"Originally Posted by panic
According to David Vizard, in a race engine with intake and exhaust lengths tuned for a harmonic resonance to assist cylinder fill, the returning exhaust pulse is stronger than the piston's draw.
Here's what that means:
"In the case of the currently-flowing header primary, the EVO-initiated positive pressure (compression) wave is reflected back as a negative pressure (expansion) wave. If the arrival of the reflected negative pressure wave back at the exhaust valve can be arranged to occur during the latter part of the exhaust cycle, the resulting lower pressure in the port will enhance the removal of exhaust gas from the cylinder, and will reduce the pressure in the cylinder so that when the intake valve opens, the low pressure in the cylinder begins moving fresh charge into the cylinder while the piston is slowing to a stop at TDC."
The quote is from this article that fully explains how exhaust systems work and how they benefit performance when properly executed:http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm
2-stroke expansion chamber design is based on the same principals applied differently. It's all basic physics and really interesting stuff.