Grayhawk,

Too late {:-) I already looked at it.  I compared it to the rough formula I have always used - % power = RPM/RPMmax * MAP/MAPmax  - and it comes within 1% of it.  I stopped there and didn't try to "reverse engineer" the formula, except I have hunch it has to do with assuming the max RPM is 2500 (the 2.5) and the max MAP is 35 inches (the 3.5).  Regardless, the formulas are not quite right.  As rpm increases, engine friction increases and volumetric efficiency decreases so the increase in power won't go up directly with RPM.  The opposite is true of MAP.  It takes a certain MAP just to power the engine (overcome friction) so power will increase more than the increase in MAP.  How much are these effects?  Without real data it would be hard to say, but in the RPM and MAP ranges that are useful in aircraft engines the factor for rpm might be something like 0.8. For MAP there is a subtractive term of about 10 inches.  The equation would then look like  %power = (RPM/RPMmax*0.8) * (MAP-10)/MAPmax-10).  Gets hard to do in your head (well, mine anyway), so I just remember that a 10% reduction in RPM will drop the power less than 10%, but a 10% drop in MAP will drop the power by more than 10%.  Then there is the effect of altitude..

Gary