EDIT: MAY 2014:
This is an article I wrote 5 years ago, since then I've learned a lot and realized that primary compression is a lot more complicated in its overall effect on an engine.
I don't necessarily agree with all the comments, but I don't really have time (and still probably don't know enough) to properly discuss or refute them. At face value this is a pretty basic article describing the basic crankcase pumping operation on a two stroke. I could take it down, but it helps to remember how far I've come and how much I still have to learn, so I'll leave it, but use it at your own risk.
I originally wrote this for a moped army forum thread located here: http://www.mopedarmy.com/forums/discuss/7/189038/189038/
With the increase of availability of moped parts here in the U.S., we've seen a huge boom in the technical sophistication of domestic tuners. With this availability, everyone seems to be pushing the envelope further, trying to squeeze out every last iota of power from our little machines.
A good example of this is the availability of 'stuffed' cranks. Many of us have purchased them for absurdly cheap through the US moped parts sellers, having only a vague idea of what it really means to 'stuff' a crankcase, why it can produce more power, and what the implications are for the rest of the engine.
Crankcase stuffing is decreasing the volume of the crankcase to improve what is called the 'primary compression ratio'- the compression ratio for the system comprising the bottom of the piston, crankcase, and transfer ports up to the edge of the piston.
Primary compression is defined as Vi/Vf which is:
volume final divided by volume iniitial.
final volume is the displacement volume of the piston plus the volume of the case. initial volume is just the volume of the case.
when doing this for a cylinder, the head volume is relatively small, so we can achieve 11:1 (11/1 = 50cc disp + 5 cc head volume/ 5 cc head vol.) or better. When we do this same thing in the case by either increasing the displacement, or decreasing the crankcase volume (say, by stuffing the crank) we can get better compression ratios...
so what? right? why does changing the compression ratio help us?
having a higher crankcase compression ratio lets us get more air in during the intake phase, and creates higher pressure pushing that air through the transfers.
This is obvious when we go back to high school chemistry and remember PV=T(nr): Pressure times volume equals temperature, if you decrease the volume of an area, the pressure or temperature MUST increase to compensate. In the very short time that the piston is going up and down, we can assume the temp cant change much.
This makes sense for the second part, the transfer pressure. The higher pressure from a smaller final volume will push the air through the transfer ports at a higher velocity, improving scavenging, and transferring more air in the very short time the ports are open. But this only makes sense if we are getting more air in the crankcase in the first place. The displacement of the piston isn't changing, so we cant change how much air gets into the crankcase right? Wrong.
The higher ratio means that when the piston goes up and creates a vaccum in the crankcase, the vaccum is stronger. Even though transfer pressures can be as high as 50 PSI, the intake pressure pushing air through the carb and past your reeds is only 1 bar, because it is being pushed in by the atmosphere. You want to get as close to a perfect vaccuum as possible to get air to transfer as quickly as possible. (This is why piston port engines can blast reeds at the top end, the piston port has much less resistance to flow, but i digress. ) Having a compression ratio of infinity, as in 0 crankcase volume, with no leaks, would create a 'perfect' vaccuum, This would provide 32 PSI across your intake tract from the air filter in. In reality this is impossible, the ratio drops very quickly with increase in volume in the case, and the potential transfer pressure drops from 32 even faster, by PV=T. The time your piston is going up is limited by your RPMs, and the amount of air that can transfer is limited by the pressure, restrictions in the intake, and time. Therefore, if you can make more pressure (by improving the vaccuum in the case) you can get more air in the case on each stroke, which means you get more air into your cylinder, and can burn more fuel.