Originally Posted by wingracer

Pumping effect: basically on a two stroke, the crankcase acts like a fuel pump and since the air is involved too, sort of like a supercharger as well. The higher the compression ratio in the case, the more the effect though higher ratios also mean less volume so you lose on volumetric efficiency so around 1.5 to 1 seems to be the magic number.

The link:

http://edj.net/2stroke/jennings/