If you doubt that, just examine the equations and curves which Lycoming publishes to determine HP from known operating conditions.

Jack,

 

I did and they are virtually useless since I can't hold a ruler steady in turbulence.  Plus, I will never understand just how the chart examples are worth anything because of the limiting caveats.  Especially things like abs dry MAP, pressure altitude, standard temperature, certain compression ratio, no ram air, certain throttle body, state of engine, old fashioned oil, regular plugs, ordinary mags, certain injector size, ............. It doesn't even take into account true air temperature adjusted for compressability.  I have never been able to match the known operating conditions in my "experimental" airplane.

 

I don't care about exact calcs because nobody can tell me what horsepower I am generating without a "dyno".  Of course, that dyno can't simulate flight conditions very well.  Oh, you can try with SFC, etc, but this is no better than these other approximations (or thumb rules, if you wish).

 

Just look at the 75% power entries in most POHs and see if the formula is close enough.