do an internet search on "exhaust scavenging."
the pulses in the exhaust create vacuum to help scavenge (or pull) exhaust out of the motor. when you lose that backpressure, you lose that 'suction' force.
that is why crotchrockets for awhile now have had valves in the exhaust to maintain backpressure at lower RPM, while allowing better flow at higher RPM.
|