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.
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.
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 24 hours.
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