Candid Cameron
Candid Cameron
Q I don’t understand any of the theory regarding airboxes. They supposedly are a buffer of sorts, providing a reservoir of air, but wouldn’t the lack of an airbox result in an even bigger supply? I understand the need to minimize induction noise and to keep the reservoir (downstream of the air restriction) as big as is practical. I “de-snorkeled” my Suzuki SV650 and love the sound (along with its LeoVince pipe), but like any “improvement,” the butt-dyno always says “faster/better.” Did I do the intake equivalent of the popular Harley gaffe wherein aftermarket pipes make more noise but less power? If so, can you shed any light on the theory behind the airbox, and how one might calculate the theoretically ideal size?
Mark Martin Colorado Springs, Colorado
A When you blow across the top of an emotv bottle and ..~. generate a tone, you are making the bottle into what is called a Helm holtz resonator. Every resonator has a mass and a spring or other restoring force, as in a pendulum, for example, whose restoring force is gravity. In the case of the bottle, the oscillating mass is the slug of air in the neck of the bottle, and the "spring" is the volume compress ihilifv nf the ir in the hnffle~
The same is true of a guitar or a violin, or of a motorcycle's intake airbox. If the natural frequency of the airbox and its inlet stack(s) is equal to the suction frequency of an engine's intake process, the engine will take air from the box at the peak of its pressure cycle; the box then refills from the atmosphere during the low-pressure part of its oscillation. The box has to have stiff walls and must be well sealed everywhere; if not, the damping effect of leakage kills the resonance. I learned this effect from the fact that a flute with a leaky pad will not sound the lowest notes.
Naturally, this pressure-boosting effect will be limited to a band of rpm-a few hundred on either side of the peak-but the boost effect can be 10-15 percent and is used by engineers for such purposes as filling in any low-torque region of the exhaust pipe, etc. By using intake, exhaust and airbox resonances, it is possible to cobble together quite a smooth powerband. Bear in mind that a sudden powerband is often perceived as being “faster” than does a more usable, smooth band. I’ve had the experience of a rider perceiving a carburetion change as “flat” when the stopwatch was showing his lap times were half-a-second quicker after the change.
When Honda NSR500s were raced on tracks that required engine rpm to be pulled down farther than usual, some teams would remove the airbox entirely. Another way to deal with this was to equip the box with something that looked a lot like the big valve on a bass saxophone and open the valve (the engine ECU did this via an electric actuator) at any rpm at which the airbox resonance worked against the engine.
Calculating the ideal volume of an airbox is a complex process that must take into account numerous factors that include, but are not limited to, engine displacement, camshaft timing, exhaust-system resonance and the desired width and location of the powerband. Engine designers generally achieve this by running all the relevant factors through a dedicated computer program that puts them in the ballpark; but even then, the final airbox configuration is fine-tuned by dyno testing and seat-of-the-pants evaluations conducted by test riders.
Kevin Cameron
