Posted for "Larry Graves" <rlg34@attbi.com>:

We may be RPM-limited, but we aren't usually pitch-limited at full throttle.
Porting/polishing, ram air induction and turbocharging/supercharging all
increase horsepower because they increase manifold pressure and allow your
engine (air pump) to process more airmass and fuel. When this happens at max
RPM/tip speed, the prop just goes to a coarser pitch and accelerates even
more air back over the plane, thrusting it forward faster. This is an overly
simplistic answer -- many books have been written on the subject.

Best,

Larry Graves