DYNOTECH                                          Volume 6   Number 5   Page 13

AIRBOXES                                               The Cellar Dweller
new tuning tool?                  Kevin Cameron

 For years, airboxes have been part of the production trash that serious racers routinely discard at once.  Airboxes were, it was believed, like mufflers and low exhaust ports – things that might be desirable in a world of civilization and wives, but are definitely contrary to the spirit of good fun.  Accordingly, we have either ripped out the aforesaid airboxes, or have gutted them of their fun-destroying (and airflow-killing) elements such as baffles, resonator tubes, and foam.  Ahhh, there’s that soothing roar again…now I can hear the intake process all the way to the crankshaft.

 But a mystery has arisen, and our hands hesitate as we prepare to gut yet another new airbox.  Sometimes, these days, engines make less power after de-airboxing than they did before.  What is going on?  Is nothing sacred?

 What has happened is that the airbox has now been added to the list of “resonant powerband aids” that formerly included only tuned intake and exhaust pipes.  Intake noise, once just an EPA-listed source of environmental hazard, can be put to work cramming air into our engines.

 Hold an empty bottle up in front of your mouth and hum up and down the scale into it.  At certain voice frequencies, the bottle will act as a resonator, and the sound will become louder.  At other frequencies, the bottle will be in “anti-resonance” with your note, and the sound will be softer.

 The bottle acts as a resonator because it has (a) a spring, which is the volume compressibility of the air in the bottle and (b) a vibrating mass, which is the slug of air in the narrow neck of the bottle.  Your humming provides a cyclical driving force, setting the air-slug to vibrating in and out in the neck of the bottle.

 This is a different kind of resonance from that found in intake and exhaust pipes, where pressure waves travel and reflect in specially-shaped tubes. 

 Your engine’s airbox is a very large bottle, and its “ram-air” pipe or pipes act as the bottle’s neck.  Your engine’s intake process, occurring hundreds of times a second, is a particularly strong form of humming into the bottle.  To boost power, design the airbox volume and the diameter and length of the box’s air supply pipe(s) such that it resonates in step with your engine’s intake process at the desired rpm.  At resonance, your engine will take air from the box only when it is above atmospheric pressure, and between intake events, the box will be below atmospheric pressure, refilling itself from its air supply pipe(s).

 How can this effect be useful?  Well, imagine that peak torque is OK, but the engine’s a little weak at closing the clutch.  No problem; just set the box resonance to give a little boost right at the rpm where it’s needed.  Or perhaps your exhaust pipes are a little sharp, leaving the engine an airless weakling right where a lot of people want to ride it?  Fill in that hole with airbox power.  Or maybe you just need to make a bigger number to fill a vacuum that’s formed between the admen and the dyno?  Stack that box resonance right on top of peak torque.

 This same situation existed in production motorcycle racing not so long ago.  Where rules permitted, racers whipped airboxes off new models as if by reflex.  This thing’s gotta make more power with K&N sock filters than with that big asthma box.  But in the late 1980s, track testing and accurate dyno work began to show less power, not more, after airbox removal.  Back on went the boxes, amid heavy shoulder-shrugging and sighing.  A litter later, even pure racebikes began to appear with “ram airboxes”.  This meant that the carbs were located inside a sealed box, fed with ram air by one or more forward-facing pipes.  Attention immediately focused on the idea that ram action would “pack” air into the carbs.  But the sliderule people noted that a forward-facing intake, operating at an unlikely 100% pressure recovery, is only worth about a 3% pressure gain at 150 miles per hour.  Yet these “ram airboxes” were regularly showing much larger gains of 10 or even 15% more torque.  Where was the extra power coming from?

 Well, some of the gain on bikes came from keeping hot, lower-density radiator air out of the carbs – but the gains were too big for this explanation, too.  But now the mystery is explained; the resonant airbox has come of age.  With the airbox humming its big, monotonous tune, in the right range of rpm, the engine performs its intake process only during the positive-pressure part of each airbox oscillation.  This is mild supercharging – and it’s essentially free.

 There are minor problems with operating in a sealed box.  The intake pipe or pipes to the box must be large enough to supply plenty of air without much pressure drop from internal friction.  But the physical laws that govern this kind of oscillator must be respected, too.  The whole device must fit on the machine.  Just putting the carbs bellmouths into the box is usually not enough.  Fuel is metered based upon the pressure difference between carb throat and the pressure above the fuel in the fuel bowls.  Putting the carbs wholly inside the box makes sure that fuel breather lines are at box pressure.  Just as in humming into a bottle, there are resonances and anti-resonances; if the airbox helps in one rpm range, it is sure to hurt in another.  Fortunately for the snowmobiler, the clutch keeps the engine at near-constant rpm on full throttle, so a singe box resonance works very nicely.

 To give maximal gains, a resonant airbox must be sealed up tight everywhere.  Otherwise leakage will damp out and kill the resonance just as surely as a leaking valve will make it impossible to blow a note on a saxophone.  Finally, strong airbox resonance still makes a lot of noise, and the EPA is still listening.  For this, it is usual to put small sub-resonators into the intake pipe(s) feeding the box, so the sound bounces between airbox and sub-resonators, with a lot less reaching the great outdoors.

 This big change in airbox design is one of both knowledge and intention.  The boxes were originally just a place to put the air filter, and a way to suppress intake noise.  The new boxes tackle both those problems, but also employ box resonance to help boost torque in some desired rpm range.

 A resonant airbox is a type of Helmholtz resonator, and there is a formula for calculating its frequency from its dimensions.  As you’d expect, the formula is not an exact predictor of reality, but it can give the user a good idea of what has to be changed, how much, to get what effect.

 There is a direct way to the same information, too.  Go to the electronics store and get a frequency generator, which is the modern equivalent of the singing teacher’s pitch pipe.  Feed its output, suitably amplified, into a small loudspeaker.  Place the speaker at the entrance to the intake pipe of the box to be tested (installed on the engine), and sweep the frequency up and down until you hit a strong resonance.  Note the frequency from the dial.  To lower the frequency, you can make the box bigger or the intake pipe longer or smaller in diameter.  To raise it, do the opposite.  This kind of testing, combined with careful dyno work, will soon reveal just what has to be done to get maximum effect where you want it.  In this formula, the frequency of the oscillator, in cycles per second, is equal to 5280 times the square root of the following quantity; area of the intake pipe’s cross section, divided by (oscillator volume times intake pipe length).  Obviously, all the numbers have to be in the same system of measurement – don’t, as I sometimes do, mix millimeters, inches, and feet to get a ridiculous answer that takes half an hour to correct.

Intake airflow through the carb and into the crankcase can also be treated with the Helmholtz formula; the crankcase volume itself is the spring, while the air in the carb, manifold, and inlet valve is the vibrating mass.  From this treatment, you can easily discover that, with the usual sizes of carburetors and the crankcase volumes used with them, it is practically impossible to make the intake system short enough.  This explains why, when an intake system is shortened, there is nearly always a gain.  Those of us who like to fiddle with numbers when it’s inappropriate to fiddle with hardware (in business meetings, at family reunions, religious observances, etc.) will find hours of enjoyment in simulating new combinations with the Helmholtz formula and a pocket calculator.

 Airboxes are no longer power-robbing annoyances, foisted upon us by the heavy hand of government or voluntary compliance.  They are useful, torque-boosting tools in their own right.  There is an interesting parallel between airbox development and jet engine experience.  Back in 1960, when loud turbojets were coming into civil aircraft usage, the complaints from people on the ground were equally loud.  In the resulting efforts to quiet those noisy machines, each time the engineers investigated and controlled a sound source, there was nearly always an accompanying gain in thrust or a drop in fuel consumption.  As some are fond of saying, knowledge is power.  Now about all that noise coming out the tailpipe.