RAM AIR is a forced induction system for automobiles. There are actually two different things which are known as ram air; Actual ram air systems, which compress air and force it into the throttle body, and thus the combustion chambers, and positive pressure systems, which increase pressure only in the airbox and improve intake efficiency. 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.
Both types of "ram air" system work because of inertia. While strictly speaking, the car is moving through the air, you can model this system as if the air were moving, and the car were standing still. The air has mass, and therefore inertia; It tends to keep moving until stopped. This force is sufficient to slightly compress the air, at which point it is fed into the cylinders.
There is a certain frequency at which air is taken into the cylinders, based on the opening and closing of the valves, and the movement of the pistons. This creates a wave harmonic effect in the intake. A positive pressure system helps improve intake efficiency by ensuring that there is always a ready source of air available to the engine, thus reducing loss due to friction. This is the kind of "ram air" system most commonly found on motorcycles and snowmobiles.
The other positive effect found in either type of system is that you have a source of colder air. Many cars draw their intake air from the underhood area, where it is heated considerably. This is most common on older, carburated vehicles, but not unheard of on newer cars. Colder air is denser, thus providing more Oxygen (O2).
Adding aftermarket ram air to a vehicle is possible, but not as straightforward as you might think. First of all, if your car does not use a MAF (Mass Air Flow) or MAP (Mass Air Pressure) sensor, and instead uses a timed system or a carburetor to control fuel delivery, ram air is likely to produce adverse results. Secondly, even a MAF sensor is ill-equipped to measure the airflow past it. Most MAF sensors are based on a hotwire system, or a bending reed. Either way, they are designed to measure the speed at which air is passing them, not the density of the air. A MAP sensor is more likely to work correctly because it measures pressure, and the compressed air will exert its force on the MAP.
No form of ram air provides very much of an improvement in horsepower. Even Formula 1 cars only gain about 20hp at 200kph. The 2002 Ram Air WS6 Firebird has only 15 more horsepower than the non-WS6 version, but it is also tuned differently from the non-ram-air version, so any claim that the power increase comes from the ram air effect is dubious at best.
On the other hand, what is true about so-called "ram air" systems is that some of them, when properly designed, may increase the pressure differential induced by the venturi effect in carburetors. Without getting into mathematics, a venturi is a restricted section of a carburetor (or any other system involving flow) which causes a reduction in pressure, resulting in a suction-like effect in which the intake air speeds up as it passes the restriction. This is what draws the fuel out of the bowl of the carburetor. Reducing the pressure should in theory reduce turbulence and thus reduce restriction, which means that there is less resistance when pulling air into the engine. This could translate into an increase in horsepower, but it could as easily turn into an increase as turbulent air attempts to flow through the smooth passages of the intakes.
- Littmann, Carl R. A Simplified Approach to Metal Tensile Strength Using Concepts of Guericke and Venturi. Carl R. Littmann, February 20, 2001. (http://www.causeeffect.org/articles/tensile.html)