| There’s a lot of anecdotal information about the benefits of a larger airbox being passed-around so maybe this would be a good time to summarize.
Chassis Stiffness
The airbox on a Ducati superbike bridges the upper tubular space-frame members so (if it’s properly mounted) it adds rigidity to the frame, particularly in torsion. Less flexing of the frame means smaller geometry changes at the steering head and swingarm attachment points.
The trend in motorcycle chassis design has been a progression toward stiffer frames, but you can actually go too far when there’s so little flex in the fork tubes, swingarm, steering head etc. that small bumps cause problems hooking-up the tires.
So, changing to a carbon fiber airbox will offer a chassis stiffness improvement and a small weight reduction because the material properties of carbon fiber material is better than the stock plastic. To give you an idea of the effect, it was reported that a carbon airbox on a 916 increased the torsional stiffness of the chassis by 20%.
Increasing the volume of the airbox has a tendency to make it less-stiff, as does segmenting-it for ease of installation/removal.
Intake Tract Tuning
The basic constraint to more engine power is the amount of air that you can pass through the engine. Once you’ve got more air, adding more fuel is trivial. A common way to improve air flow is to pressurize the incoming air using a supercharger or turbocharger.
Another way is to increase the size of the valves, throttle bodies and use higher-lift cams to allow more air in and larger exhausts to move air out.
You can also pressurize the intake charge at high bike speeds using air runners coupled to an airbox, but at 160mph you’ll only get a small 3psi increase. Further, larger volume airbox doesn’t offer any ram-air pressurization improvement.
There’s also another way.
You can use the intake air pulses (and the exhaust gas pulses) from the other cylinders to pull (and push) more air through the engine.
The first step is to enclose the air intakes of all the cylinders so they can send pressure pulse to each other. If the timing is just right, the pressure pulse from one cylinder will push air into an adjacent cylinder.
This timing is controlled by the size of the airbox.
An easy way to remember this is, if you blow air across the mouth of a bottle you’ll get a tone. Actually, a tone is a sound of a given frequency with a bunch of harmonics thrown-in. If you fill a portion of the bottle with liquid such that you decrease the volume of air it contains, the pitch (frequency) of the sound will increase.
BTW, if you fill the bottle with foam from an old air filter - you’ll get NO tone - which is the primary reason not to use any after-market air filter that take up space in the airbox.
When you change the size of the airbox, you change airbox resonant frequency (and its harmonics), just like you do with the bottle.
Further, the frequency that the air inside the airbox vibrates and the frequency of the pressure pulses of the other cylinder’s intakes can be paired-up for improved air flow and power - but only for a selected engine rpm range.
If you add a larger airbox, be prepared to do some dyno development work. You’ll change the amount of air going into the engine at every rpm, so the fuel map will need to increased or decreased to optimize the air-fuel ratio at each point.
On stock bikes, the chosen airbox volume is used to minimize mid range rpm dips in the torque curve, but on performance-version bikes a larger airbox is added to enhance red-line power.
So, if you decide to put a larger airbox on a stock bike that’s used primarily on the street, you’re likely degrading power in a more frequently-used rpm range to gain somewhat higher maximum power (if, and only if, you do the dyno development work.)
A larger airbox is a good trade-off on a race bike that lives at high rpm, but a questionable choice for a street bike.  |