Here is rough way to calculate the power, probably within 10% and maybe 5%:  Starting with HP=Torque(ft-lb)xrpm/5252, replace torque with MAP:  HP=MAPxrpm/K where K is a constant ( 30" and 2700 rpm produces 180 hp giving a K of .00222, for example). But then none of that is really accurate.  Torque changes more than proportional to MAP because there is some friction HP to be accounted for.  You could subtract something from 30 - a number between 5 and 10 would be appropriate.  Then HP changes slower than a simple rpm multiplier would suggest, partly because engine friction increases faster than rpm.  Then torque will change with ambient temperature - a change of 50F will change torque by about 10%.  That isn't even too accurate because the air will be heated on its way to the cylinder - a number of maybe 5% per 50 degrees would be closer (1% per 10 degrees).  And all that is true only if you are running best power mixture.  When LOP the best measure of power is fuel flow - I think Paul has a number for this, which I forgot.  What I do is memorize a number for 75% power.  Turns out it is about 24"-2400 rpm (picked a "square" number because it is easier to remember).  For every inch the manifold pressure changes the rpm has to change 100 to compensate, so 75% is at 23"-2500rpm and 25"-2300rpm).  But because of the errors mentioned above 21"-2700rpm will NOT produce as much power as 26"-2200rpm.  Then add or subtract 1% for every 10 degrees the temp changes from 60F (NOT from standard conditions at that altitude).  Or one could create a chart with the above information to which one could refer.  Or, as in my case, with a NA engine (Naturally Aspirated, not "Normally" as some would say.  After all, the turbo enthusiasts would insist that boosted engines are more "normal".) I need all the power I can get most of the time, so it really doesn't matter - I can't do anything about it anyway.

Gary,

 

Try this calculator and see what the dew point does to % power.

 

 

Grayhawk

 

PS:  I have an inexpensive instrument that I stick into the cabin air inlet in flight to measure the dew point or % humidity.  Temperature isn't enough.

 

PPS: The sq rt of .75 is .866.  86.6% of 30" is 26" and 86.6% of 2700 is 2300.  But since I fly at 2500 rpm, using your adjustment, then a MAP of 24" would approximate 75% power.  Since I also like to fly at WOT and anytime I see a MAP that low, I must be at or below 75% power.  Normally that would be at or above 6500 MSL.  Since I also use ram air induction at around 175 KIAS at that altitude, I have to go up another 1000 feet and fly in the opposite direction (towards O'Hare) in order to conduct the GAMI lean test.  Bummer.