Airbox Removal

I recently added my $0.02 to the airbox removal discussion on the SV/DL Riders Google Discussion group. 

This discussion is interesting to me, because for a long time I believed that engines always ran better with open breathing.  Scoop more air into the throttle body, remove the exhaust system, and you get more power: instantly.  …only it just ain’t so. Kevin Cameron showed me the error of my ways in his column.  Here is my edited input.

Waves of energy that get pushed back into the airbox while the intake valves are still open, but the crank passes BDC.  Airboxes are sized so that certain frequencies of waves (the ones that are causing problems on the dyno) cancel themselves out: the high energy meets the low energy causing interference: just like noise cancelling headphones.  This cancelation stabilizes the air,  stable air isn’t being pushed out, so more air gets into the engine.  These waves of energy are also what causes induction noise, but the airbox design is actually meant to improve performance where the waves would hurt it; the quiter noise is likely just a side effect.

So what do those little individual filters that you can attach directly to the throttle bodies do?  They also cancel out waves, but at even higher frequencies, unfortunately that negatively affects the bike’s torque and horsepower curves.  The throttle bodies are small (there are shorter distances between the walls, so shorter wavelengths get cancelled out), so the engine works more efficiently at extremely high RPMs, but worse at other RPM’s.   These filters helped racers on old race bikes, because those racers were keeping their engines running at high RPMs anyway.  Those engines also died a lot.

This is also why ram air isn’t on every vehicle.  Ram air is only useful when it’s tuned (like a musical instrument) to the frequencies that don’t keep the air out.  The “big funnel effect” only goes so far.

If you want to change the use of your bike from what the engineers were thinking (such as a show bike, or a racer that no longer rides on the street) and you have a lot of dyno time then you might be able to improve performance, but I would keep this part of the bike the same otherwise.

This begs the question: if the airbox design is so great then why are things like the suspension not?  Well, better suspensions cost more money, but airboxes are dirt cheap after the engineering and tooling is amortized.