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This month I wanted to share an exert from the Race and Performance Engine Building chapter in my book, The Four Stroke Dirt Bike Engine Building Handbook. If you've been wondering how high compression pistons work and if they are right for your application, read on!

Piston upgrades are normally considered when changing the compression ratio is desired or larger valves are installed. In both instances the shape of the piston is altered either to reduce the volume in the combustion chamber or to allocate additional room for larger valve pockets.

The compression ratio defines how much the original air/fuel mixture which was sucked into the engine is compressed. The following equation shows how an engine’s compression ratio can be calculated.

The swept volume is the volume that the piston displaces as it moves through its stroke. Mathematically this volume can be determined using the following equation.

The clearance volume is the volume of the combustion chamber when the piston is at top dead center (TDC). While manufacturers specify what the compression ratio should be, due to subtleties in manufacturing, parts vary slightly from engine to engine so finding the exact clearance volume of your engine actually requires measuring the clearance volume.

Undoubtedly you have probably heard that raising the compression ratio will increase the power of an engine. This is definitely true, however you should be aware of the other consequences that come along with this.

The more the air/fuel mixture can be compressed before it is combusted, the more energy which can be extracted from it. The reason for this is due to thermodynamic laws. In summary, the temperature difference between the combusted mixture when it is hottest and coolest determines the power and efficiency of the engine. The hottest point of the mixture will arrive shortly after the mixture has been ignited and the coolest point will occur around the point where the exhaust valves open. Since the temperature of a gas increases as its volume decreases, it is easy to see how increasing the compression ratio increases the overall combustion temperature. Something less obvious is that because the gases are compressed more, they will expand more and actually be cooler by the time the exhaust valves open.

If increasing the temperature of the compressed mixture is good, you might be wondering what keeps us from raising it higher and higher. Detonation, which is a by product of the additional heat and pressure in the combustion chamber, is the main reason the compression ratio can’t be increased beyond a certain point. Detonation occurs after the spark plug has ignited the air/fuel mixture. Normally once the spark has ignited the mixture, the flame will propagate outwards from the spark plug evenly in all directions. When detonation occurs some of the remaining air/fuel mixture situated towards the edges of the combustion chamber spontaneously combusts before the flame reaches it. When this happens a large spike in combustion pressure occurs. If severe enough detonation can cause engine damage in the form of pitting on the piston crown, broken ring lands, and scuffing of the piston from overheating.

To combat detonation there are a few different parameters which can be tweaked to help alleviate the problem. The air/fuel ratio can be altered along with the engine’s ignition timing to change the peak combustion temperatures, a fuel with a higher octane rating can be used which will be more resistant to detonation, and upgrades to the cooling system can be carried out to help keep the combustion chamber cooler.

Along with increasing the likelihood of detonation as a result of increasing the compression ratio, the engine will also produce more heat. The cooling system must absorb this additional heat and be able to adequately cool the engine, otherwise overheating and detonation may be problematic. Radiator size, thickness, and the speeds at which you ride at all play a big role in how efficiently the cooling system operates.

Now that you have an understanding of how high compression pistons affect performance, you can consider if this will be a good modification for you. Aftermarket pistons are usually offered in a few different compression ratio increases. You will want to look closely to see if any high octane fuels will be required to use in conjunction with the piston and if any cooling system improvements are necessary.

For racers looking to extract all the power from their bike, adding a high compression piston is one of the things that will be necessary. If you do a lot of tight woods riding, hare scramble racing, or enduros where low speeds are the norm, you may want to shy away from raising the compression ratio as the cooling system will have difficulty dealing with the increased heat at low speeds where airflow is limited.

I hope you enjoyed this excerpt on piston modifications and how they affect an engine. If you liked this write up and are interested in learning more about performance options and four stroke engine building, pick up a copy of my book. Right now the book is on sale at 20% off our list price when you order within the next two weeks.

You can grab your discounted copy off our site here: The Four Stroke Dirt Bike Engine Building Handbook Or on Amazon: Amazon Book Store

Thanks for reading and have a great week!

-Paul

Great article from Paul as usual.  Speaking from experience I would really think twice before going to a hi-comp piston.  I ride an 01 KTM 520 EXC that the previous owner added the factory 570 stroker kit to.  Nice bike but was virtually unrideable in tight woods/technical situations here in Colorado in the heat of the summer.  Sure some of that may be due to altitude but a factory fan kit, coolant overflow tank, and oversize water pump later (\$250+) the bike now runs very cool.  Ironically I have a buddy with a stock '02-520 EXC (no cooling fan-no overheat issues) and I notice no difference between the two off throttle and in 1-2 gear riding.  Sure my bike is scary fast 4-5-6 but who cares as I rarely ride in these gears.  Just my .02.

Awesome post, as always.

Thanks guys, glad you enjoyed it!

Glad to be on board I have a 685 kit in my KLR made a big diff

It should be noted engines don't suck in air/fuel mixtures. Atmospheric pressure does the work by way of the engine creating low pressure via displacement.

Compression ratios are typically described by swept, static, and total volume.

It should be noted there is a direct correlation to valve timing and displacement in any compression ratio calculation. The effective or corrected compression is the true goal.

Hi octane fuels are a trade off. The additives typically reduce the effective thermal output of the combustion process. There is simply more energy in lower octane gasoline. Though effectively utilizing it requires more sophisticated measures.

Ignition timing and air/fuel ratio/charge density all are relative in the compression scheme. Increased thermal/combustion efficiency requires a corresponding change in tuning.

Once these principles are addressed, real gains can be achieved. This separates the men from the boys. Imho.

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