The Purpose of the Air Induction System for a Honda Motorcycle

Most motorcycle engines suck a mixture of gas and air into the combustion chamber by vacuum. Of the four separate strokes in a four-stroke motor, (intake, compression, ignition/power and exhaust) the intake stroke--with momentum from the power stroke--creates the vacuum. The vacuum pulls air through the air intake, with cause fuel to be atomized into a combustible mixture. Many Honda motorcycles, the CBR line in particular, use a special air-induction system to build power and responsiveness.

Forced Induction

• Motorcycle engines that use vacuum for air intake are called "normally aspirated." Other engines push air into the engine, pushing more gas/air into it, revving it faster and creating more horsepower. Turbo-chargers and superchargers are the two most common types of forced-air induction in cars, but motorcycles seldom use them. Honda's CBR line uses ram air induction to effect "forced aspiration."

Plenum

• A plenum is an container connected to a carburetor; it collects and distributes air to the cylinders. An enclosed metal air filter on old V8 car engine is an example of a plenum. It collects air and connects to the carburetor, which connects to an intake manifold. Plenums are intended to be pressurized, and that's what ram air induction does: pressurizes a plenum (or airbox).

Pressure Differentials

• The atmosphere has an ambient or "normal" pressure. If, for example, there are two rooms in a house, both sealed air-tight and they both have ambient pressure, if you open the door between the two rooms, no air will rush from one to the other to equalize the pressure; the pressure is already the same. But imagine forcing air into one of the rooms with a powerful air pump. It would create pressure in the room. Because the pressure is greater than the ambient pressure, it will look for a way to escape. If you open the door to the other room, air from the pressurized room will rush into the unpressurized room until the air pressure has equalized between the two rooms. Now, imagine the pressurized room is the plenum, the non-pressurized room is the cylinder, and the door is the intake valve; open the valve and a gas/air mixture will rush in.

Ram Air Induction

• Ram air induction is just another way to create positive pressure in a plenum. It has a slight supercharging effect. One or more air scoops or cowls, facing forward, gather air as the motorcycle accelerates. The air is funneled into a tube or chamber, then fed into the plenum. If air is fed into the plenum at a rate faster than the engine can consume it, it will create positive pressure and more horsepower. Superchargers and turbochargers are capable of doubling an engine's power. A ram air induction simply takes advantage of wind to add a little positive pressure in the plenum. It's an inexpensive way of achieving mild supercharging with no moving parts. It can achieve power gains of approximately 3 to 7 percent.