[admin]

Isn't that why Yamaha have the exup exhaust valve to limit or alter the cross section of the exhaust.

Two stroke exhausts are often called expansion chambers and their shape alters the pressure of the exhaust pulse and then reflects the pulse to act as a barrier to stop unburnt mixture leaving the exhaust port. Very technical stuff. This tends to work best at a certain rpm, which is why 2 strokes often have very narrow power bands.

I think the same holds true for 4 strokes but the gains are less impressive.