On May 21, 5:21 am, Stealth Pilot
wrote:

my docs say "rated horsepower" which makes me think that there is a
calculation used to derive the figure not an actual 2750rpm
measurement. the 74hp at 2500 rpm seems to be measured.

They normally take a brake horsepower reading, with the engine at
redline RPM and full throttle. Torque times RPM times 6.28 divided by
33,000 gives HP.

BUT:
The O-200 makes its 100 hp at 2750. The C-90 its 90 hp at 2475.
Everyone knows that drag increases by the square of the increase in
speed, so the O-200's prop theoretically has 19% more drag than the
C-90's. The C-150's prop is narrower and I think a bit shorter than
the prop on the Aircoupe I flew with its C-90, so its drag might be a
bit less to start with, but overall the losses will still be higher at
2750.
So adding RPM to gain hp is a poor way to go. That's why PSRUs
are better than direct-driving a tiny prop at 4000 RPM. And it's also
why a 110-hp Corvair, if it's running at 3300 or some such RPM, isn't
going to outdo an O-200.

Dan