Ram Air

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 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 that 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. While ram-air may increase the volumetric efficiency of an engine, they can be difficult to combine with carburetors, which rely on a venturi-engendered pressure drop to draw fuel through the main jet. As the pressurised ram-air may kill this venturi effect, the carburetor will need to be designed to take this into account; or the engine may need fuel-injection.

By now private research had come up with figures, quite unimpressive for some:No OEM pressurised airbox could manage a single psi of 'boost' even at top speed.The best ones would max out at less than 0.5 psi, with 0.2 psi more common at around 100mph.

So is THAT it? 0.2psi? big deal

But look carefully and let's see what that is compared to: A non-ramair airbox breathes from the side or a low-pressure area, which is probably also affected by engine heat. A ramair box is stuffed with fresh air exactly at ambient temperature. So what's the difference then?

As a rule of thumb, for every 4C that air temp goes up, 1% of power is lost. It's easy to have the airbox temp go up by 4C, sometimes a lot more. A ram-air box is always cooler. More importantly, a non-pressurised airbox is typically under vacuum, and under load at high revs that is several psi worth of vacuum. People think that it's atmospheric pressure in there, but it isn't. Just calculate how many liters of air are demanded to flow when the throttle is wide open, and you'll realise that the airbox is empty within milliseconds. It takes time to replenish these reserves, typically almost a second (with the throttle closed). If the throttle is kept wide open, the 'replenishment' rate is far lower than the engine's air consumption, so the airbox remains in deep vacuum all through the acceleration process. The higher the engine speed, the deeper the vacuum.

That's where a RAMair intake comes in. It doesn't pressurise more than a few tenths of a psi during cruise conditions, but when the throttle is whacked open, the vacuum appears a bit later - maybe a second later. That feels like better throttle response, because that's what it is. Even then the vacuum is always lower, because now the replenishment rate is much closer to the engine's consumption rate.

It's under these dynamic conditions that RAMair shows it's teeth, but that's near impossible to replicate on a dyno. Natural 150mph airflow cannot be replicated by fans, even big ducted ones. Natural airflow isn't preheated, either, unlike fans.

When this replenishment rate is combined with the lowest possible intake temperatures, 5% power increases are typical, with over 10% in some cases. The smaller the airbox (compared to the engine's bhp) the easier it is to feel these gains, and at 100mph part-throttle response can be much improved because the airbox won't be in deep vacuum at all.

Sources: Wikipedia, Max-boost