"Doing it Right" or "How to Build a Functional Café Racer"

teazer said:
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.
teazer said:
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.
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 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.

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.
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.
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!
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?
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://powroll.com/P_HONDA_VINTAGE_100-125.htm for example
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 .

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.
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.
... 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
scm said:
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

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".
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.
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.

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.
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.
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. :)
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.

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?
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.
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