Ram air system.

A ram-air intake is any intake design which uses the dynamic air pressure created by vehicle motion to increase the static air pressure inside of the intake manifold on an engine, thus allowing a greater massflow through the engine and hence increasing engine power.

The ram air intake works by reducing the intake air velocity by increasing the cross sectional area of the intake ducting. When gas velocity goes down the dynamic pressure is reduced while the static pressure is increased. The increased static pressure in the plenum chamber has a positive effect on engine power, both because of the pressure itself and the increased air density this higher pressure gives.

Ram-air systems are used on high performance vehicles, most often on motorcycles and race cars.

At low speeds (subsonic speeds) increases in static pressure are however limited to a few percent. Given that the air velocity is reduced to zero without losses the pressure increase can be calculated according. The lack of losses also means without heating the air. Thus a ram-air intake also is a cold air intake. It should be noted that in some cars the intake is placed behind the radiator, where not only the air is hot, but the pressure is below ambient pressure.