Once the engine is spooled up, (with the ideal exhaust and zero back pressure
muffler) the exhaust is leaving at supersonic velocity and leaves behind it a
slight vacuum. So the overlap that makes it a dog down low, is making it a
Bear in the operating range. The low pressure in front of the inlet runner pulls
mixture down the face of the rotor near the 100% mark, and instead of a loss
we get a bonus amount of mixture. That is called "blow down." The added
velocity can overcome more of the reverse flow and overfill the chamber.

This is why when there were no noise restrictions the racers ran full megaphone
exhausts. The expanding area used allows the "zero restriction muffler" idea to come
into play.
Bill Jepson

 
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