The airbox. Appearing to be no more than a weatherproof container for an air filter, the modern airbox is actually a highly sophisticated, but underappreciated, component that’s often the target of crude efforts to boost a motorcycle’s performance.
Besides preventing as much dirt or water intake as possible, airboxes have several other important jobs, not the least of which is to quiet the intake tract so that the maximum allowable sound that reaches the Thai Department of Land Transport meters can come from the exhaust system for a customer-pleasing impression.
Yet the inside of an airbox is actually a very noisy place, so that’s not easy to do. Because a twist of the throttle opens passages that are alternately sucking air to high velocity and then slamming a valve against the flow to create sharp sonic reflections, not only is all this whisling/honking activity loud, it also makes the airbox housing pulsate strongly. So think of the airbox first as a silencer, its efficiency will largely depend on rigid construction, a large volume and a precisely sized opening to the atmosphere. To appreciate the importance of size, imagine the throttling effect on response and peak power if an undersized airbox can’t supply an engine’s maximum appetite for properly conditioned air – meaning, air that has been filtered and straightened for a non-turbulent entry into the fuel mixer. And the bigger the engine, the harder adequate airbox space is to find on a motorcycle. Research suggests that a single cylinder test engine should have an airbox volume 10 to 12 times greater than its displacement for a good compromise between its size and effectiveness. But even that surprosing multiple is likely not enough for optimum performance.
Although airbox volume is rarely noted, back when powerful 600cc supersports got major updates every couple of years to stay ahead of well-televised competition on race track, the 2004 Honda CBR600RR boasted of a 15-liter airbox, or a displacement/airbox volume ratio of 25:1! So it’s very likely that many airbox setups are seriously undersized for the engines they serve, making the temptation o ‘improve’ things hard to resist. Removing restrictive-looking snorkels, cutting oversized openings or fitting aftermarket air filters may appear to be the easy answer, but if the airflow into the carburetor or throttle bodies then becomes hotter, turbulent or otherwise inconsistent across the rev range, even extensive re-jetting or dyno remapping, can fail to fill in the holes, and the not-uncommon result is a fraction more top end power, at the cost of a noticeable loss in driveability and low/mid-range power, plus or course, more noise.
And on those rare engines, like the best supersports, that have generously sized airboxes tuned to perfection, any change at all can easily spoil some incredible complex engineering and cost you power. We’ve even seen an expensive ‘high flow’ aftermarket panel air filter noticeable decrease driveability.
And. Just like with exhaust systems, the latest airboxes on everything from dual-sports to supersports may now incorporate rpm-sensitive valves meant to relieve airbox restriction at higher rpm and boost low/mid-range response. Yet the Internet is full with how-to-videos that show how to carve all that clever acoustic tuning away with a Dremel tool.
Because they are commonly positioned atop the engine, the airbox is also exposed to high heat, which must be prevented from raising the temperature of the incoming air any more than absolutely necessary. Their typical, slightly heavy, fiber-reinforced plastic construction satisfies the need for a container with sound-damping as well as insulating qualities, and the rule of thumb is that for every 7 degrees of added air temperature, power will fall by 1 percent. Likewise, any method that reduces air temperature by the same amount, like an effective cold air box or perhaps a facy gold-foil-covered insulation blanket around the airbox, can boost power.
A ram-air system, although very similar in parts, is not to be confused with a standard airbox. The name was originally coined in the ‘60s by Pontiac, and while many automotive systems are, in truth, just cold-air boxes, a proper ram-air system is meant to capture the air pressure generated by high vehicle speeds in a sealed airbox to achieve a supercharging effect. By expanding this air in a plenum chamber, its velocity is decreaded, which causes its static pressure, and therefor density, to increase. But unlike turbo or supercharging, the air is not heated in the process, so the effect has no downside costs.
And because the precise shapes of the air passages can be so important, it seems Kawasaki’s ram-air systems have been particularly effective as its aero division’s engineers are on-call to assist its motorcycle divisions. According to tests, Kawasaki’s ram-air will begin to make a horsepower difference at speeds of 112km/h and up, with a maximum advantage of almost 10 percent at 273km/h. But what’s probably more important at road-legal speeds is that ram-air introduces an intoxicating intake honk to the motorcycles sound, and because this sound is projected away from the exhaust, it doesn’t negatively affect the Thai Department of Road Transport sound testing, either.
Perhaps the most subtle science employed in an airbox is resonant tuning. Any time you create a volume with a neck, like a bottle or an airbox, you have created a potential resonator, think of pushing a youngster on a swing. It will be comparatively difficult to start the swing moving, but once it has achieved its natural frequency, just the slightest push is all that’s needed to keep it swinging strongly. In the same way, an acoustic system at resonance will have almost no resistance to airflow, and the engine designer can tune this effect to optimize intake efficiency at engine speeds where the engine’s valve overlap is out of sync with its rpm. Smoothing power delivery.