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Author Topic: "Doing it Right" or "How to Build a Functional Café Racer"  (Read 154923 times)

Offline veloracermike

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #20 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. 
« Last Edit: Aug 04, 2012, 15:55:17 by veloracermike »
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Offline legendary_rider

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #21 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.
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Offline Sonreir

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #22 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.


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.

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.
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Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #23 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.

Offline snmavridis

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #24 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!

Offline juan@crqcycles

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #25 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!

Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #26 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

Offline kopcicle

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #27 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 .

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If it leaks after all this? I'm gonna pull a "Brad" and bulldoze the fucker!

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Offline kopcicle

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"If a man wants to carry two cats home by their tails, by all means let him. He’ll learn things that he might not have otherwise even guessed, and the experience will be one he’ll not soon forget!"
~S. Clemens

If it leaks after all this? I'm gonna pull a "Brad" and bulldoze the fucker!

Quote from: Scruffy;1025553
As an equal opportunity bigot, I'll insult any brand, any style, any time...:wink:
-Scruffy

http://tinyurl.com/TheRedBike

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Offline Sonreir

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #29 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:
  • Displacement - A bigger engine creates more power than a smaller one.  Add cubic centimeters to your displacement by increasing the stoke, bore, or both.
  • RPMs - Power is force over time and so power is derived from torque.  Spinning the engine faster means more power so long as you can keep the torque up and the losses down.  Probably a losing game in the long run, but it can be fun to chase for revs.
  • Reduce Parasitic Losses - Make it lighter, make it smoother, make it balanced, make it slippery.  All things take energy to move and so keep the energy losses to a minimum.

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:

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.


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.
Sparck Moto - http://www.sparckmoto.com

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