The basic idea of motorcycle ram-air is to provide more air for the engine without the added weight and complexity of a forced-induction system like a turbo or supercharger. As the bike moves forward at speed it pushes a wedge of pressurised air before it, some of which is channelled into fairing-mounted ducts.
|Kawasaki ZX-10R’s air scoop is worth 10-15 BHP at high speed|
Although the phrase ram-air makes you think it’s an intake system with ducts out in the airflow that simply shovel more air into the inlet tracts, it’s a little more subtle than that. Once past the intakes the cross-sectional area of the ducting increases as it is routed into the airbox or plenum chamber. This reduces the air velocity and as the air slows down its dynamic pressure drops, but the static pressure, and therefore its density, increases and boosts the engine’s volumetric efficiency.
To use this increased gas charge, fuel injection systems run different fuelling maps in higher gears at high engine rpm with air pressure sensors in the intake that adjust fuel injector duration to suit.
In terms of out-and-out performance, a ram-air system only really comes into its own at speeds above 120mph, where as much as 10-15bhp extra can be produced on a modern superbike armed with a huge central-mounted ram-air scoop like a Kawasaki ZX-10R or BMW S1000RR.