Quote:
Originally Posted by Gurkha
http://en.wikipedia.org/wiki/EXUP
this is beneficial to the performance of the engine at high RPM, but detrimental to performance at low RPM. Closing of the valve then creates back pressure inside the exhaust system, forcing down the piston
Back pressure is a must in four stroke as well although 2 strokes would die without an expansion chamber.
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I call BS on this. If you change the exhaust on any engine, you will have to change the mixture to suit (especially for a carb).
If four-stroke engines need back-pressure, why do the fastest engines out there run with no mufflers?