see title...

 

There is more overlap between the intake and exhaust, so at low speeds some of the intake charge can "leak out" the exhaust port. The less air you have to compress, the less compression you'll have.

Once the motor gets up to speed it's less of an issue - the rotor is turning fast enough that there isn't enough time for the overlap to cause intake dilution. You can also, in some instances, get a scavenging effect where the exiting exhaust gasses help suck in more intake air, giving a fuller charge at higher RPM.

Dale

 

ooOOoohh.... i get it.... thanks dale

~Jeremy