to my experience with other IC engines (cars...Gasp!) and how you put it, would make a fine teaching manual. Ever think about doing that (putting this in book form)?

Nice job!
Robb
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Nov 09, 2012, 19:48:36
Isn't the optimum angle 14 degrees? (included angle)

I think it depends on the type of collector.  If I remember correctly, the merge style uses a sharper angle than the baffle type.  Somewhere around 10° (plus a few for merge, minus a few for baffle), I think?

Quote from: crazypj
BTW, like your math on the carb sizing for 360, I had hell of a time getting people to listen when I said 30mm instead of 32mm Mikuni's, I knew they didn't want the truth that 28mm was optimum on 350/360  ;D

Thanks.  :)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Nov 09, 2012, 19:53:20
I am well and truly impressed! Very nice write up, clear and easy to understand. I knew a lot of what you wrote due to my experience with other IC engines (cars...Gasp!) and how you put it, would make a fine teaching manual. Ever think about doing that (putting this in book form)?

Nice job!
Robb

To be perfectly honest, I don't think I know quite enough to to write a book.  Believe it or not, a lot of the info here is the "basic" stuff.  It's a quick summary to help people understand what's going on.

If you're interested in reading about these things in more detail, I highly suggest any of the performance tuning handbooks by A.G. Bell.  William Denish does a very good book about tuning HDs, but the same details apply to other engines, more or less.

Also... a little off topic as to what we've been discussing so far, but Corky Bell wrote a book called, "Maximum Boost".  It's all about turbocharging and it was the first book I ever picked up that had to do with engines.  I got into bikes and engines and such because I was interested in how turbos worked.  So I read that book and kind of worked my way backwards...
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: BLSully on Nov 10, 2012, 10:07:35
+1 for Maximum Boost. Even if you don't plan on doing a Forced Induction  setup, Corky explains lots of complicated stuff in a relatively easy to read format.

Sent from my Galaxy Nexus using Tapatalk 2

Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Nov 10, 2012, 11:10:09
SPD claim that their 12 and 15 degree collector angles make most power, so I suspect that the 14 degree figure is probably correct. The larger the collector angle, the larger the flare angle needs to be in the transition.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: gregajo on Nov 13, 2012, 15:56:09
Quote
To be perfectly honest, I don't think I know quite enough to to write a book.  Believe it or not, a lot of the info here is the "basic" stuff.  It's a quick summary to help people understand what's going on.

If you're interested in reading about these things in more detail, I highly suggest any of the performance tuning handbooks by A.G. Bell.  William Denish does a very good book about tuning HDs, but the same details apply to other engines, more or less.

Hi, thanks for your posts and the reference of the book by Bell.  I will definately check it out.

I have been wondering about where to get information to learn how to "do the tune".  The concept of "tuning" seems to cover a great deal of different aspects of a motorvehicle and a motorcycle in particular; everything from engine internals to apsects of intake and exhaust to frame design and suspension.  It seems that "tuning" is both a science and an art and as such appears to be something that some people grow to understand well while others struggle with it.  I know that a great deal of this information exists on this forum site as well as others out there.  Maybe there is a way to put some of the expert advice and vast experience that exists among this forums members into some kind of book?  It would be an interesting (and challenging) project. 
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Nov 13, 2012, 16:18:19
The key to understanding tuning is to keep in mind that engines are a compromise - or a series of compromises.  What you have to do is to define your objective for the project and then understand the starting point and develop a plan on how to get from A to Z.

So you want more power eh?  How much more?  Do you want top end or all the way through the rev range stump pulling power? how large is your budget? Do you have access to a good machine shop?  Are parts available?  Is there enough metal in the right places to make your vision possible? and so it goes on.


No point starting with rusted solid CB250 and set an objective of 100HP for example. Or even 50 or 40 or maybe even 30. As you start down the path you have to continuously ask where you are going.

You may for example want to just clean things up and blueprint teh motor to get teh best out of what teh manufacturer designed.

Or you may want to add more pulling power or more top end. More torque needs more efficiency and larger bores are an easy way to go, as is higher compression.  Cams help to add back top end, so does porting.

Some engines respond well to changes and others not so much.  It's an ongoing process of development to get a bike to satisfy your needs and like any good relationship that takes work and time and understanding.

The hardest part is making sense of all the info out there - some good and much of it bad or not relevant.

Get a copy of the AG Bell book and read it cover to cover. repeat and then start reading everything you can about your bike and what other people did and see if you work out what worked and what didn't and pretty soon, you'll have a plan. It will change with time, but you will have a plan. 
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Nov 13, 2012, 17:39:55
If you have the later CB250 (same engine as CB360 but smaller bore) it's possible to take them out to almost 400cc (I have a CB390  ;D )
It is a hell of a lot of work though
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Nov 16, 2012, 21:55:21
Applications - Forced Induction
OK... so we're going to diverge a bit.  I picked up this baby earlier in the week:
(http://www.dotheton.com/forum/index.php?action=dlattach;topic=15707.0;attach=67174;image)

Understandably, I currently have boost on the brain, and so I want to write a bit about forced induction.  When you gotta write, you gotta write...

Forced induction, for those unfamiliar with it, is the addition of compressed air into the engine in order to raise volumetric efficiency.  By introducing air at a pressure level that is higher than atmospheric, we can add more fuel, which gives us more power.  This takes one (or more) of three different forms:  Turbocharging, Supercharging, and Nitrous Oxide.

There may be no replacement for displacement, but volumetric efficiency isn't a bad substitute in my opinion.  Dollar for dollar, you're unlikely to find another modification that generates as much horsepower as forced induction.  That's not to imply this is a cheap prospect though, only that it has high value when planned and executed well.

One of the other things I really like about forced induction is that its a bit of a "game changer".  Many of the rules and best practices we've been covering over the past few pages don't apply.  Well... let me clarify...  They still apply, but the priorities change.  In theory, if you double the amount of air your engine can ingest, you can also double your horsepower (this never works in practice, but stick with me).  So are you really going to worry about intake lengths to get an extra couple of percentage points at a specific RPM when it's possible just to up the boost and generate even more?  Probably not.  I'm not saying that boost is king and should be the only consideration, but there's no denying that it will change your approach to your build.  Think of it as a reshuffling of priorities.  Boost can mask all manner of ill designs, but it shouldn't be thought of in that way.  As with all engine modifications, they work best with a decent foundation and that foundation often means following the principles we've laid down so far.

Finally, the feel of a forced induction engine is something that everyone should experience at least a few times.  It has a very non-linear torque curve and that's damn good for generating smiles.

For the purposes of this post, I'll be sticking to turbochargers.  Superchargers are similar in operation in that they add compressed air into the intake, but how they get the energy to compress the air does differ.  Turbochargers rely on compressed exhaust gases to spin a turbine that then compresses the intake air (at a slight loss in efficiency).  Rather using the exhaust gases to spin a turbine, superchargers use power from the engine (usually a belt that's run directly from the crankshaft) to spin the turbine.  Efficiency wise, turbos hold the edge.  Superchargers are often easier to fit and implement, however.  They also don't usually suffer from lag (I'll get to lag in a bit).  For the rest of this post, it's safe to assume that the information applies to both turbos and superchargers, except where exhaust stuff is concerned.  I'll cover nitrous oxide in a future post as the way it operates is much different than turbos or superchargers and it comes with its own set of concerns.  One of the nice things about NO2 is that you can use it in conjunction with turbos and superchargers, but we'll cross that bridge when the time comes...

Selecting a Compressor
OK... so one of the first considerations in a turbo build is the size of the turbo, itself.  There are about a dozen different variables that affect a turbo's operation and often you have control over many of them.  Turbos can often have the same housing size but then the turbine and compressors will be sized differently.  Even with similarly sized turbines you get things like trim and diameters for the inducer and exducer.  I'm not going to go into those things in a lot of detail, but just be aware that you often have some measure in customizing a turbo to fit your needs.

What I will talk about is called a "compressor map".  This map displays the efficiency of a given turbo (and all its related variables) in producing a certain level of boost for a certain mass of air.  Depending on your engine displacement, the amount of air your engine desires can be measured.  For instance, a three liter engine will use more air than a one liter engine, regardless of the specs of the turbo.  However, it's theoretically possible to boost the intake pressures on the one liter engine so that it's using the same amount of air that the three liter engine would normally ingest.  The ability of the turbo to produce this level of boost is what's measured in a compressor map.

Before we go too far down this road, we need to know how much air our engines use before the turbo comes into the equation.  The first step is to figure out how much volume of air your engine is using.  It may be handy to get this into an Excel sheet (or similar) because you'll need to tweak this number and recalculate a bit later on.  Luckily this is fairly simple.  First, take your redline RPM value and divide by two (for two strokes, just use redline).  Multiply this value by the displacement of your engine and then convert to cubic feet.  For my engine we have 6500 RPM * 378cc * .00003531466672 (this is the conversion from ccs to cubic feet) = 86.76 cubic feet per minute.

Now that we have volume, we need to calculate mass.  As you may recall, temperature affects volume (more specifically, density) and so in order to find mass, we need to know temperature.  Fortunately, we can go back to the Ideal Gas Law to figure out all this stuff.  The equation for the IGL is P*V = n*R*T.  P is the absolute pressure (14.7 PSI at sea level), V is the volume of the gas, n is the number of moles of the gas, and T is the absolute temperature in Rankine. Finally, R is the gas constant and will can use a basic value of 10.73 for this (NOTE:  This constant changes depending on the units.  10.73 is for cubic feet PSI).  Through basic algebra, we can find the value of any of the variables so long as we know the other three.

For calculating mass, we rearrange the equation so that n = (P*V*29)/(R*T).  The "29" in this equation is the molar weight of air and it used for clearing some of the units.  By plugging in the variables our equation now becomes n = (14.7*86.76*29)/(10.73*531.67).  The resulting value gives us 6.48 pounds of air used per minute at sea level and 72°F.  This is purely academic, but it's an interesting (to me, at least) fact that pounds are only interchangeable with kilograms on earth.  Pounds are actually a measure of force whereas kilograms are a measure of mass.  Mass stays the same regardless of gravity, but pounds do not.

Anyway... back to compressor maps.  We now know that at redline, and in an ideal world, my 360 will be consuming 6.48 pounds of air every minute.  This calculation doesn't take into account volumetric efficiency, however.  Most engine will not be sucking in their full displacement's worth of air on every intake stroke.  Volumetric efficiency for most machines tends to hover around 82% for single valve engines, 85% for multivalve, 90% for "built" engines, and between 95% and 100% for full race spec.  Taking the 6.48 value and multiplying by .9 gives us a more realistic value of 5.84 pound per minute.

Now this is a value with which we can work.  And by "work", I really mean "apply more math".  The process is to now take our "actual pounds per minute" and convert this to a "corrected pounds per minute".  The purpose in correcting the air flow is to take into account the change in the temperature and pressure of the air as it passes through the turbo.  In this case, we're going to assume a slight vacuum of -.5 PSI (mostly due to the expected air filter over the turbo inlet) and an ambient air temp of 72°F.  The formula for corrected flow is cf = ((actual flow)*(air temp in rankine / 545)^.5)/(absolute pressure in PSI)/13.949).  The 545 value is a constant used for correction to standard temperature (85°F) and the 13.949 value is for correction to standard pressure.  Following the math through we now have a corrected air flow of 5.67 pounds per minute.  NOW we can do something with this number.  Take the compressor map for our turbo and now draw a vertical line at the point on the X axis at which our pounds per minute value corresponds:
(https://scontent.fsnc1-5.fna.fbcdn.net/v/t1.0-9/402460_10151325100975159_1093795403_n.jpg?oh=89f999b896a9dd50ad412a02a051ecdd&oe=58A6F009)

Plotting for our cross line from the Y axis is the simple part.  The pressure ratio can be calculated using this formula:  PR = ((desired PSI + 14.7)/(initial PSI + 14.7).  For these purposes I'm going to set my desired PSI at 10.  Note that this is PSI measured at the turbo outlet, NOT measured at the intake manifold.  In reality, you lose pressure as the compressed air makes its way along the twists and turns of the intake tract.  I like to assume a 3 PSI loss for intake inefficiencies and so boost at the manifold is likely to be around 7 PSI.  The "initial PSI + 14.7" portion is exactly the same value as we used in the corrected air flow pressures and so we'll use 14.2 (assume -.5 PSI for air filter, etc).  The pressure ratio we have is 1.74.  Graph this line on our compressor map, and we have the following:
(https://scontent.fsnc1-5.fna.fbcdn.net/v/t1.0-9/12687_10151325119175159_1032840229_n.jpg?oh=90d6c42e6a9288e6f96b86f0a5412bed&oe=58692984)

So for our purposes, it looks like our turbo is operating at the 69% efficiency range.  This is pretty decent, but it's only half of the story.  It's where we're at a redline and we'd really like to know where we are in the rest of the RPM band.  The first step is to trace our way to the left from the intersection of our two lines until we get as far left as possible on the map.  This far left line is called the surge limit and the turbo is incapable of operating in a predicable manner if the pounds per minute drop any further.  This second vertical line we're drawing is the pounds of air per minute necessary to hit our desired level of boost.
(https://scontent.fsnc1-5.fna.fbcdn.net/v/l/t1.0-9/382008_10151325135265159_912215843_n.jpg?oh=548ed0e99807780c62fd3179364d5ab1&oe=586CED2E)

This value corresponds to 3.32 pounds per minute.  Working backwards through all of our earlier math we can use this to calculate the RPM at which full boost becomes available.  I'll spare you the boredom of that task; the result is simply 7,600 RPM.  So... we know already that our turbo going to come on late, which isn't too bad for a bike but would probably give some drivability problems on a car.  Now that we know when max boost comes in, we need to figure out when initial spool up starts to take effect.  Spool up is when the turbo starts making usable boost, but isn't yet at the boost limit.  This is also the point at which your smile starts to form as you roll on the throttle.  For this portion, we take our maximum mass air flow and multiply it by .2 (this is a rough calculation, but it's good enough for this step).  This gives us a value of 1.13 pounds per minute.  This is also the point at which we can start drawing a line from the X axis to the point of maximum boost (3.32 pounds per minute).  We'll call this the spool threshold.  Finally, draw a vertical line from the point at which our spool threshold intersects the 1.2 pressure ratio (1.2 pressure ratio is often considered the minimum amount of usable boost.  Any less and it's not likely to give any real performance benefit). Our graph now looks like this:
(https://scontent.fsnc1-5.fna.fbcdn.net/v/t1.0-9/533558_10151325153725159_1652696389_n.jpg?oh=36f2332e05750ddf72af34adfac48739&oe=58662360)

We can see two things right away from our most recent graphical additions.  First, the boost threshold line stays within the islands of the compressor map.  This is VERY important.  Any significant drifts to the left or right of the islands means you have a turbo of the incorrect size.  Not only will the turbo fail to perform well, it's also possible to damage your engine and/or turbo through surging and added heat.

Second, our new vertical line tells us the point at which usable boost comes in.  This corresponds to 1.74 pounds per minute.  Again, working backwards though all the math (I hope you have an Excel sheet open) we come to an RPM value of 3,600, which is damn near perfect.  In an ideal world, this value will be 33% of your redline.  Since I'm redlining at 11,000 RPM, 33% of this would be 3,630.

Finally, in a perfect world, I'd probably select a turbo even slightly smaller than this.  You can see much of the graphed line falls in the white islands of the map.  These areas are usable, but are of lower efficiency.  To shift our graph to the right, we need more displacement, better volumetric efficiency, or a smaller turbo.  Selecting a lower level of boost would also work, but we do sacrifice power if we go down that route.

OK.  We've now verified that the selected turbo is a good match for our engine.  If it's not, we need to go shopping for turbo parts.  Consult a professional to see what can be changed in order to get the characteristics you need.  Also, remember, that all of these calculations have been dependent upon temperature and pressure.  BOTH of these things change from day-to-day (but not drastically), so this has only been a rough guide to see if our selection was correct.  Ideally, you want to recalculate based on your altitude, etc.

Plumbing
This section is going to be fairly short because most of the rules we've already covered in previous posts still apply.  There are a few considerations, however.

First, is the inclusion of an air plenum into our intake.  The purpose of the plenum is to help stabilize the air pressure so that the turbo can operate in a smoother fashion.  Basically, a plenum is just a way to expand the volume of the intake tract so that when your intake valve opens and starts letting air into the cylinder, the pressure within the intake tract doesn't drop too much.  Without a plenum, you're losing power and causing the turbo to change turbine speeds as the pressure fluctuates.  Not good.  The rule of thumb is that for four or more cylinders, your plenum should be at least as much volume as your engine's displacement.  Double the size if you're running two or three cylinders.  Quadruple it if you're running a single.  This value is the minimum size of the plenum.  The maximum is simply the minimum times 1.5.  The reason to have a maximum is because this is all volume which the turbo needs to fill.  More volume means a longer fill time which means that your turbo is less responsive to throttle changes.  This delay between the time you crack open the throttle and the time boost starts being generated (assuming you're within the usable RPM range) is called "lag".  It's not a good thing and it's not fun.  Keep it to a minimum.

Secondly, the intake tract would do well to include an intercooler.  As we've discussed before, a cool intake charge is a dense intake charge.  Increased density of air means more power.  So, intercooler = more power.  Intercoolers are generally rated by efficiency and that takes into account both the flow restriction caused by the intercooler (no way around this, but do try to minimize it) and its ability to shed heat in relation to the ambient air temperature.  Aim for 70% or better from your intercooler, with an emphasis on flow over cooling ability.

The last main plumbing consideration is the exhaust plumbing.  This is a real game changer as far as designs go.  There are several considerations when designing an exhaust for your turbo.  First, make it short and small.  This area needs to be pressurized before the turbo will spool.  Having a long exhaust with a large diameter pipes makes this process take longer and contributes to turbo lag.  Second, if possible, adjust the length of the headers so that the exhaust pulses reach the turbo in an even and measured fashion.  This mainly applies to 180° twins and all of the triples.  You may need one of the pipes to be longer/shorter than the other in order to facilitate this.  In my own design, the right cylinder's exhaust header will be about twice the length of the left in order to time the pulses appropriately.

Finally, the tail pipe after the turbo should be free flowing as possible.  We're a lot less concerned with things like inertial tuning and acoustic tuning when it comes to turbos... It's all about flow.  It's not uncommon to see just a short, open pipe to channel the exhaust away from heat-sensitive components.

Fueling
OK... when it comes to fueling you basically have three options.

The first and, by far, best option is electronic fuel injection.  Carbs are just not as precise as an EFI system, especially for things like turbos.  Chances are, not a lot of us will go down this road because of the cost and difficulty in implementing them on our old bikes and so I won't really cover it.

Option number two is a draw through carb system.  This means a single carb handles all the fueling and the turbo sits in between the carb and the engine.  The turbo sucks the air through the carb and the fuel is delivered to the cylinders via a splitting-type manifold.  This is probably the easier carb method to implement, but you lose a lot to gain that ease.  First, forget about using an intercooler or plenum.  The fuel will pool in these components and lead to undesirable side effects (such as your engine exploding) .  Secondly, the transitions between the throttle positions tend to be more extreme and this can lead to uneven running.  Draw through systems are very hard to tune.  Finally, having all that air flow through a single carb can cause icing.  As the fuel evaporates, it causes the carb to cool.  Often this cooling can be severe enough in order to freeze the moisture in the air.  Stuck throttles are not fun.  I don't recommend option number two.

Finally, option number three is a blow through system.  This allows us to continue to make use of individual throttle bodies (e.g. carbs) for each cylinder and also allows for the inclusion of an air plenum and intercooler into our design.  This choice does require some changes, however.

The first of these changes is a fuel pump.  Your fuel pump must be capable of delivering at your maximum boost + 5 PSI.  In my case of 7PSI, I need a fuel pump that can put out at least 12PSI.  In addition, you'll need something called a rising rate fuel regulator.  The job of this part is to ensure that the fuel is delivered at a constant pressure in relation to the air pressure.  This regulator will sense the pressure in the air plenum and always ensure the fuel pressure is a bit higher (you can set this level and I'm going to opt for +3PSI over sensed air pressure).  Without a regulator and fuel pump in place, the boost from the turbo will push the fuel right back up the lines and into the tank.

The next changes may include modifications to the carbs themselves.  All carbs will require pressure lines running from the plenum to the float bowls.  This ensures a strong signal across the venturi and keeps the fuel pressurized at the same ratio it would normally see without the turbo present.  Generally these air lines are run directly from the carb overflow tubes, which would have to be plugged, otherwise.  Also, some CV carbs may require modification so that the area underneath the diaphragm is sensing the appropriate level of boost.  This often involves drilling and more air lines run from the plenum.  Get familiar with the operations of your carbs before making any serious changes.  Finally, there's the jetting.  The nice thing about a turbo is that boost usually only comes on after 3/4 throttle.  This means 90% of your tuning is just going to be selecting the correct size main jet.  Under boost you're going to want to keep your air/fuel ratio at around 12:1.  Either invest in a wideband O2 sensor or some dyno time (or maybe even both).  Get the fueling wrong and you're going to be in for some expensive repairs.

Other Considerations
There are a few other things you're going to want to review before making any significant changes to your bike's operations. 
The first thing is to look at your cam...  Turbos prefer high lift and shorter duration.  The idea is to open the door to the cylinder as much as possible and allow the higher pressures to fill it.  You're less reliant on intake velocities and now more reliant on intake pressures.  Don't hold the door open too long or all of your pressure comes right back out (or even goes out the exhaust without being combusted).  The stock cam isn't a bad choice for a turbo engine.

Also, take a look at compression and timing.  Turbos will add a lot of heat to an engine and so we need to be aware that other modifications that also add heat don't stack up.  I'd recommend not exceeding more than 10:1 static compression and even that's a bit on the high end.  I tend to be conservative when it comes to boost, though.  I don't see a point in killing a bike's low RPM operations just to generate higher numbers in a range at which I rarely ride.  Keep your compression at at least 8:1.  For timing, less advance will be necessary because the pressurized intake charge is now more dense.  The more dense the charge, the faster it burns.  The faster it burns, the sooner peak pressures are generated.  So in order to take this into account, we dial back the advance.  The rule of thumb here is 8° of retard for every 15 PSI of boost.  If you have an electronic ignition, these can often be linked with pressure sensors to handle the timing automatically.

Conclusion
Well... this post was longer than I intended and I even covered less than I hoped.  Maybe I'll expand on it at some point...

Just one closing thought before I sign off for the day...  You can calculate the rough increase in power using the following formula:

Final power = initial power * (1 + (boost/14.7 * turbo efficiency))

So for my 360 this becomes:
Final power = 38 (guesstimate after dropping the compression to 9:1) * (1 + (7/14.7 * .69))
Final power = 50.48 horsepower (this doesn't take into account the inclusion of an intercooler, which would also give a bump of around 21% to the efficiency of the turbo, giving a different possible value of around 55 horse power)

To put that 55 horsepower number into perspective, that's 387 horsepower per ton.  The Lamborghini Gallardo Superleggera comes in at 344 hp/ton...
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Nov 17, 2012, 01:42:48
I have to read through a few times, interesting stuff
 I have to get 360's done so I can get back to XS700, 800 and 860.
700 is getting CX500 turbo (low compression pistons, around 6.4:1)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sideswipe on Dec 09, 2012, 01:32:00
Hey mate really loving this article especially the mathematical proofs behind it (like working out how long my intake should be and such) really will be a brilliant resource when i start planning to beef up the CB900

love it mate keep up the good work.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: stroker crazy on Feb 10, 2013, 06:21:54
Sonreir, I know this is a bit of a step back in this thread, but I've just read and re-read your post on exhaust design and I would like to ask a few questions.

I am in the process of improving the breathing of the SkyTeam 'Áce': http://www.dotheton.com/forum/index.php?topic=43357.0 (http://www.dotheton.com/forum/index.php?topic=43357.0)
The intake side is fairly easy to deal with but the exhaust appears to be severely restrictive.  After getting my head around converting your mathematics into the metric system I think I now have some understanding of general principals.
I have only worked with two-stroke bikes for the last thirty years so items like 2cat and EGR came as something of a surprise!
Calculations indicate quite a long pipe so I was wondering if it would work if the pipe diverged in two after the header, and shorter secondary tubes were used?  This would also allow the use of two minimally unrestrictive mufflers, hopefully making the resulting noise less obvious.
Incidental to this, I was wondering if there was any appreciable gain to be had by blanking of the (external) EGR?

Would much appreciate your input on this.

Crazy

Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: stroker crazy on Feb 10, 2013, 06:49:56
Ignore the previous post - its lateish and I've had too many beers!
Obviously splitting the pipe increases its effective diameter, not the length.

I think I should go to sleep now!

Crazy
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Feb 11, 2013, 19:58:00
Crazy, the issue is that most times the calculate intake and exhaust lengths are too long, so we use 1/2 or 1/4 of the calculated length to take advantage of second, third or even 4th order reflections.  In the real world, those lengths are only "ideal" for a certain rev point plus or minus a bit., so I wouldn't sweat it.

You can use the same calculations as a 2 stroke but arranges slightly differently to allow for different timing of events.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: stroker crazy on Feb 12, 2013, 05:58:49
teazer, my current intentions for the 'Ace' exhaust are these:

Cut the original pipe at start of CAT and at silencer
Weld in new straight section about 6 inches longer than original (I will probably be shunned for going against the trend for short pipes!)
De-restrict silencer internals
Make a combined clamp/mount to join silencer to pipe and attach both to bike
Make a strut to connect rear of silencer to frame under seat hump

Might do a Photochop later to illustrate the result.

Thanks for the inspiration provided by this thread,

Crazy
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Feb 12, 2013, 13:37:28
I like that plan.  Pipes that are too long peak early but that extra length doesn't hurt too much unless they are excessively long.  So a long pipe means potentially slightly more low down, slightly less up top but the same rev range as before.  It should (hopefully) tip the power curve a little to make for more fun on the street.

That all assumes that there are no obvious restrictors built into the head or pipe that also need to be addressed.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Texasstar on Mar 01, 2013, 10:49:13
It's an optimization exercise and not a maximization trip.



I will be passing that wisdom to my son Teazer. I can't believe this doesn't have more views! Y'all are great!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: eklimek on Jun 01, 2013, 12:15:03
Thank you.

Ed
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: cyclefreak on Jun 01, 2013, 14:15:02
Yep, love this thread!



Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Texasstar on Jun 04, 2013, 11:01:22
 Sonreir what about application streamlining? Going fast? What fairings have worked best for different sized bike historically, race application, etc?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jun 04, 2013, 11:25:01
The 'best' streamlining doesn't usually look so good, Hayabusa is very efficient but makes bike look like a tank.
Yamaha R1 is far less efficient but 'looks' faster
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jun 04, 2013, 11:38:21
For ideal streamlining, look no further than mother nature.  A fluid of higher viscosity passing through a fluid of a lower viscosity at a high rate of speed will take a very familiar form:

(http://www.clipartpal.com/_thumbs/pd/weather/water_drop.png)

The reason for this shape is that this is the path of least resistance.  If the wind resistance were to increase any further, the rain drop would be forced to change shape and/or break apart.

Chances are, the only rain-drop shaped fairing you're going to get is one to make yourself.  For a bike, you're looking for a full fairing that covers as many exposed parts as possible (including the rider).  It should come to a taper at the back and at no point should there be any jagged edged or exposed machinery.

As you can probably imagine by now, that's not likely to be possible.

For flat-out top speed, you want something looking a bit like this:
(http://www.bikeexif.com/wp-content/uploads/2010/04/speedweek-3.jpg)

You can see that the rider is fully encased by the fairing and the fairing comes to a point in the rear.  Though you can't see the front side, you can bet it's as circular as possible (including around the forks and front tire).  There's likely to be an air inlet for the engine, but that's about it.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Texasstar on Jun 04, 2013, 11:56:58
Airtech says of the 3 the VsF1 is the best and it looks like your water drop.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jun 04, 2013, 12:02:58
I'd pick the TD32, personally.  Obviously, I've never taken any of them into a wind tunnel or done any other sort of empirical tests, but I like how it has mountings for a windshield which will help the rider get down into the "bubble".  A clever father/son team with some fiberglassing materials could probably modify it to close up the bottom gap.  ;)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: cyclefreak on Jun 04, 2013, 12:54:35
Aerodynamics is nothing new. Back in the day going fast was much more important than "looking" fast like many bikes today.(http://img.tapatalk.com/d/13/06/04/e8y3aqaz.jpg)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jun 04, 2013, 13:52:10
I'd pick the TD32, personally.  Obviously, I've never taken any of them into a wind tunnel or done any other sort of empirical tests, but I like how it has mountings for a windshield which will help the rider get down into the "bubble".  A clever father/son team with some fiberglassing materials could probably modify it to close up the bottom gap.  ;)

 By the look of things the TD32 comes with pieces to infill the center section
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Texasstar on Jun 05, 2013, 08:30:23
Oh We love those Fibonacci curves are so beautiful...we don't give a rats ass how it looks we want to go fast! If we want Lucky to look good we will take off her bra and take her for a romp in the country.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Jun 05, 2013, 18:45:50

For flat-out top speed, you want something looking a bit like this:
(http://www.bikeexif.com/wp-content/uploads/2010/04/speedweek-3.jpg)
.

That would be Brett deStoop and his 232MPH GT750 Suzuki triple two stroke.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Texasstar on Jun 06, 2013, 10:13:14
Kop made some calls to airtech for us.


I got an email from Dutch that said " I spoke to Kent about the bike and we've narrowed it down to 2 choices:

Cool looks + some streamlining: CR931 http://www.airtech-streamlining.com/hondaz/CR931962-63.html
All out high speed attempt: PEEL1 http://www.airtech-streamlining.com/ducati/250350SINGLE.htm

So we love the peel1! All out speed! He also said we need to go with a taller Vesco tail to keep the air on longer.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Texasstar on Jun 06, 2013, 10:14:30
That would be Brett deStoop and his 232MPH GT750 Suzuki triple two stroke.
deStoop and now deZeeuw. Teazer do you have any Dutch blood in ya?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: AgentX on Jun 11, 2013, 00:40:22
Sonreir what about application streamlining? Going fast? What fairings have worked best for different sized bike historically, race application, etc?

To dovetail onto this, when does streamlining come into play, vs the additional weight of the fairings?  I am guessing the streamlining only matters at higher speeds.

For those of us riding at back-road speed on vintage machines, does a fairing really help us compared to the lighter weight of an unfaired bike, vs someone riding at track speeds?

Is a half-fairing or nose bowl a useful compromise in aerodynamics and weight, or just an ergonomic consideration for keeping wind blast off the rider?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Texasstar on Jun 11, 2013, 01:07:18


You can see that the rider is fully encased by the fairing and the fairing comes to a point in the rear.  Though you can't see the front side, you can bet it's as circular as possible (including around the forks and front tire).  There's likely to be an air inlet for the engine, but that's about it.
[/quote]found a pic of the front
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jun 11, 2013, 01:42:37
To dovetail onto this, when does streamlining come into play, vs the additional weight of the fairings?  I am guessing the streamlining only matters at higher speeds.

For those of us riding at back-road speed on vintage machines, does a fairing really help us compared to the lighter weight of an unfaired bike, vs someone riding at track speeds?

Is a half-fairing or nose bowl a useful compromise in aerodynamics and weight, or just an ergonomic consideration for keeping wind blast off the rider?

Yup.  From a performance standpoint, a fairing is a benefit above a certain speed and a detriment below it.

About 40 years ago, a study was conducted by the US government to help determine the speed limits during the oil crisis.  They settled on 55mph because this was the average break-even point for most vehicles on the road at the time.

Though I haven't read much about the topic, I would expect motorcycles to do a little worse due to their poorer coefficient of drag.  Maybe 50mph or so?

Regardless, it's not uncommon to see fairings removed from bikes (especially the tiddlers) for tracks with only few straightaways.  Because motorcycles weigh so little in comparison to cars, any increase in weight represents a greater percentage change than our four-wheeled cousins would see.  Aerodynamics certainly matter for top-speed runs, but weight is still a serious consideration.  When overall performance is being evaluated.

Furthermore, you may find that a fairing reduces your drag enough such that top gear is no longer tall enough.  The most common way to attach this problem is to swap out the final drive to lengthen all the gears.  Well now you've gone and made acceleration worse as well as adding weight from the fairing.  It's definitely a balancing act.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Texasstar on Jun 11, 2013, 07:46:54
Sonreir I have been reading the posts of http://www.landracing.com/forum/index.php/topic,7612.0.html
And they said more time needs to be spent on developing the tail like Bret did on his bike.

However they like other forums have criticized Bret's design because it is sucking him off the bike...so he duct taped his hump on his leathers and fixed the problem. (Love this guy)
"Most of us that have raced LSR bikes have know for some time that the "back end" is what it is all about
When we used the windtunnel back in the early 1970's for the 125c.c. Can-Am effort we saw what a
special long tail did for Aero.  What we did not know at that time was how to get the chassis to work
with the long tail........Brett and Joe got the bikes to handle....they have solved this problem.  Also until resently the rules did not allow for a "long back end"....Thankfully the "officials" have seen the error in their "old rules" and the sky is now the limit for a sit-bike"
The complete string is worth a read...I have attached a pic of Joe Amo's bike.


Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Texasstar on Jun 17, 2013, 13:13:45
Sonreir, I have been reading Phil Irving's book "Tuning for speed" and since e85 was not available at that time I was wondering if you could address how to tune a street motorcycle for increased performance with e85.  There seems to be a dispute about the octane rating of e85.  In Texas they say it is 114 but Wiki says 95.  So if we decide to increase our compression to 12:1 are there any other things we need to do to take advantage of e85?  I guess what I'm trying to say is can you address tuning for different fuels for speed?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Worst cb650 ever on Jun 17, 2013, 13:28:58
I can't speak to the octane, but I do know that e85 has about 30% less power per gallon, so you generally need to flow about 30% more fuel to make the same power as regular gas. 

On forced induction (turbo / supercharged) vehicles, the e85's high octane and cylinder cooling effect allow running higher boost on an otherwise stock vehicle, but larger fuel injectors and a higher capacity fuel pump will still be required. 

I wonder if just running 93 octane pump gas would give you what you are looking for without having to deal with the detriments of e85?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Nebr_Rex on Jun 18, 2013, 01:22:49
 Gas - 14.7/1 ,high btu
 E85. - 9.7/1   ,lower btu
 With the extra fuel being used it's about a 10% increase with the correct air fuel ratios.
As for octain rating I've heard 104.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Worst cb650 ever on Jun 18, 2013, 10:05:56
Good to know, thanks!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: gas_fiend on Jun 21, 2013, 02:12:30
this is a great thread! makes me want to build an engine so badly
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: alex2445 on Jul 14, 2013, 05:28:31
Thanks Sonreir I'm gonna keep this forum in my favorites or something, that way when I wanna rebuild an engine a lot hotter, I can look at this and do it  ;D
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Mydlyfkryzis on Aug 16, 2013, 00:16:49
To dovetail onto this, when does streamlining come into play, vs the additional weight of the fairings?

funny thing, for top speed, weight is not a facor, only streamlining. the Coefficient of drag is way more important then weight.  Weight keeps you from accelerating fast.  But 2 cars of different weight, but the same aero, will have the same top speed. the lighter car will get there a little sooner, but the maximum speed would be the same.

With A heavy, streamlined car, and light, un-streamlined car, the streamlined heavy car will be faster. Top speed is all about co-efficient of drag and frontal area. 

Weight affect acceleration only. Aero affect tops speed. The wind resistance is also a square law. It take 4 times the horsepower to go 2 times as fast.

So a motorcycle that takes 50 horsepower to do a hundred, will need 200 Horsepower to do 200.  That increase is solely wind resistance and a little rolling resistance. How long it takes to get to 200 would be a factor of weight.  To do 400 MPH takes 800 HP.... That's for steady state. to accelerate quickly to that point needs a lot more HP too.   

I didn;t even get into lift  and those forces.



   
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Aug 16, 2013, 01:23:16
And to expand further, horsepower is the other consideration.  So top speed is purely a function of horsepower and drag.

At faster and faster speeds, remember that the drag of the air still must be overcome by friction to the ground (for wheel driven vehicles), so even with an infinite number of ponies, your top speed will be limited by how well your wheels grip.  On the salt, this is a major consideration.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Aug 16, 2013, 03:23:08
Thrust 2 had to use 'solid' wheels with studs, plus aerodynamics that created about 7 tons of down pressure to be able to drive past sound barrier
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: kruck on Oct 02, 2013, 01:46:27
:popcorn:

sent from a pit stop

Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: goose07 on Oct 30, 2013, 22:07:26
Everything you need to know to build the type of engine or bike is right here,yous have covered alot of areas all I can say is, good luck with your builds, its all a learning curve! Trial and error.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DesmoDog on Nov 02, 2013, 01:43:16

Weight affect acceleration only. Aero affect tops speed. The wind resistance is also a square law. It take 4 times the horsepower to go 2 times as fast.

So a motorcycle that takes 50 horsepower to do a hundred, will need 200 Horsepower to do 200.  That increase is solely wind resistance and a little rolling resistance. How long it takes to get to 200 would be a factor of weight.  To do 400 MPH takes 800 HP.... That's for steady state. to accelerate quickly to that point needs a lot more HP too.   

It's worse than that. Wind resistance is proportional to the square of velocity, true. But that's only a force you are dealing with, not power required.

Power is the ratio of work/time.

Work = force x distance, so power required changes with (force x distance)/time.

Wind resistance is the major force you are trying to overcome. It is proportional to the square of velocity so basically you have Power required is proportional to velocity^2 x distance/time.

Distance/time = velocity.

So... power required is proportional to velocity^2 x velocity, or velocity^3.

That means it takes roughly 8 times the power to go twice as fast, assuming wind resistance is the major force you need to overcome.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DesmoDog on Nov 02, 2013, 02:54:08
Caveat #2 - Detonation and Preignition
One of the effects of additional compression is additional heat.  This heat is not only what provides the increase in power, but it can also cause two other issues.  These issues are detonation and preignition.  Both of these problems will destroy a motor in short order (especially preignition) and so neither are acceptable.

Detonation is the spontaneous combustion of the remaining fuel/air mix after the normal combustion process is nearing completion.  This is caused through the heat and pressure initiated during the combustion process and as both heat and pressure rise, it will get to a point that the molecules within the mix are pounding into one another so violently that they ignite, themselves.  Death by detonation usually results in broken rings or ring lands.

Preignition differs from detonation in that it's not so much as a spontaneous combustion of the mixture.  Preignition is a begin to the combustion prior to ignition from the spark plug.  Preignition usually occurs when a part or parts of the combustion chamber heat up too much.  This can be anything from an excess of carbon deposits (not usually an issue on a freshly assembled engine), damage to the exhaust valve, or an overheated spark plug.  What happens in this case is that whatever causes the preignition has heated up to a point where it actually starts the combustion of the fuel/air mix before the spark plug fires.  This causes cylinder pressures to rise too early (sometimes when the piston is still approaching TDC) and so peak cylinder pressures occur too early in the cycle.  This causes greatly increased stress on engine components and will usually kill an engine a lot earlier than detonation will.  Engine failure due to preignition will almost always result from holes in a piston.

Though there are many ways to combat detonation and preignition, those will be saved for a later post.  For now, just be aware they can be potential problems and the most common method of dealing with these issues is to use high octane fuel.  Consider premium gas to be the only acceptable fuel for a high-compression engine.  Better to push the bike home than fill it with regular.  Also, it's important to note that high compression engines are MUCH LESS tolerant to running lean than the stock factory offering.  Most stock engines will run all day long on a lean mix, but a high compression engine at WOT will blow up right in your face as soon as the float bowls start to get even a little shallow.

Caveat #3 - Combustion Speeds and Timing
This particular issue can be hit or miss depending on how you've achieved your increase in compression, but it's unlikely you'll be able to avoid these effects all together.

First up, it should be known that increasing compression increases the combustion speed of the mixture within the cylinder.  This is generally considered a good thing. <snip>

I'll admit I haven't read every word of every post on every page so maybe this got addressed… but I missed it if it got discussed.

In your description of what detonation is, you left out the concept of a delay period. this gets important later so let me ramble a bit here. The mixture in the cylinder is exposed to high temp and pressure. It can only resist this temp and pressure for a given amount of time before it detonates, as in , there isn't a given combination where it explodes, there is a given amount of time for various conditions before it explodes.

A common misconception I see is people thinking that detonation occurs because the flame front moves too fast. this isn't true. Detonation is sudden, everything goes BANG rather than burns. This creates a shock wave that bounces around in the cylinder and makes the knocking noise which identifies the condition. it also knocks the boundary layer off the piston, which removes the insulating properties of the boundary layer and lets the piston heat up, which weakens it which leads to damage. There is no flame front when a cylinder detonates I used to have a text that had pictures of detonation happening. you could see the flame, then BOOM. nothing.

Which was all supposed to tie in to something I saw in another post - that high octane gas burns slower than low octane gas.

My challenge is for someone to post a technical reference, that means not Hot Rod magazine or Wikipedia, etc, that claims there is a significant difference in the flame speeds of high and low octane pump gasoline, or that flame speed is the method used to control detonation.

Here's the thing. Making the gas mixture burn slower would make detonation WORSE, not better. It goes back to the delay period. You want the mix to burn as fast. Hence all the other mods get get "more complete combustion" like dual plugs, swirl in the intake charge, MSD ignitions, that kind of stuff. It's all designed to burn the mix faster. You want to burn it all before the delay period expires and it goes boom. Why would it make sense to make the gas itself burn slower? A higher octane gasoline resists autoignition due to high temp and pressure longer than a lower octane gas does. it gives you more time to burn the mix but does not burn it slower. You want to reduce detonation? Lower the temp, lower the pressure, burn the mix FASTER, or make the gas more resistant to autoignition. 

To address a few misconceptions I see (not necessarily here) Slow flame speeds do not prevent detonation, they make it worse. Fast flame speeds do not cause detonation, they reduce it. Multiple flame fronts coming together do not cause detonation.

Why should anyone believe me? No reason. So here are some sources I found earlier (and posted on another site) concerning this. Read the references, note the sources, and think about it. Make your own decisions.

*************************

I've seen this basic write up on a couple different race fuel supplier sites.
http://www.whitfieldoil.com/www/docs/171.284/vp-racing-fuel- (http://www.whitfieldoil.com/www/docs/171.284/vp-racing-fuel-)
Quote
Octane number is not related to flame (burn) speed either. Variations in octane quality are independent of flame speed. There are some high octane gasolines in the marketplace with fast flame speeds and some with slow flame speeds. It depends on how they are put together. We prefer fast flame speeds because we know that a properly tuned engine will make more power on this type of gasoline than one that has a slower flame speed.

Another link from a fuel supplier
http://www.pinux-products.com/octane-rating/ (http://www.pinux-products.com/octane-rating/)
Quote from: pinux products
It should be noted that octane rating does not relate to the energy content of the fuel (see heating value), nor the speed at which the flame initiated by the spark plug propagates across the cylinder. It is only a measure of the fuel’s resistance to autoignition. It is for this reason that one highly branched form, or isomer, of octane (2,2,4-trimethylpentane) has (by definition) an octane rating of 100, whereas n-octane (see octane), which has a linear arrangement of the 8 carbon atoms, has an octane rating of -10, even though the two fuels have exactly the same chemical formula and virtually identical heating values and flame speeds.

<snip>

Octane rating has no direct impact on the deflagration (burn) of the air/fuel mixture in the combustion chamber. Other properties of gasoline and engine design account for the manner at which deflagration takes place. In other words, the flame speed of a normally ignited mixture is not directly connected to octane rating. Deflagration is the type of combustion that constitutes the normal burn. Detonation is a different type of combustion and this is to be avoided in spark ignited gasoline engines. Octane rating is a measure of detonation resistance, not deflagration characteristics.

Check out Section 6.3 of this university paper:
http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html (http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html)
Quote
The antiknock ability is related to the "autoignition temperature" of the hydrocarbons. Antiknock ability is _not_ substantially related to:-

The energy content of fuel, this should be obvious, as oxygenates have lower energy contents, but high octanes.
The flame speed of the conventionally ignited mixture, this should be evident from the similarities of the two reference hydrocarbons. Although flame speed does play a minor part, there are many other factors that are far more important. ( such as compression ratio, stoichiometry, combustion chamber shape, chemical structure of the fuel, presence of antiknock additives, number and position of spark plugs, turbulence etc.) Flame speed does not correlate with octane.

Sunoco Race Fuels says this:
http://www.racegas.com/article/10 (http://www.racegas.com/article/10)
Quote from: Sunoco
Naturally aspirated race motors with large combustion chambers spinning at high RPMs really like high-octane, fast burning fuels.  They need the octane to prevent uncontrolled combustion, and they need a fast-burning fuel so that the flame front can span the large bore of the combustion chamber quickly.  <snip>You might be surprised to learn that some of the highest octane fuels may also be some of the fastest burning fuels!

Any opinion can be supported by links to internet sites. But it also makes no sense that a slower burning mixture would be better for controlling detonation. Flame fronts, even fast ones, are a controlled burn, detonation is an explosion.
*************

No flames intended here, I've just seen the slow burning gas claim so often and no one ever questions why we would do so many mods to make the mixture burn faster, then put a slow burning mixture in there.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: GTVSaviour on Nov 11, 2013, 10:51:59
Reading through this lot - new to 2wheels so this thread is a massive help. Off to continue reading and printing as I go....... definitely going into the garage with me for those tea drinking (hey - I'm english) moments.

Thanks a million

John
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: calebacm on Nov 11, 2013, 13:24:39
I'm looking at doing some engine and tranny mods to my CB400. Where would be a good place to start?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Nov 11, 2013, 13:32:31
Start by setting some goals.  You need a yardstick for success before you can undertake any work.

What is the reason for the engine and transmission mods?  Better acceleration?  High top speed?  Both?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: calebacm on Nov 11, 2013, 13:40:42
The goal would be a happy medium of both. I'm running stock right now and there's not a lot of low-end power but there's an okay amount of high end. I've taken it to a straight and my top speed before it really could not give me any more was 102. And God was that terrifying. I thought it was going to explode underneath me. So if I could get some more low-end out of it and be able to push it more would be great. I'm in the process of lightening it up too, which I think will help some.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Nov 11, 2013, 13:50:42
Take it a step at a time and do the easy stuff before you do anything that can't be undone or takes a lot of time and money (especially if this is your first time undertaking performance modifications).

Personally, I would these things, in rough order (again, one at a time and not all at once):
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: calebacm on Nov 11, 2013, 14:27:18
Thanks man. I'm definitely starting small. I'll come back with any questions that I have.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: mrlvlagic on Jan 03, 2014, 12:12:18

A café racer without performance enhancements is just a tractor with a body kit.  Do you really want to be one of those kids in a 1.6L Honda Civic with glowing lights under the body panels racing his gutless wonder from stoplight to stoplight?  If the answer is, "yes", you can probably stop reading now.  ;)

(Snip)

That speaks volumes... I think I need to make that one of my prorities
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Jan 04, 2014, 11:22:14
I'll admit I haven't read every word of every post on every page so maybe this got addressed… but I missed it if it got discussed.

In your description of what detonation is, you left out the concept of a delay period. this gets important later so let me ramble a bit here. The mixture in the cylinder is exposed to high temp and pressure. It can only resist this temp and pressure for a given amount of time before it detonates, as in , there isn't a given combination where it explodes, there is a given amount of time for various conditions before it explodes.

A common misconception I see is people thinking that detonation occurs because the flame front moves too fast. this isn't true. Detonation is sudden, everything goes BANG rather than burns. This creates a shock wave that bounces around in the cylinder and makes the knocking noise which identifies the condition. it also knocks the boundary layer off the piston, which removes the insulating properties of the boundary layer and lets the piston heat up, which weakens it which leads to damage. There is no flame front when a cylinder detonates I used to have a text that had pictures of detonation happening. you could see the flame, then BOOM. nothing.

Which was all supposed to tie in to something I saw in another post - that high octane gas burns slower than low octane gas.

My challenge is for someone to post a technical reference, that means not Hot Rod magazine or Wikipedia, etc, that claims there is a significant difference in the flame speeds of high and low octane pump gasoline, or that flame speed is the method used to control detonation.

Here's the thing. Making the gas mixture burn slower would make detonation WORSE, not better. It goes back to the delay period. You want the mix to burn as fast. Hence all the other mods get get "more complete combustion" like dual plugs, swirl in the intake charge, MSD ignitions, that kind of stuff. It's all designed to burn the mix faster. You want to burn it all before the delay period expires and it goes boom. Why would it make sense to make the gas itself burn slower? A higher octane gasoline resists autoignition due to high temp and pressure longer than a lower octane gas does. it gives you more time to burn the mix but does not burn it slower. You want to reduce detonation? Lower the temp, lower the pressure, burn the mix FASTER, or make the gas more resistant to autoignition. 

To address a few misconceptions I see (not necessarily here) Slow flame speeds do not prevent detonation, they make it worse. Fast flame speeds do not cause detonation, they reduce it. Multiple flame fronts coming together do not cause detonation.

Why should anyone believe me? No reason. So here are some sources I found earlier (and posted on another site) concerning this. Read the references, note the sources, and think about it. Make your own decisions.

*************************

I've seen this basic write up on a couple different race fuel supplier sites.
http://www.whitfieldoil.com/www/docs/171.284/vp-racing-fuel- (http://www.whitfieldoil.com/www/docs/171.284/vp-racing-fuel-)
Another link from a fuel supplier
http://www.pinux-products.com/octane-rating/ (http://www.pinux-products.com/octane-rating/)
Check out Section 6.3 of this university paper:
http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html (http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html)
Sunoco Race Fuels says this:
http://www.racegas.com/article/10 (http://www.racegas.com/article/10)
Any opinion can be supported by links to internet sites. But it also makes no sense that a slower burning mixture would be better for controlling detonation. Flame fronts, even fast ones, are a controlled burn, detonation is an explosion.
*************

No flames intended here, I've just seen the slow burning gas claim so often and no one ever questions why we would do so many mods to make the mixture burn faster, then put a slow burning mixture in there.

Correct that the only thing that octane number tells us is the resistance to detonation which is an uncontrolled explosion for all intents and purposes.

But what's missing is the fact that many additives that improve knock resistance burn slower than regular gas.  So a high octane gas may resist knock and still burn slower than regular gas. But that doesn't mean that ALL race gas burns slowly.  Some burn fast and some are slower burning.  Specific gravity gives some indication as to relative flame speeds, but the vital data about fuels such as energy content and flame speeds are rarely quoted.

The simplest example of high octane slow burning is Methanol which may need as much as 20 degrees more ignition lead and yet still resists knock better than gas. It's better toi think of race gas as a chemical soup which is why it's so expensive and why there are so many choices.

We found on the dyno that street 93 octane makes slightly more power in our small Hondas than VP C12 and there is less unburned hydrocarbons.  It never ceases to amaze me how many fuels VP and Sunoco offer and they keep coming up with new ones. And let's not forget that we want a fuiel that works well at slow speeds as well as high speeds and we want crisp throttle response as well as good power flat out and those need different chemicals to change those characteristics.

Check out the RVP and distillation curves to get an idea of how a fuel will work at lower speeds and when you crack the throttle.  There's no direct correlation as far as I can tell, but it tends to indicate a probable response.

Shell used to have an F1 web site where you could play with different blends of fuels for different circuits to see what they used and why. They had it down to three components to suit different parts of the track. But that was just for the one engine they were supporting IIRC.  Some engines are nice open combustion chambers and are detonation resistant and other designs burn long and slow and are prone to detonation.

I have read many articles on race fuel and I still have no idea how much I don't know :-)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Mydlyfkryzis on Jan 04, 2014, 20:43:50
Desmodog has some of the statements a little off.  Detonation by definition is a fast flame speed....The difference between burn and detonate, in a pure definition, is flame speed...

However, when he states
Quote
Making the gas mixture burn slower would make detonation WORSE, not better. It goes back to the delay period.
he is contradicting himself.  If the burn is slow, it is by definition, not detonation.  However, if I had to guess, I believe Desmodog is referring the the extra heat caused by a too slow burn that can cause the remaining mixture to actually detonate....

Here is a non-wiki reference, from Mikuni on it:
http://www.mikuni.com/tg_detonation.html (http://www.mikuni.com/tg_detonation.html)

(I bolded and underlined the specific definition)

Quote
5: Detonation ("Spark Knock")

Detonation, often called pinging, is nothing less than a series of small explosions that take place within an engine's combustion chambers. It can be extremely destructive, breaking pistons, rod bearings and anything else from the pistons down that a large hammer could damage. It is best avoided.

Pinging is a descriptive name for detonation. Pinging is that high pitch ringing sound that an engine sometimes makes when the throttle is opened with the engine under load. It sounds as though the cooling fins are ringing as they do when you quickly run your finger nail over their edges.

Pinging indicates trouble. Trouble that does damage. That damage can be quick and catastrophic but usually isn't. Most often, detonation occurances are small in energy and the engine is able to absorb the punishment, at least temporarily. However, over time, even light detonation does harm; weakening pistons and overheating the top piston rings.

Severe detonation can destroy an engine literally in a heart beat.

HOW IT HAPPENS

After a spark ignites the air/fuel mixture in an engine's combustion chamber, the flame front travels across the chamber at a rate of about 5000 feet per second. That's right, one mile per second.

Flame front travel for detonation is closer to 19,000 to 25,000 feet per second; the same rate as in dynamite. The difference between normal combustion and detonation is the rate at which the burning takes place and therefore the rate of pressure rise in the chamber. The hammer like blows of detonation literally ring the metal structures of the motor and that is what you hear as pinging.

Detonation occurs when the air/fuel mixture ignites before it should. Normal burning has the flame front traveling from the spark plug(s) across the chamber in a predictable way. Peak chamber pressure occurs at about 12 degrees after top dead center and the piston gets pushed down the bore.

Sometimes and for various reasons a second flame front starts across the chamber from the original source of ignition. The chamber pressure then rises too rapidly for piston movement to relieve it. The pressure and temperature become so great that all the mixture in the chamber explodes. If the force of that explosion is great enough --- the engine breaks.

WHAT CAUSES IT

Anytime the combustion chamber pressures become high enough, detonation occurs. Anything that creates such pressure is the cause of detonation.

Here is a list of possible causes, it may not be complete:

* Timing - if the spark happens too soon, the chamber pressure may rise too high and detonation results.
* Gasoline - if the gasoline burns to quickly (a too-low octane rating), high pressure and detonation are likely.
* Glowing objects - a piece of carbon, a too hot spark plug or other glowing object can start burning too soon. Pressure rises too high and detonation can happen.
* Cranking pressure - Any given combustion chamber has a maximum pressure (before the spark is struck) beyond which detonation is likely.
* High engine tempertures - High chamber temperatures raise cranking pressure and promote detonation.
* Lean jetting - Weak air/fuel mixtures can result in very uneven mixtures within the chamber, uneven burning, pressure spikes and detonation.

Note that each of these possible causes are relative. That is, there is no absolute timing, mixture strength or ignition timing that is going to guarantee detonation. Equally, there are no absolute settings that guarantee that detonation does not occur.

Motorcycle manufacturers, Harley-Davidson included, spend a great deal of time and money fine tuning their engines to eliminate or nearly eliminate detonation. When we change the engine design in the direction of detonation by, say, raising the compression pressure with domed pistons or milled heads, we increase the chance of detonation actually occurring.

Gasoline quality helps determine whether or not an engine is going to detonate. The higher the octane rating, the lower the chance of detonation.

Modified engines often have had several engine design changes that, combined, increase the likelyhood of detonation. High compression pistons, thin head gaskets, some alternative ignitions, some exhaust system designs, etc.

Stock street bike carburetion is very lean for emissions purposes. When the air cleaner and/or exhaust system are replaced by less restrictive components, this stock jetting becomes impossibly lean. The engine does not run well and detonation is likely at some throttle settings. Re-jetting or wholesale carburetor replacement (Mikuni!) is the cure for this particular problem.

If one fits high compression ratio pistons together with an early closing (mild) cam, the cranking pressure may become high enough that serious, engine-deadly detonation is likely. How much is too much you ask?

Well (Rule of Thumb here), Evolution engines are fairly safe against detonation if the cranking pressure remains at 180 psi or less. The TC88 motor can dodge detonation if the pressures remain at 190 psi or less. Keep in mind that these maximums are for fairly stock engines; no porting, no chamber work and no squish areas.

A well shaped combustion chamber with squish effect is much less likely to detonate than most stock examples. The main reason the TC88 engine can withstand higher cranking pressures than the Evo is its better chamber design.

Cranking pressure here refers to the number one gets by conducting a normal compression test. This test is done by removing the spark plugs and fitting a compression gage in one of the spark plug holes. The throttle is then held open and the engine cranked with the starter until the gage needle stops climbing. The resulting number is the cranking pressure.

Ignition systems are important. If the spark plugs fire too soon, the combustion pressure may rise too quickly bringing on detonation. The main reason for having an advance curve built into an ignition system is to avoid detonation. The correct timing for any given engine design (and state of tune) varies with rpm and throttle setting.

Hot spots is more than a night club. If your engine has been running rich or burning oil, it may have thick bits of burned-on carbon. This carbon build-up can literally glow and, under the pressure of compression, start burning before the spark is struck. This leads to severe pressure excursions and, often, detonation.

Lean carburetion can lead to detonation. Uneven combustion in over-lean air/fuel mixtures can escalate pressures and bring about sudden explosive burning. Also, lean mixtures elevate chamber temperatures which, as you now know, can lead to dreaded detonation.

If all this leads you to think that your engine is in imminent peril, then we have succeeded. Detonation is a terrible thing to happen to your expensive Harley engine. The pressures of those explosive events can be enough to hammer rod bearings, pistons and rings into useless junk.

If you hear the tell-tale ringing of detonation next time you open the throttle on a hot day or at low rpm or after a tank of questionable gasoline, back off the throttle and ride carefully until you can find and render harmless this demon visiting destruction upon your motor.

Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DesmoDog on Jan 05, 2014, 10:40:25

The simplest example of high octane slow burning is Methanol which may need as much as 20 degrees more ignition lead and yet still resists knock better than gas. It's better toi think of race gas as a chemical soup which is why it's so expensive and why there are so many choices.


But methanol is not gasoline.... I typically put the disclaimer in posts on detonation that I'm talking about pump gas only, not other fuels. Forgot to here.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DesmoDog on Jan 05, 2014, 11:06:02
Desmodog has some of the statements a little off.  Detonation by definition is a fast flame speed....The difference between burn and detonate, in a pure definition, is flame speed...

However, when he states  he is contradicting himself.  If the burn is slow, it is by definition, not detonation.  However, if I had to guess, I believe Desmodog is referring the the extra heat caused by a too slow burn that can cause the remaining mixture to actually detonate....

A slow burn is not detonation. True. A fast burn is not detonation either, if by "burn" we mean something caused by a flame front. Detonation is auto-ignition. the mixture is consumed due to temp and pressure, not being exposed to a flame.

Normal combustion is propagated by a flame front. I think everyone agrees with that? When detonation occurs, this flame front does not suddenly speed up to five times it's normal speed. When detonation occurs, the unburned mixture auto-ignites, independent of the existing flame front.

So either you consider detonation to be a "fast flame speed" (which I do not) or you consider it to be auto-ignition.


Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jan 05, 2014, 11:52:37
Desmodog has some of the statements a little off.  Detonation by definition is a fast flame speed....The difference between burn and detonate, in a pure definition, is flame speed...

However, when he states  he is contradicting himself.  If the burn is slow, it is by definition, not detonation.  However, if I had to guess, I believe Desmodog is referring the the extra heat caused by a too slow burn that can cause the remaining mixture to actually detonate....

Here is a non-wiki reference, from Mikuni on it:
http://www.mikuni.com/tg_detonation.html (http://www.mikuni.com/tg_detonation.html)

(I bolded and underlined the specific definition)



Interesting read, Rich, but a lot of that information goes against things I've read elsewhere.  I'm not saying it's wrong, but I do advocate further research into the topic.

Just a few points:

Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Jan 05, 2014, 12:39:50
But methanol is not gasoline.... I typically put the disclaimer in posts on detonation that I'm talking about pump gas only, not other fuels. Forgot to here.

Understood.  I was using it as an extreme example to illustrate the disconnect between flame rates and Octane rating. 

BTW, VP make specific mention of the fact that Octane level is only related to knock resistance.  They do not offer any data on flame rates or BTu/# numbers.  Billy Alvaarson says that flame rates are in the 30-50k Meters/sec range but doesn't quote his source.  Mikuni say 19-25K ft/sec which works out to 5-8k m/s  So the data is a little confusing at this point.  Suffice it to say that it is not One number but a range and some are slow and some are fast burning. 

Detonation by comparison is essentially an explosion where flame rates are in the supersonic range

edited to add the K after the metric numbers that I managed to leave out before.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jan 05, 2014, 13:42:53
Understood.  I was using it as an extreme example to illustrate the disconnect between flame rates and Octane rating. 

BTW, VP make specific mention of the fact that Octane level is only related to knock resistance.  They do not offer any data on flame rates or BTu/# numbers.  Billy Alvaarson says that flame rates are in the 30-50M/sec range but doesn't quote his source.  Mikuni say 19-25Kft/sec which works out to 5-8m/s  So the data is a little confusing at this point.  Suffice it to say that it is not One number but a range and some are slow and some are fast burning. 

Detonation by comparison is essentially an explosion where flame rates are in the supersonic range

25,000 ft/sec is 7620 m/s.  :P
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DesmoDog on Jan 05, 2014, 13:43:06
Understood.  I was using it as an extreme example to illustrate the disconnect between flame rates and Octane rating. 


Got ya.

I don't do such a great job trying to explain myself at times but I think you and I (and Sonreir?) agree on things, even if I do muddle an explanation here and there.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Jan 05, 2014, 20:22:32
I think we are all on the same page. :-)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: LArzFroMArz on Jan 11, 2014, 13:24:50
Interesting comment by Sonreir, something to the effect of "I don't know enough to write a book". I like his style of writing so I decided to strip his posts (and a few pertinent other posts) so I could read it offline. Once the 'fluff' posts are out I get a Word document over hundred (that's 100) pages at over 44 thousand words. Not all contributions are Sonreirs' to be fair but the bulk is what he posted. I was told once, "Readins good"... I look forward to this one. Love to see the "article" assembled and made a locked sticky for just the pertinent thread info (less the fluff).
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Jan 11, 2014, 23:43:41
Larz from mars,

CB92 tribute, 65 Cb77, say more please.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jan 12, 2014, 03:34:39
25,000 ft/sec is 7620 m/s.  :P


Just a few points:
  • The flame front travel speeds quoted by Mikuni seem WAY too fast.  Most other sources I've read say somewhere between 20 and 80 m/s.

At 10,000rpm you have 5,000 firing strokes.
60 / 5,000 = 0.012 seconds
It's why high dome pistons are a bad thing, they make combustion path way longer than it needs to be necessitating excessive ignition advance (XR750 needs around 55+ degrees BTDC to reach 9,000rpm - 4,500 firing strokes)
The burn rate really doesn't increase much with temperature or pressure so the time available for flame travel is 'fixed' by the bore diameter and spark plug placement (why several motors used dual plug heads and found more power plus better fuel economy)
someone else can do the math  ;)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jan 12, 2014, 12:11:56
That's interesting.  I was always under the impression that increased compression and turbulence did have an effect on burn rate.  I had always assumed that this was the reason that full ignition advance is usually achieved relatively low in the RPM range.

Any more info on this?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: LArzFroMArz on Jan 12, 2014, 12:38:34
At Teaser... I don't want to jack this thread. Tell me you where you want me to put it and I'll post about it there. Essentially the tribute bike will a Ca160 based frame, knee-tuck tank made from CL72 scrambler tank. The 160 is being modified for 5 speed and hopefully some more grunt. Other than it it'll be red and silver... and fun and 1/10 the cost of real CB92
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Jan 12, 2014, 13:06:05
@Larz please start a new build thread.  It would be very interesting to read.

@PJ/@Sonrier, as far as I know, burn rates increase with turbulence (squish) and with RPM Honda discovered that as engine speed increases, flame rates also increase basically in proportion.  So at 20,000 PRM it takes less time to burn as say 10,000 which is just as well because there is only half as much time for that burn to take place and still reach peak cylinder pressures by 15 degrees ATDC.  If burn rate stayed the same, peak pressure would happen later and later in the cycle.

Let's say that at 10,000 burn starts at 35 BTDC and peak is at 15 ATDC, for a critical burn over 50 degrees. At 20,000 it would take 100 degrees of burn to reach peak pressure but of course the piston would now be at 65 degrees ATDC and could not peak because the volume into which the burn occurs is increasing faster than combustion pressure. 

In reality Honda discovered that they could use fixed timing on their GP bikes and flame rates increased nicely with RPM.

PJs point is spot on about high domes.  High domes leave a combustion chamber shape like the skin of half a tennis ball that's long and thin and is a very inefficient combustion chamber shape and requires huge advance to get the fire going.

That is why some of us do not use very high dome pistons but use lower domes and flatten the combustion chamber and build in an effective squish band to increase turbulence and flame speed.

Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jan 12, 2014, 23:01:59
Honda have been developing engines which more or less detonate above a certain rpm for at least 25 yrs. (they were only in Belgium for about 10yrs until the engine development was 'leaked')
 In all probability something close to detonation always happened and actual flame travel was never really known just assumed?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Jan 13, 2014, 01:47:12
I remember there was an SAE paper or similar way back when.  Or at least I think I do....
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: doctorlumen on Jan 20, 2014, 23:27:37
Can you guys expand on the "domed piston" concept a bit? I recently finished a CB450 build using some highly domed pistons from Charlies Place thinking I was helping the motor along some. But no?
This is the single greatest motorcycle forum thread on the internet. Thanks for posting it up!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jan 21, 2014, 00:33:22
A high dome ~70mm piston will react as though it's a 'flat' 90~100+mm piston. the flame has to travel from spark plug, up one side, over the top and down the other side.
 They actually work very well in low rpm situations and improve torque. ('low' around ~3,500rpm max)
Smaller bore engines have less total distance from plug to edge of combustion chamber so resist detonation better, particularly 4 valve/cyl with central plug and squish or quench area's
I've always found it better to use flatter pistons and reduce combustion chamber volume by milling head (sometimes pretty difficult to do)
Flame speed can only increase so far due to temperature, swirl, tumble, etc before it becomes spontaneous explosion, it is in effect 'static' speed only loosely related to engine speed
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Mcgoo on Jan 30, 2014, 13:29:35
Love this thread - definatelly needs a re-read or two, but I can feel one of those "brain-growth" moments coming on.. Love learning new (to me) concepts/facts that have a practical purpose.  :D
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Jan 31, 2014, 13:02:26
A certain vintage road race team bought a cam from me some years ago for their CB77 based racer.  It had huge tall piston domes and would not rev. That's when they asked if I could get them a cam like the one I raced with, but their motor would not rev.  It just went flat and adding more timing made no difference.  The flame took too long to raise pressure in the cylinder.

They stripped the motor and machine the crowns down a bit - slight improvement.  Then they repeated that until the crowns were quite low and it was revving properly and making power.  On that motor, the combustion chambers is only 54mm wide but the piston is 64 or more mm wide, so it had tall bumps on the pistons to raise compression. More compression = more torque and more power but only if it can burn fast and not detonate. 

That's one reason why tall dome pistons work well at low speeds.

Modern race bikes run to 20,000 revs and you can pretty much bet that they do not have tall dome pistons. Old bikes, not so much :-)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jan 31, 2014, 19:25:26
Kinda kicks the faster burn at higher rpm into touch?
5,000ft/sec isn't going to go much faster at high rpm, fuel can only burn so fast before it's more likely to detonate
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jan 31, 2014, 19:48:17
The thing is, the fuel doesn't really need to burn very fast.

Let's say you have an engine with 60mm stroke revving at 20,000 RPM.

One revolution means the piston goes up once and down once; 120mm.  120mm * 20,000 = 24,000,000mm (or 2400 meters ever minute).

2400 meters is 7800 feet or so.  So if your fuel is burning at 5000ft/sec (which I think is a pretty darn high number, personally), it's still 50 times faster than your piston is moving.

The number I've read (Bell, I think?) quoted at 80m/sec for flame front travel? And with a 50mm bore, the flame should travel across the cylinder in less than a millisecond.  One millisecond at 10,000 RPM is 60 degrees of rotation, though, so the domes definitely matter.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jan 31, 2014, 23:40:21
Ah, but, you need max pressure to occur roughly 14 deg ATDC which gives far less time to complete combustion
I think I may have meant 15,000ft/min as well, (trying to remember without looking anything up  ;) ) the slower burn means pressure is pushing on piston for relatively 'long time'
Diesels work by creating high pressure from compressing a fixed amount of air then add fuel from around TDC to keep things burning longer, usually to around 60 deg crank rotation (that's why high performance turbo diesel race trucks blow so much black smoke - mostly carbon from partially burned fuel)

*80m/s is roughly 265ft/sec
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 01, 2014, 00:01:48
And 15,000ft/min is 76m/sec.  :D
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Tijmen on Feb 19, 2014, 10:22:43
Thanks for this write-up! Especially for the great explanation of optimizing the "intake tract", but it's left me with two questions:

How do I determine the intake duration of the camshaft? For now I used the 221 degrees you used in your example, also because it sounds like a pretty reasonable number, because I couldn't find it anywhere in my Haynes manual and also can't think of a way to calculate it. This is kinda crucial for me because using the 221 degrees I found I need a 22mm carb instead of the stock 26mm CV carb currently installed on my Suzuki GN125. It seems like a pretty big jump, but I found quite a couple of 125cc bikes running carbs this small...

I'm looking for a new carb because I want to get rid of the airbox (I know, I know) and I think a roundslide carb like the VM22 will handle pods or UNI-like foam filters much better, which brings me to my second question. The airbox is what currently gives the intake length, right? So if I would put my pod on a pice of tube to reach a 400mm intake and capture the 4th wave would that lessen the negative impact pod filters have? And does the length of the filter add to the total length of the intake tract? Thanks again!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Feb 19, 2014, 13:39:37
26mm CV carb flows much less air that the equivalent round slide and that one is probably closer to 22-24 of effective area, so that's not an issue. It isn't impossible to get the bike to run OK with a pod and those carbs, it's just harder to them right that a slide carb. 

The airbox is not part of the intake length.  Pressure waves "see" that as an open space. Where it makes a difference is that the air in their is still and not blowing and turbulent.   

To put the whole inlet tract length thing into perspective, it's worth probably 2% power at the peak compared to a similar intake of the wrong length, and loses almost as much off peak and getting jetting wrong can cost 10-20%.

If you really want more power, then a larger bike is probably the easiest route, but failing that, higher compression, larger piston, 24mm carb with velocity stack inside a large capacity filter are things to consider.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Tijmen on Feb 19, 2014, 17:23:34
26mm CV carb flows much less air that the equivalent round slide and that one is probably closer to 22-24 of effective area, so that's not an issue. It isn't imp[ossible to get the bike to run OK with a pod and those carbs, it's just harder to them right that a slide carb.

And the only difference would be that slide carbs are easier to dial in, or would they offer a performance improvement too? I know I have to be more careful with the throttle on a slide carb, but I suppose more control would lead to (slightly) more power?

The airbox is not part of the intake length.  Pressure waves "see" that as an open space. Where it makes a difference is that the air in their is still and not blowing and turbulent.   

To put the whole inlet tract length thing into perspective, it's worth probably 2% power at the peak compared to a similar intake of the wrong length, and loses almost as much off peak and getting jetting wrong can cost 10-20%.

If you really want more power, then a larger bike is probably the easiest route, but failing that, higher compression, larger piston, 24mm carb with velocity stack inside a large capacity filter are things to consider.

A larger bike is out of the question, I'll be counting the days until I can upgrade my license but until then I'm SOL.

After researching pistons and compression ratios for an hour I feel like I'm in way over my head, so that might be a project for next winter, but I want to have as little downtime as possible and think half a year of wrenching and riding will help a lot with big "surgeries" like this.

And which of these four filters do you mean?
My plan was doing something like the last one, but with a UNI-filter. Mainly because my gut-feeling tells me to.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DesmoDog on Feb 19, 2014, 22:47:30
How do I determine the intake duration of the camshaft?

With a dial indicator and a degree wheel.

You need to know what lift the measurements need to be taken at. For example I'll say 1mm. Use the dial indicator to keep track of lift, degree wheel to keep track of degrees. Turn the crank until the lift is 1mm, note the degrees on the wheel, turn the crank until the lift drops back to 1mm, and note the degrees on the wheel. The difference between the two is the duration.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Feb 20, 2014, 01:44:21
Slide carbs fall in a hole when you open the throttle too fast because there's an immediate drop in charge velocity, and as velocity drops, so does the amount of fuel picked up.  Consequently it bogs for a moment until it picks up steam.

With CV carbs the slide is operated by pressure differential and as you bang open the throttle, the slides stays where it is until conditions change.

But a slide carb tends to flow better than the equivalent sized CV carb and sometimes that allows the motor to run harder.  A carb that's bigger than the motor needs makes less power and runs badly compared to a correctly sized carb.  That's a bit of a generalization but think of it this way.  If you were to breathe through a small straw and you sat still, no problem.  if you get up and walk around, you need more air.  As you run you need a bigger straw. You eventually reach the point where you are getting all the oxygen that you need, and what's holding you back now is your lung capacity or legs or whatever, but having more air available won't help.

It's the same with engines, bigger carbs only help if they are the limiting factor.  As you make the engine run harder with bigger cams or big bore pistons, it needs more air and at that point a change to larger carb will help.  So bigger carbs will not always make more power if that's the only thing that changes. 

Hope that all makes sense

The best of those is #3 where there's a proper velocity stack and it's protected by a filter.  It would be better if that filter were larger, but who's counting.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Tijmen on Feb 20, 2014, 04:55:38
Alright, thanks! A filter at the end of a stack would be even better than over it because it would allow more air in, right? Like this: http://i.imgur.com/fHlekbl.jpg


And are pistons model specific, or would every piston with the right bore fit in the cylinder? I suppose the linkage to the crank could be different? I found a high compression 57.0 mm high pressure piston for a Yamaha 125 bike, and don't see why it wouldn't work, seems like a pretty easy upgrade with a good bang for buck ratio.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Feb 20, 2014, 11:11:54
Piston mods are specific, you may have to machine parts for clearance. You can't use two stroke pistons in fourstroke motors (unless you want an oil burner  ;D )
 There are very few direct swaps when doing big bore conversions.
 You should be able to go to around 145cc though, it's major work to go much bigger.
You will need better valve springs to cope with higher rpm, they float, touch something, break off heads and destroy motor
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Feb 20, 2014, 15:08:49
The key to the "filter over stack" approach is to ensure that the open end of the stack can get enough clean air from the sides to clean up flow.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Tijmen on Feb 20, 2014, 16:09:36
Piston mods are specific, you may have to machine parts for clearance. You can't use two stroke pistons in fourstroke motors (unless you want an oil burner  ;D )
 There are very few direct swaps when doing big bore conversions.
 You should be able to go to around 145cc though, it's major work to go much bigger.
You will need better valve springs to cope with higher rpm, they float, touch something, break off heads and destroy motor

Sounds like making this a winter project wasn't a bad plan at all, thanks for the info!

The key to the "filter over stack" approach is to ensure that the open end of the stack can get enough clean air from the sides to clean up flow.
Cheers!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Mydlyfkryzis on Feb 26, 2014, 17:07:45
Bear in mind, a CB350 is 325cc, about 20 Cubic inch. STOCK, is develops 36Hp at the crank, with is 1.8 HP per CI.   That's pretty darn good..... Think of a Mustang with a 5 liter V8 (305 CID).  That would be a 550 HP 305 in the same state of tune as the 350....

A modern 2014 Mustang, with fuel injection, variable valve timing, computer controlled ignition is rated 420 hp from 305 CID.  Go back to 1967, a Mustang with the 390 CID (6.4 Liter) was 320 HP. (0.8 HP per CID)...thought to be pretty powerful in the day.

Just illustrating that a CB350 is a pretty highly tuned little engine. Of course you can improve it a little, but it is a lot of work to get the little bit more....
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 28, 2014, 12:45:25
It's not uncommon for CV carbs to be much larger in diameter than slide carbs, so 26mm to 22mm may not be as crazy as it sounds.

I'm not sure on the duration for the GN125, but hopefully someone else can chime in.  You'll definitely want to use the correct numbers for your bike.

Finally, the length of a pod filter should not be included into the intake tract, but a velocity stack should be.  It's the point at which the intake opens to ambient pressure which is important.  So in a bike with a stock airbox, you'd measure from the intake valve to the airbox, not from the intake valve to the opening in the air box.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Feb 28, 2014, 13:44:55
I've always treated CV carbs as about 80~85% flow of slide carb. (or, to put it another way, CV is always 'too big' by 15~20%)
 36mm CV about equivalent of 32mm round slide or 29mm 'smooth bore' (actually closer to 31mm and 28mm but they don't come in those sizes)
The smaller the mechanical slide carb you can use and get the flow you want will always produce better throttle response than going 'too big' on carb venturi
As for airbox not having an effect on tuned length, it depends on the intake snorkle, if it's smaller than 1.5 times area of one carb, it will have the same effect as increasing intake length, the airbox will be acting as an Elmholtz resonator
https://en.wikipedia.org/wiki/Helmholtz_resonance
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DucatiDave on Mar 07, 2014, 14:24:00
   First off, thanks to Matt, Teazer, PJ and all the others for passing on this info. I knew some of the basics but not the theory behind it. What a wealth of info.
   We do these builds because we can, and I get that. My question is what is the dollar cost to HP gain to actual real world increase. If I am riding, and next to me is a "built" CB350 vs my stock CB, does he pull alittle ahead, or does he walk right away? Anyone can twist the throttle on the straight road. I want to be able to out brake and out corner the other guy. That is where I am putting my emphasis, handling and braking.
   For me, I am re-building a 73 CB350 with 9k miles. My thoughts were to make the motor reliable. Upgrade to electronic ignition, Ricks stator, Bore-tec cam chain tensioner, and Mikuni VM30's. I thought about doing a whole motor build but came to the conclusion that at the end of the day it is still a small motor. Would like to know others thoughts on that.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: stroker crazy on Mar 07, 2014, 16:59:39
the dollar cost to HP gain

There comes a point where the law of diminishing returns kicks in.

It is usually cheaper to go for a larger motor than try to wring the last fraction of a horse from a small one.

That said, it can be an interesting exercise to see how much can be extracted from any given motor!

Crazy
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DucatiDave on Mar 09, 2014, 13:59:25
Stroker, I agree, there is no replacement for displacement. And you can only go as fast as your wallet is fat. So to invest alot of extra money into a small motor and still have small motor performance seems alittle excessive. I am not saying one way is right or wrong, I am a gear head and appreciate the time, skill and effort that people put into their engines. That is why I am not going the engine upgrade route. I am looking at the bike as a cohesive piece of rolling art. Carefully crafting the bike from front to back. Looking at every part that I change and make sure it flows with the other changes I have done or have in mind. I built a "best in show" Bobber, in my garage. The only things I did not do was paint, powder coat, and chroming. (I used Kraft-tech frame) The judges said they chose my bike bike because from front to back the bike flowed, all the little details were thought out well. Doing it right is a great title for this thread, but how is everyone taking that phrase. Does it mean building a Cafe Racer this way, and that is the right way. Or Doing it right, don't short cut it. Do the job the right way the first time. I am sure everyone knows what a "rat" bike or car is. There are 2 ways to make a rat ride. Build it haphazzardly, tie stuff on with bailing wire so every time you ride it something falls off. Or build it with craftmanship and have the look that you desire.
Ah if we only had unlimited funds!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: mannydantyla on Apr 08, 2014, 14:52:59
Applications - Intake Tract

OK.  Up next I'd like to ramble on a bit about intake tracts.  By "tract" I mean everything before the cylinder (including the valves, valve seats, ports, carbs, and filters).
...

Excellent! Thank you for this!

https://www.youtube.com/watch?v=RA5PRj7KPkE
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: raptormeat on Sep 05, 2014, 22:05:51
come on guys its not rocket appliances
http://www.youtube.com/watch?v=wAIe9QtRKlc
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Nov 14, 2014, 15:07:53
Nitrous Oxide

It's been a very long while since I've done one of these posts, so I apologize in advance for the lapse. If any others want to take up the mantle and help pick up the slack, please do. ;)

This is going to be another one of those posts that's largely academic, since few of us will ever go this route. It's fun to discuss and the knowledge is useful, though, so I'll proceed.

The chemical formula for Nitrous Oxide is N2O, but it also often goes by the name, "NOS", which is the brand name of Nitrous Oxide Systems, one of the more popular kit offerings and parts suppliers. N2O means two atoms of Nitrogen and one atom of Oxygen. This is significant for several reasons. Primarily, the limiting factor of producing power in most engines comes down to how much Oxygen you can stuff into the cylinder(s). You need between 12 and 14 times as much Oxygen (by weight) as you do gasoline for combustion, so adding fuel is rarely a problem. Air generally found in our atmosphere is constituted of a lot of different gases, of which Oxygen represents about 1/5. In N2O, the Oxygen represents a little over a third. What's interesting is how this Oxygen is freed up. On its own, N2O is a non-flammable, relatively inert, gas. When heated to around 570°F, is begins to break down. The Nitrogen and Oxygen separate and we now have accessible Oxygen which can be put to use in our combustion. Finally, the way the N2O is stored becomes quite a bonus as well. Compressed N2O is stored in liquid form in gas tanks. When the N2O is delivered from the tanks, it decreases in pressure drastically. And because temperature and pressure are interrelated, a drop in pressure also means a drop in temperature. Releasing N2O into your intake tract drops temperatures by a significant portion. This, of course, leads to a denser intake charge, resulting in even more air making it into the cylinders.

Unfortunately, to which many a poor soul can attest, tweaking with the amount of Oxygen entering the cylinder without adjusting the fueling can lead to a bit of heart ache. Burned pistons and valves are not uncommon as well as even larger failures that stem from detonation and pre-ignition. With all of the extra Oxygen available, we need to add the fuel to match or we quickly develop a very lean running condition.

How that fuel is added is how each Nitrous Oxide system is generally described. N2O systems come, usually, in one of two flavors:  Dry Shot or Wet Shot.

In all cases, the actual installation involves locating your N2O bottle somewhere on the bike and routing the lines to an electrically activated pneumatic solenoid.  The solenoid then controls the flow of N2O gas into a set of injectors mounted into the intake tract.  The start of the flow of the N2O usually triggered by a button near the hand controls.  Many systems also include a safety that prevents the system from being used unless the bike is at wide open throttle.  Use of the N2O at partial throttle positions usually ends up being fatal for the engine.  To further combat any issues arising from the addition of more fuel and Oxygen, ignition timing is usually retarded a few degrees when the N2O is in use.

Metering of the N2O is usually handled by a pressure regulator as well as jets.  Like the jets you find in carbs, these jets control the amount of N2O that is provided.  And like carbs, the size of the jets are usually represented by a number (which may or may not be arbitrary).  The combination of the pressure of the N2O system as well as the jet size is usually summarized with a rough estimate of the horsepower added to the engine when the system is in effect.  15 shot, 20 shot, 40 shot, and even 75 shot all refer to the estimated horsepower added.  As with most things, more power usually means more cost, more planning, and more difficulty in engineering.

Dry Shot
A Dry Shot Nitrous Oxide System is so named because there is no auxillary addition of fueling to compensate for the addition of N2O. Dry shot relies solely on the vehicle's existing fuel delivery system to administer additional fueling. For carbbed engines, the N2O is added upstream of the carbs. The idea is that the denser charge passing through the carbs will pick up more fuel. While this is true, it's usually not enough. The same process is used in fuel injected engines, though usually with a greater effect.

A Dry Shot system is usually the easiest to install and maintain, but also the most prone to damaging the engine and its components. Also, Dry Shot usually results in less overall power than the other types. This is purely due to the failure to add enough fuel to fully utilize the extra Oxygen.  Because of the lack of additional fueling, the amount of N2O that can be added is usually minimal.  Things must remain matched.

Another common tactic is to intentionally richen the mixture by installing oversized main jets into the carbs.  Since the main jets have the most effect at WOT and the use of N2O requires WOT, this throttle position is reserved for when the user wished to take advantage of the Nitrous system.

Because of the balance of risk involved in using a Dry Shot system versus the low gains, Dry Shot systems are not usually favored by those looking for real power or longevity, though they are much more common in modern fuel-injected engines where electronics can account for the gas addition without many other changes.

Wet Shot
A Wet Shot system is quite a bit different than a Dry Shot system in that all of the fuel necessary to utilize the additional Oxygen provided by the N2O is added at the same time, along with the N2O, usually with fuel injectors located in each intake port, after the fuel metering (so after the carbs or MAF/MAP of an EFI system).

To add this fuel, it's usually necessary to engineer a fairly complex system.  If your bike didn't have a fuel pump before, it definitely needs one now.  A fuel pressure regulator also becomes a necessity.  Fuel injectors must be added to each port, after the carbs, so now the button press to release the N2O into the system now also actuates fuel injectors.

It doesn't sound like a lot extra, but all of these added parts on an already small machine usually requires quite a bit of forethought to pull off successfully.  Furthermore, the electricity needed to run the fuel pump, solenoid(s), and injector just may not be available on some bikes.

Conclusion
Nitrous Oxide can be a game changer.  Compared to most other performance options, it is generally very inexpensive to install, but it also costs money over the long term.  Refilling N2O bottles can get spendy, especially for those of us addicted to torque and horsepower.  A poorly executed system can also result in a lot of broken parts and it's not difficult to find videos on YouTube of installations that turn their operators into human fireballs.  Well executed systems, however, can result in a lot of extra power for very little investment of time and money and usually do little to long term reliability of an engine.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Nov 14, 2014, 16:59:17
A 50+ hp CB350 will probably cost in the region of $7,000 for engine work.
That will be at the wheel and not crank though so pretty much double power output
It's very easy to get 32~34 at wheel but costs pretty much double to go over 40bhp at wheel (should still be under $1500.00)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Nov 14, 2014, 18:21:14
And a 20 shot wet nitrous kit runs about $1000 or so.  ;)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Nov 14, 2014, 19:23:44
And a 20 shot wet nitrous kit runs about $1000 or so.  ;)

.................and could be fatal on a stock motor  8)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Nov 14, 2014, 19:50:45
Yeah.  But what a ride until things break.  ;D
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Jan 30, 2015, 23:49:13
Thanks for this thread, it's friggin' heavy reading, but super informative! Bookmarked.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jan 31, 2015, 16:52:10
Hopefully it will stay here (or be archived somewhere?) for posterity  ;D
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 01, 2015, 01:25:16
Ok, ive managed to read through this rather intense (to me, anyway...) information, and definitely plan on using some of it in my build. One thing i would like to clarify... you did go into 2 into 1 systems, but i want to keep my cx500 as a 2 into 2, Which wasnt really covered. will it be an issue if i make my pipes slightly longer, to ensure the muffler extends past the rear shocks so as not to blacken the springs? I will be removing the original headers and having stainless pipes made up the same size as the original headers. Im not home now, so cant do actual measurements...  i have a more or less scale drawing, if that helps... before you give me a hard time, i realise the angles on the headers are too sharp, after reading this thread i went and changed my drawing a bit to have more realistic header curves 😀
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Feb 01, 2015, 13:55:54
Unles your going to run it over 12,000 rpm I've always found a longer exhaust provides easier for carb mods and better overall power. You know CX motor will kill it's cam chain tensioner if you plod around at low rpm? (below about 6K)

 I would probably have flanges turned up and convert to spoked wheels, really don't like Com-Star's
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 01, 2015, 16:18:03
I think the cam chain tensioner is shot, its a bit "clattery" i will check it and the cam chain when i pull the motor for its clean out and inspection. I will likely fit a new did chain as a matter of course. I am looking into a set of gl1000 spoke rims, or possibly a conversion (conversion is a last resort, shipping and customs will probably cost more than the kit itself, i would have to have one made locally ) fortunately i dont hate the comstars, so its not going to kill me to keep them. Thanks for the input 😀 i tend not to ride at low revs, so the new tensioner should be ok...
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 01, 2015, 16:23:41
Ok, ive managed to read through this rather intense (to me, anyway...) information, and definitely plan on using some of it in my build. One thing i would like to clarify... you did go into 2 into 1 systems, but i want to keep my cx500 as a 2 into 2, Which wasnt really covered. will it be an issue if i make my pipes slightly longer, to ensure the muffler extends past the rear shocks so as not to blacken the springs? I will be removing the original headers and having stainless pipes made up the same size as the original headers. Im not home now, so cant do actual measurements...  i have a more or less scale drawing, if that helps... before you give me a hard time, i realise the angles on the headers are too sharp, after reading this thread i went and changed my drawing a bit to have more realistic header curves 😀

Take a look at the Acoustic Tuning sub-section and just ignore the portion about the headers.  Just apply the overall lengths and diameter of your calculations to the entire pipe for each cylinder.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 01, 2015, 18:51:42
coolness, thanks. I will do. At the moment, i am away from home for another 6 weeks, so cant do any physical work on the bike. But now, im thinking it is a good thing, it has forced me to do a lot moreresearch andplanning than i would have,  saving me from mistakes i now knowi would have made, and thinking of other ways of accomplishing my goals. My build will be all the better for it. I have restored a bike before, but this will be my first custom job, totally different. Easier in the sense that parts are more available, more dificult in that mods need to be planned, you cant just follow a garage manual! 😀
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 01, 2015, 20:47:09
Another question which shows how little i actually know about engines... when i restored my suzuki, i had a lot of smoke, so got first over size pistons and rings, then had a rebore done to accept them, problem solved, and a just noticeable power increase (maybe my imagination, but i swear it feels quicker...) was that nessecary? Could i merely have replaced the rings, or pistons and rings with new standard size? Just something for me to keep in mind if my cx decides to start smoking...
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 02, 2015, 00:09:43
Smoking can be caused by a number of things, one of which is the rings.  If you just decided to replace it and that solved your problem, you got lucky.

Often, if the rings are loose enough to be causing smoking issues, it's not uncommon for that to also cause a loss of compression. Increasing compression will DEFINITELY cause a noticeable increase in power.
Title: Re: &quot;Doing it Right&quot; or &quot;How to Build a Functional Café Racer&quot;
Post by: datadavid on Feb 02, 2015, 00:20:38
That is decided by measuring your cylinder bores, quintin.
You look for width and out-of-round with a 2-point micrometer.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 02, 2015, 02:06:22
I was having compression issues too, but much better now, sitting around 8.6 now, number 2 is around 8.8, dont remember what it was before, but bad...
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 02, 2015, 02:11:25
How are you arriving at those numbers?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 02, 2015, 02:18:32
Im assuming bar, thats what the mechanic said (not the one who did the rebore, and i did the rebuild myself) i dunno how trustworthy he is though, when he did my carbs, they were worse than before he started. The guy who actually fixed my carbs also said compression was good though, and also called the previous guy an idiot
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 02, 2015, 02:20:23
Compression ratio is a calculated, not measured, value.

The compression you measure is in PSI.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 02, 2015, 02:26:26
South africa, we work in metric, he measured it via the spark plug holes
Title: Re: &quot;Doing it Right&quot; or &quot;How to Build a Functional Café Racer&quot;
Post by: datadavid on Feb 02, 2015, 02:31:51
We do in sweden as well quintin, thing is psi i a finer scale.. my comp gauge is in both psi and bar.
My uncle used to live in south africa, he had a cobra replica built there, and started a bobber build with revtech engine. When we got the stuff up here me and my father just stared in disbelief at the horrible stuff that was done to the machines. Imagine a 7 liter v8 hanging in one (!!!) Bolt.
Gas tank was completely loose, wiring was an abomination.
In the engine they tried using metric bolts in inch threads, and there was no correct ISO standards on any bolts..
So.. i dont give much for south africas builders..
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 02, 2015, 03:05:30
I agree, there are some really sloppy and bad techs, but some of us take pride in our work. (Im no tech, just an enthusiast, but i did successfully get my totalled bike back on the road in near showroom condition with very little help)  If i was to build a bike to sell, i would take the same care as i did on my suzuki. Forcing imperial and metric together is unforgivable, if it feels too tight, then it is, re check and tap or cut if nessecary. Better still, get the right part... its difficult, but not impossible to find imperal parts in south africa, especially with the internet!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: datadavid on Feb 02, 2015, 03:13:18
Yea of course I've seen some superior restorations as well! I remember some guy restoring a couple of bikes from the 20's, they were awesome..
One was an enfield v-twin if i recall correctly. Absolutely beautiful.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 02, 2015, 04:01:27
Important thing is i eventually found a mechie i can trust, and after over a year of fighting my carbs, the suzuki is running like she should, even has a crazy power band which wasnt there before... much happiness. Now i have found this forum with so much info on it, my cx can't be anything other than beautiful. (Unless i build like a monkey...)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 02, 2015, 09:55:13
OK.. if that's BAR and not ratios, that makes more sense.

Still... that's a very low reading.  You want about 10.5 to 11.5 BAR on a fresh engine.  Anything below 10 is definitely a problem.
Title: Re: &quot;Doing it Right&quot; or &quot;How to Build a Functional Café Racer&quot;
Post by: datadavid on Feb 02, 2015, 10:25:54
Maybe engine was cold with dry barrels?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 02, 2015, 10:43:13
Possible.  But I would hope a mechanic would know better than to do that (proving there were other options).
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 02, 2015, 15:15:33
Im assuming bar, thats what the mechanic said (not the one who did the rebore, and i did the rebuild myself) i dunno how trustworthy he is though, when he did my carbs, they were worse than before he started. The guy who actually fixed my carbs also said compression was good though, and also called the previous guy an idiot

I refer to this... i dont know how good the first guy was, but he gets a vote of no confidence from me, he wont be working on my bikes again... sad thing is, it is a mate of mine's shop, and i think his choice of mechanic is a liability, not an asset. Ive told him as much. Apparently they get a lot of returns from unhappy customers, and are starting to get a bad reputation.
Title: Re: &quot;Doing it Right&quot; or &quot;How to Build a Functional Café Racer&quot;
Post by: datadavid on Feb 02, 2015, 15:17:59
And if its rebuilt and not yet run in, comp will probable lower than with seated rings
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 02, 2015, 15:32:09
Rebore, new pistons and rings, barely run in, maybe about 600 miles since open heart surgery, evertyhing is still shiny and new...
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 25, 2015, 12:23:43
Everything should have been bedded in within the first 20 miles or so, assuming you've run it hard.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 25, 2015, 19:41:35
Nope, no hard riding, all gentle 300rpm region. Ive been told by a lot of people who know more than me that i have to take it easy during run in or i will ruin the rebore and new rings. i was also having carb nightmares, and still charging issues, but carbs are finally done, and once the charging fault is done, i can finally get some actual performance out of her.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: stroker crazy on Feb 25, 2015, 20:07:37
Ive been told by a lot of people who know more than me that i have to take it easy during run in or i will ruin the rebore and new rings.

It ain't necessarily so!

Crazy
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 25, 2015, 20:29:19
Nope, no hard riding, all gentle 300rpm region. Ive been told by a lot of people who know more than me that i have to take it easy during run in or i will ruin the rebore and new rings. i was also having carb nightmares, and still charging issues, but carbs are finally done, and once the charging fault is done, i can finally get some actual performance out of her.

I don't subscribe to that theory.  I believe the exact opposite, actually.

If you don't run it hard in the few 20 miles or so, the rings will never seat as well as they could have.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 26, 2015, 16:25:27
Too late now then, i guess. But i will wind it a bit next time i ride her, like she was designed to do!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 26, 2015, 17:07:41
Actually... the window of opportunity is probably gone.

If/when you do another rebuild, let the engine warm up and progressively load it.  In my opinion, the best break in location is on the track.  Let the engine have full throttle and progressively increase the RPMs over a series of runs.  Try to stay off the brake and let the engine's compression handle it.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Feb 27, 2015, 01:19:59
No.  What you need are revs but no load.  Think machining.  Too much load and not enough revs hardens the bores and they never seal well. 

I'm sure there are more other opinions out there :-)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: stroker crazy on Feb 27, 2015, 02:54:18
What you need are revs but no load.

Try to stay off the brake and let the engine's compression handle it.

Another opinion!

Plenty of revs, compression braking, definitely avoid load at low revs.

Crazy
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Feb 27, 2015, 09:50:57
I'm not sure I agree, teaz.  The rings don't have a ton of tension on them.  You need the compression of a load to help them seat.  It's the compression in the chamber that slips behind the rings and causes them to exert force on the walls, no?

Without that load it would be like trying to plug a bathtub with only a few cups of water in it.  You need that extra water to cause the plug to fully close the gaps.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Feb 27, 2015, 18:21:51
So, if i read correctly, the general idea is to keep in lower gear and ride around revving the hell out of the engine? Totally the opposite of everything ive been told in the past... i will love that, but im sure my neighbourhood will go ballistic! 😂😆😲
so, suggestions here,,, the smoke isnt very much, and only on number 2, do i leave it, or pull the motor again and get another set of rings? If so, can i do just the one pot, or do i do all 4? (I made sure all rings werein the right way up, and in the correct order, just in case anyone was wondering...)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DesmoDog on Mar 04, 2015, 22:58:43
Try to stay off the brake and let the engine's compression handle it.

I agree that being too easy on the engine during break in can/will result in the rings not seating well. Been there, done that in fact.

However... the above quote seems to imply that compression causes engine braking. If compression caused engine braking, diesels wouldn't need Jake brakes. Engine braking comes from trying to pull flow past closed throttle plates, which diesels don't have. They have more compression than gas engines, but they don't create manifold vacuum, so they don't engine brake.

If you accept that, it follows that when the engine is braking there's a vacuum in the cylinder. When there's a vacuum in the cylinder, the rings aren't getting pushed against the cylinder walls from combustion pressure like they are when under power, so I question just how much engine braking contributes to break in.
Title: Re: &quot;Doing it Right&quot; or &quot;How to Build a Functional Café Racer&quot;
Post by: datadavid on Mar 04, 2015, 23:23:47
So, if i read correctly, the general idea is to keep in lower gear and ride around revving the hell out of the engine? Totally the opposite of everything ive been told in the past... i will love that, but im sure my neighbourhood will go ballistic! 😂😆😲
so, suggestions here,,, the smoke isnt very much, and only on number 2, do i leave it, or pull the motor again and get another set of rings? If so, can i do just the one pot, or do i do all 4? (I made sure all rings werein the right way up, and in the correct order, just in case anyone was wondering...)
4? So you are not talking about your cx anymore?
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: teazer on Mar 05, 2015, 11:10:54
Too many revs or too much load too soon is bad.  Progressively increase the load but use the whole rev range.  If you run it too light on load and too low revs for very long it will never bed in.

Agree that the track is a good place to bed it in.  Compression braking/engine braking just means shutting the throttle and letting the engine slow you down.

Title: Re: &quot;Doing it Right&quot; or &quot;How to Build a Functional Café Racer&quot;
Post by: Quintin Snell on Mar 05, 2015, 20:08:10
4? So you are not talking about your cx anymore?

Nope, i also have a Gs1000g which i rebuilt, and the number 2 piston still has some intermittent smoking. Not bad, but after that much work, there should be none...
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Quintin Snell on Mar 05, 2015, 20:10:13
Sorry, i seem to have hijacked this thread, i can start a separate one...
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: DohcBikes on Jun 27, 2015, 12:04:43
With a fresh rebuild, assuming already that everything is properly assembled, pre lubed, and has arrived at operating temperature while slightly above idle to build and maintain oil pressure:

-Never rev a new engine with no load on it.

- It is cylinder pressure that forces the rings into shape, and the piston travel that matches them to the bore. High RPM under load is the best way to achieve even cylinder pressure and high piston speed.

- after the engine is warm, you need to get to it. Be ready to ride, accidents can be more likely when focusing only on the task of seating the engine components at high rpm, road hazards still exist. Have a place to ride it like it needs to be ridden, a track if possible. Firing it up and whomping it up and down your block is dumb.

-Safely enter a rural highway or track. Briskly throttle and get out of first gear, take it to a grand under redline through second, third, and fourth gear. You dont want to be in the lower rpm when you get into the next gear up, get those shifts made quickly.

- At a grand under redline in fourth, let the throttle snap shut and roll down through the rpm until it gets to a safe rpm to downshift, repeat through third and into second. Using no clutch to downshift is recommended.

-After the initial runup, go WOT to redline through as many gears as you can without endangering yourself or others. Do not miss any shifts. Let the trottle snap shut, roll down through the gears safely to second, and repeat.

Your engine is ready. Go ride it however you want to now. Run it the way you want it to run.


Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jul 03, 2015, 19:17:33
And DO NOT lug the engine.
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jul 13, 2015, 12:48:15
I warm up 'new' motor at 2,000 rpm to make sure the oil is being thrown everywhere it needs to go, minute or so at first start isn't so bad as there will be assembly lube on all moving parts
Too much idling is really bad though, usually causes problems with small end of connecting rod which tends to 'go dry' and scuff and/or have partial seizure (seen it many many times)
I also chamfer the bottom edge of piston thrust face to direct more oil inside piston (rather than have the factory square edge)
I think it also helps cool piston crown?
 Not as good as real oil jets but better than nothing  ;)
I think it may have been mentioned previously, it isn't a good idea to slather oil all over piston, rings and bore as your much more likely to get glazing of rings and cylinder.
It will bed in eventually but can easily take 20~30,000 miles
 I use only a tiny bit of oil on piston skirt thrust face and a smear on the front below rings
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Tune-A-Fish© on Jul 13, 2015, 13:13:03
I don't know how a cycle motor off the showroom floor ever gets past the first 1K miles with all the stoopid furkers ridin em  ::)

The definition of torque is when you push down on a boner your feet come off the ground  ;D

Yup that's all nothing to add... great stuff here though and I learned a lots :o
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: crazypj on Jul 15, 2015, 17:05:48
In the 70's Honda made a films of bikes straight ff the production line being run through the gears and taken to red line before being dismantled and shipped
The main reason for 'break in' period was/is to try and get new riders used to new bike and it's foibles rather than a chance of engine damage
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sideswipe on Oct 20, 2015, 22:12:40
I always wondered about the 'ideal' break in process - but without any tracks nearby I'd be concerned about running it through very many gears to redline!!  :-X
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: hooligan998 on Oct 21, 2015, 01:24:49
Sub'd for future reading.  Great info so far!
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Tune-A-Fish© on Oct 21, 2015, 08:22:26
I always wondered about the 'ideal' break in process - but without any tracks nearby I'd be concerned about running it through very many gears to redline!!  :-X

Leme just say... If you just rebuilt the ZX14... ramp up on the nearest I-? wring it to red through 6th gear your now at a silent cruising speed of about 210MPH... Luckily the bikes computer wunt allow for such fuckery lesst you swapped it out for a race chunk  :o
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Psycrow on Jul 09, 2016, 14:53:12
I will admit strait up I have not read the entire thread YET but I felt it was important to comment on some of the things the  OP stated even though it was 2012 he made many great points and responses have added may more and I whole heartedly support the positive tone his post took however I have a big issue with one of his main points. I seriously think the definition of "Cafe Racer" needs to evolve somewhat.. cafes racers of their day were brittish twins modern in their own time and cut and modified to remove the unnecessary and mod the essential for maximum performance. If we apply that to today it would be doing the same to a late model GSXR.. if your going to extend the term beyond its origins an include 70's and 80's UJMs then you can't apply other rules to the term either.  What am I going on about? The attitude that if you don't over bore and port and polish your doing it wrong... I am getting so sick of this attitude and it's what keeps me from certain other boards with Cafe in the name....I strongly agree with function over form and the importance of making a bike mechanically sound before dumping money into esthetics HOWEVER.
Even with mechanical knowledge sorceing performance parts for these older UJMs are getting difficult. I see no requirement for someone posting a build thread focused on  estitics while also addressing mechanical issues. I think it's okay to take an old ugly bike from the past and keep it on the road and current with trends of the time and not chase some idea of performance that will never be achieved by the standards set by today's bikes. If someone takes a runner and tunes it up replaces worn parts and rotting rubber then starts making it look trendy where is the problem? Please do not look down on these people or call them less because they can't afford or have the desire to chase HP... can't it simply be a cafe because of the style the bike is aspiring to?

Psy

Sent from my SM-G903W using Tapatalk
Title: Re: &quot;Doing it Right&quot; or &quot;How to Build a Functional Café Racer&quot;
Post by: datadavid on Jul 09, 2016, 16:00:08
I will admit strait up I have not read the entire thread YET but I felt it was important to comment on some of the things the  OP stated even though it was 2012 he made many great points and responses have added may more and I whole heartedly support the positive tone his post took however I have a big issue with one of his main points. I seriously think the definition of "Cafe Racer" needs to evolve somewhat.. cafes racers of their day were brittish twins modern in their own time and cut and modified to remove the unnecessary and mod the essential for maximum performance. If we apply that to today it would be doing the same to a late model GSXR.. if your going to extend the term beyond its origins an include 70's and 80's UJMs then you can't apply other rules to the term either.  What am I going on about? The attitude that if you don't over bore and port and polish your doing it wrong... I am getting so sick of this attitude and it's what keeps me from certain other boards with Cafe in the name....I strongly agree with function over form and the importance of making a bike mechanically sound before dumping money into esthetics HOWEVER.
Even with mechanical knowledge sorceing performance parts for these older UJMs are getting difficult. I see no requirement for someone posting a build thread focused on  estitics while also addressing mechanical issues. I think it's okay to take an old ugly bike from the past and keep it on the road and current with trends of the time and not chase some idea of performance that will never be achieved by the standards set by today's bikes. If someone takes a runner and tunes it up replaces worn parts and rotting rubber then starts making it look trendy where is the problem? Please do not look down on these people or call them less because they can't afford or have the desire to chase HP... can't it simply be a cafe because of the style the bike is aspiring to?

Psy

Sent from my SM-G903W using Tapatalk
I just spent a ton of money porting, boring and polishing crap from the 70's and now you're telling me thats wrong??😨😢😤 i must go pick on someone with a non runner now..
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Psycrow on Jul 09, 2016, 16:17:25
I just spent a ton of money porting, boring and polishing crap from the 70's and now you're telling me thats wrong??😨😢😤 i must go pick on someone with a non runner now..
No I never said that was wrong... if that's what you want to do have at it and post a thread I'd love to read it! I'm saying stop looking down on and insulting the "body kit on a tractor" crowd or you'll be sitting here on a dead forum debating with the 15% that care about how you ported your head...
If you want to mod and throw cash at an old small displacement bike to see just how far you CAN push it that's fine but ...chasing HP gains on a CB360 for the sake of HP gains is a false economy..... sell and buy a 550... oh wait ... I meant a 750... oh... screw it I guess I'll get a Busa and strip it to look vintage....

Psy

Oh and according to some of these opinions (not mine) if you paid someone ELSE to do that work for you.... you ARE doing it wrong....

Sent from my SM-G903W using Tapatalk
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: Sonreir on Jul 09, 2016, 16:49:20
If you were in it for the economy, you wouldn't be building a café racer to begin with. You'd just buy it from someone else. ;)
Title: Re: "Doing it Right" or "How to Build a Functional Café Racer"
Post by: datadavid on Jul 09, 2016, 16:54:34
No I never said that was wrong... if that's what you want to do have at it and post a thread I'd love to read it! I'm saying stop looking down on and insulting the "body kit on a tractor" crowd or you'll be sitting here on a dead forum debating with the 15% that care about how you ported your head...
If you want to mod and throw cash at an old small displacement bike to see just how far you CAN push it that's fine but ...chasing HP gains on a CB360 for the sake of HP gains is a false economy..... sell and buy a 550... oh wait ... I meant a 750... oh... screw it I guess I'll get a Busa and strip it to look vintage....

Psy

Oh and according to some of these opinions (not mine) if you paid someone ELSE to do that work for you.... you ARE doing it wrong....

Sent from my SM-G903W using Tapatalk

Just kidding of course.. i do have a build thread though!

And in order to do all machine work yourself you would still have to buy equipment for millions, if you dont make your own mills and lathes and so on.. so i just pay some old dudes who wants me to take over their businesses eventually since they have no kids and im about the only 30 something in this land to have a genuine interest in vintage crap..
Btw all my bikes are tractors because of how i ride them😂 too many locked joints and vertebrae to properly control a vehicle here. If only someone would have told me to get an education and a cushy job instead of working steel all my life.