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. Ram-air has been a feature on some cars since the late sixties, but fell out of favor in the seventies, and has only recently made a comeback. Modern parachutes use a ram-air system to pressurise a series of cells to provide the aerofoil shape.
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 to:
- Air filter
- Cold air intake
- Warm air intake
- Short ram air intake
- Diffuser (automotive)