The formula is a little misleading in that it does not take in to
consideration all of the variables necessary to calculate true %HP.  It is a
nice formula to use for general discussion.  I would not recommend using it
for setting fuel flows or for flight operations.

> industry standard (verifable in any STC POH) is: (RPM/2700)*(MAP/29)=HP

Joe,

 

Actually, I made a small mistake, it should be:

 

((RPM/2700)*(MAP/29))*100=%HP as rated at sea level, standard conditions.

 

This was the formula frequently used by aircraft manufacturers to develop their POH tables.  It is close enough for fuel flows and flight operations.

 

This formula has finally helped me manage my engine better - an engine of unknown horsepower because of "adjustments" - by reference to actions relevant to % horsepower.  I carry just such a printed and plasticized excel spreadsheet with me at all times so I can get a "best estimate" of power.

 

Using the Lycoming Engine manual and spending about 10 minutes carefully charting 25 squared for a 320-D, the chart came up with 80.6% power at 3000 feet pressure altitude (+ or - depending on the pen line width) where as the formula came up with 80%.  Additionally, the notation from the engine manual says that there is a "...1% correction for each 10F variation from the standard temperature.  The interesting part of the chart is that it shows a decrease in HP at lower altitudes if the standard temperature is used?  BTW, my very effective induction ram air recovery is not considered in the engine manual nor are the high compression pistons.