Originally Posted by
usmc-sgt
The old rule of thumb is to keep your manifold pressure lower than your prop rpm such as crusing at 20" MAP and 2300 RPM. Also the same reason they say when you add power you go from right to left such as increasing your prop rpm to 2500 first and then adding throttle to get your MAP to 25" and the same with descent to ensure that you are always in that safe envelope they claim.
In actuality when you look into the POHs for most constant speed prop aircraft you will find many power settings where best performance and the factory reccomended setting requires an over square condition such as maybe having MAP at 25" and RPM at 2400.
Definitely still a good practice during climbs and descents, but cruise, the AFM/POH will have far better info. I usually pick the most optimum setting for what I am doing that has the lowest RPM. Often times provides for a quieter ride with the same BHP as a low MP, high RPM setting.
On another thought, I would always use the best power setting on the mixture which was usually slightly rich of peak. However, I read somewhere, can't remember where (Pilot Magazine?) of someone who at low altitudes would use slightly rich of peak, then at high altitudes, slightly lean of peak. His philosophy being that in the thinner air you will have less BHP so you would not be doing the damage that you would be doing at high BHP values at low altitude, and in turn would be getting better fuel economy without sacrificing maintenance costs. Thoughts?