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Author Topic: "Doing it Right" or "How to Build a Functional Café Racer"  (Read 127815 times)

Offline miken5678

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #30 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.

Offline scm

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #31 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
someone built it anyway ...

Offline Sonreir

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #32 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".
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Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #33 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.

Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #34 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.

:-) 

Offline Sonreir

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #35 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.
« Last Edit: Sep 12, 2012, 13:25:23 by Sonreir »
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Offline Sonreir

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #36 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.
« Last Edit: Aug 29, 2012, 16:19:12 by Sonreir »
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Audaces fortuna iuvat.

1977 Honda CJ360 - Café SOS - Stage One™, Café SOS - Stage Two™
1976 Puch Maxi - APuchalypse Now
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Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #37 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. :-)

Offline Sonreir

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #38 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.

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:

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.
  • High compression engines are usually easier to start from a thermodynamic standpoint, but are not as easy to kick over
  • Aluminum heads and/or liquid cooling will usually withstand one full compression point higher than air cooled or iron headed engines can endure
  • Undersquare engines can handle more compression than oversquare
  • A good rule of thumb for an engine running on pump gas (premium, of course) is not to exceed 200 PSI on a compression tester
  • High compression engines have an AWESOME feel.  They want to run and rev and are much more responsive to changes in throttle position.
  • If you plan on running a hot cam, an increase in compression will almost always be required.  This helps to gain some of the bottom end power back
  • If a change in gasket is required, copper is the most commonly used material.  It's a bitch to seal against aluminum though, so skimming a bit of metal from the sealing surfaces is highly recommended
  • If you've gone too far on your compression it is entirely possible to then use thicker gaskets to help lower the compression a little bit.  Retarding the ignition timing and running rich on the fueling are also options for combating too much compression, but neither are very desirable.  Thicker gaskets can help when clearances become to tight, as well.

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

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Offline Sonreir

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #39 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.
Sparck Moto - http://www.sparckmoto.com

Audaces fortuna iuvat.

1977 Honda CJ360 - Café SOS - Stage One™, Café SOS - Stage Two™
1976 Puch Maxi - APuchalypse Now
Suzi T500 Cobra Resto

Custom Gauge Graphics
Custom Wiring Harnesses

DTT Red, White, and/or Black 360 Club - Better than those Blue guys