Cowl flaps

From FlightSim

Cowl flaps are movable panels on the engine cowling which control the amount of air exiting the engine compartment. Since most reciprocating aircraft engines (piston engines) are air cooled, the amount and location of air passing over the engine has a large effect on engine cooling. While most low powered aircraft don't have cowl flaps, higher powered ships must have a way to allow the pilot to have some control over engine cooling, since it might get too hot on the ground or in a climb or it might get too cool in cruise or descent.

Thus cowl flaps can be opened for more cooling and closed for less cooling.