...this paragraph pretty much tells the tale:
"
MAP and RPM
To achieve a certain amount of power there are a number of MAP and RPM combinations available to the pilot that will have the same result. The lowest SFC, however, is reached with the
highest MAP and lowest RPM for that power setting. Low RPM reduces friction in the engine and improves the air intake (volumetric efficiency) and a high MAP also contributes to a good air intake (less restriction by the throttle valve). Limits are placed on MAP/RPM combinations by the engine manufacturer regarding the prevention of detonation. Just go by the book numbers and no harm is done."