2 stroke exhaust pipe

The two stroke exhaust pipe is a very important tuning tool of a two stroke engine. A stock two cycle engine is usually tuned to make reasonable power across the power band. Aftermarket pipes are called expansion chambers because of the way the pipe expands in the center. A 2 stroke engine has a very strong exhaust pulse that occurs as the piston passes the flat top of the exhaust port. The quick pressure pulse moves quickly down the pipe and creates a shock wave as it exits the end of the pipe. Part of this strong pulse wave travels back up the pipe and if timed correctly bounces off the piston as it travels downward.

 

As the piston travels downward and the secondary wave starts to travel away from piston again it creates a vacuum there. When the piston uncovers the exhaust port the vacuum pulls the exhaust out of the cylinder and part of the fresh intake charge into the cylinder. This creates a supercharging effect and dramatically increases the power. The timing of the pressure wave is extremely critical and generally only makes more power in a smaller band of power. Although the power band becomes more narrow and usually higher in the rpm range, it is worth it because of the large amount of power this enhancement will add. The reason the pipe is expanded is to simulate a much longer pipe. A correct length pipe would be about 30 feet long if not expanded. This would not fit too well on your motorcycle or snowmobile. An expansion chamber will not work on a four stroke engine because the combustion only occurs every other rotation not every as a two stroke engine. Also the pressure wave is not as strong in the four stroke as it passes the exhaust valve. The silencer can be replaced but will show gains similar to a muffler on a four stroke.