Why Manifold Pressure for Fixed Pitch Props?
Many would think that the manifold pressure sensor is most suited for constant speed prop equipped airplanes. While this may be feasible with production airplanes which include power setting charts, kitplanes typically do not have this information, if for no other reason than the vast combinations of engines, props, and factors affecting drag. This leaves the fixed pitch propeller user with the need to determine engine power when cruising at altitudes below about 8-9000 feet. Above this altitude, wide open throttle produces less than 75% power, making leaning safe. Below this altitude, the manifold pressure is necessary for accurate determination of the power setting to allow for safe leaning.