Engine wear and fuel savings can be achieved by reducing RPM and increasing manifold air pressure (MAP) to maintain horsepower. Modern engines have approved power ranges for which this practice is allowed. These ranges are in the POH but can also be found in the engine manufacturer's handbook.
Seventeen page article on engines and performance.
Excerpt from the document:
Power produced is proportional to the air density at the intake manifold, the cylinder displacement and compression ratio, the number of cylinders, and the RPM. Of those items, only the air density at the intake manifold and the engine RPM alter, or can be altered, during flight. (With a normally aspirated engine and a propeller whose pitch is not variable in flight, the throttle controls manifold pressure, which then determines RPM.)
A traditional four-stroke light aircraft engine, such as the Lycoming O-235, has an individual cylinder displacement of 950 cc, a compression ratio of 7:1 and a maximum design speed of 2600 RPM, at which its rated 110 bhp is produced — in sea-level ISA conditions.
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