A ram-air intake is any intake design which uses the dynamic air pressure produced by vehicle motion to increase the static air pressure inside of the intake manifold on an engine, therefore allowing a greater massflow through the engine and therefore 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 whereas the static pressure is increased. The increased static pressure in the plenum chamber has a positive effect on engine power, both due to 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 performance cars. Ram-air was a feature on some cars in the sixties, falling out of favor in the seventies, but recently making a comeback. whereas ram-air may increase the volumetric efficiency of an engine, they can be complex to combine with carburetters, which rely on a venturi-engendered pressure drop to draw fuel by the main jet. As the pressurised ram-air may kill this venturi effect, the carb will require to be designed to take this into account; or the engine may require fuel-injection.