Hacked about, chopped up and often simply discarded, the airbox is one of the least understood and also most under-appreciated assemblies on a motorcycle. It's right in the front line and ripe for attack usually involve butchery at best and disposal at worst.
At their most basic, airboxes are little more than somewhere to keep the air filter. But as motorcycle designs have evolved to deliver greater all-round efficiency through better performance, less noise and reduced emissions, the airbox has had an increasingly sophisticated role to play as an integral part of the air/fuel system and even to aid crankcase breathing. It's also still somewhere to keep the air filter.
The primary purpose of the airbox is to provide the fuel system with clean air in suffient quantity for the incoming charge. Engines don't like dirty air, so the filter element prevents the ingress of abrasive airborne particles. An engine also prefers to have a reasonably constant supply of air. To this end, airboxes have been increasingly designed to act as plenum chamber, providing still air at a fairly constant pressure whether the motorcycle is traveling slowly or quickly. This means that fueling can be consistently maintained; something that's certainly desirable for a four-stroke and absolutely essential for a mixture-strength-sensitive two-stroke.
That's because a four-stroke pulls the fuel-air mix into the combustion chamber in a far more positive way that the crankcase-induction two-stroke does, if the airbox wasn't acting as a plenum chamber, mixture strength would suffer as airspeed and pressure around the engine fluctuate.
For decades, conventional practice was to have a vertical or near-vertical airbox hanging off the similarly oriented carburetors, feeding horizontal or near-horizontal inlets. This relied on a combination of basic atmospheric pressure, plus the pull of engine induction to supply the air. This of course, put the carburetor and airbox in the hot spot behind the ingine, out of the way of the oncoming air the motorcycle was carving through. Suzuki saw a way to make something from a lost opportunity with their early Ram-Air system, as seen on the two-stroke GT series of the early '70s, but rather than pressurizing the airbox, the head-mounted shrouds caught and directed more cool air to the area behind the engine where the carburetors and airbox resided. The shape of the shroud meant that the air pressure would be equalized to an extent as speed increased.
Increasingly inclined engines and downdraught carburetors saw airboxes begin to move toward the top of the engine until they assumed the position under the fuel tank we're familiar with now. This orientation makes the forward-facing intake an obvious move. Systems incorporating ducts and scoops and even using the frame – for example, Kawasaki's ZZ-R1100 – as part of the air induction system emerged. These made it possible to reliably deliver higher pressure within the airbox.
Suzuki's SRAD series provides an example of further sophistication. These models had a flap in the airbox to influence flow in accordance with engine speed. This remained closed until engine speed increased significantly, when it would be opened by a rod attached to a diaphragm that was connected to a vacuum take-off on the inlet manifold. The amount of vacuum exerted on the diaphragm was dictated by an ECU-controlled solenoid valve. It's all a long way from a plastic or steel box with a foam filter inside. The system offered the promise of more accurate control of air speed and pressure, dependent on the demands of the engine.
Many early era motorcycles also use the airbox to take care of crankcase emissions. These are created by the changing pressure within the crankcase, created by the pistons traveling up and down as well as gasses blowing by the piston rings. Therefore crankcase emissions are mainly comprised of oily air and unburnt fuel. Larger oil particles are intercepted by baffles in the crankcase but the rest of the contaminated gasses are sent via a crankcase breather to the airbox by way of an oil/air separator, which may be discrete or part of the airbox. Some of the oil from the mist goes into a catch tank; the remainder is recirculated via the airbox into the combustion chamber.
The more sophisticated the airbox, the more susceptible it is to the ill effects of meddling. At one time if you fancied fitting a freer-flowing filter, it was a relatively simple matter to rejet the carburetors to compensate. Ever higher-revving and more powerful engines are highly sensitive to departures from factory specifications, although there are still advantages to be gained if you know what you're doing; go to a good dyno operator or make sacrifices in return for gains elsewhere in the rev range.
It's also a fact that the drive towards lower emissions and noise often led to unwelcome compromises in power delivery.
If you really think you do need to deviate from the stock factory settings we'd seriously suggest a visit to the dyno to quantify the gains you think you're making.
Whatever you do, ripping off the airbox and fitting pods is no longer really an option – if it ever was.