prop rpm question

I believe
there is more to it than just VE. I don't believe that bearing friction
is linear with RPM for example. Also, speed of the flame front becomes
and issue at higher RPM. I believe the drop-off in torque with RPM is a
function of a number of factors.

Yup you're right, there's more than just volumetric efficiency, but
flame front speed in these slow engines is still around 100 feet per
second, while average piston speed won't be much over 40 or 50 fps with
the midpoint travel being somewhat higher. The intake and exhaust
systems present more drag at higher RPMs and start to affect the
performance, and in many modern auto engines four valves per cylinder
are used to ease breathing.
I wonder if the new direct-drive diesel aircraft engines have much
higher torques in the right places?

Dan