More from Kevin Cameron:
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Any hi-fi enthusiast knows that woofer enclosures work best when the resonant frequency of the enclosure is nicely centered on the speaker's response range. The enclosure usually consists of a sealed volume with the speaker installed in one of its walls, and an opening, called a reflex port, cut into the enclosure. A resonant system consists of a mass, which vibrates back and forth against the restraint of something flexible, like a spring, with an excitatory force to drive it. In the case of the speaker enclosure, the mass is the air in and within one diameter's distance of the reflex port. The spring is the compressible air inside the enclosure. The system is set into vibration by the amplifier, driving the speaker cone back and forth as a piston.
In the case of an engine's intake airbox, the mass is the air in the airbox inlet pipe(s). The "spring" is the compressibility of the air in the box. The excitatory force - a very powerful one - is the endless sequence of strong engine intake suction pulses from the carburetors. The airbox must not have any significant leaks, as the throttled, back-and-forth airflow through them acts like a hand on a vibrating bell (anyone who's ever tried to play low notes on a valved wind instrument knows what a killer leakage is). The airbox inlet pipe is usually made with a smooth bellmouth on either end to reduce flow losses. Carburetors or throttle bodies must likewise seal positively to the box. When a system like this gets to humming, the pressure inside it vibrates rapidly plus and minus 10-15% of atmospheric pressure. In fact, the humming is so powerful that in many cases a sub-resonator is placed near the atmosphere end of the inlet, to prevent radiation of this powerful honking sound to the outside. EPA objectors are always waiting there with calibrated sound meters and spectrum analyzers at the ready.
How can you adjust the resonant frequency of your airbox if you raise your engine's peak-torque rpm with pipes or porting? One way is to invest $30,000 or so in professional wave dynamics software like Ricardo "Wave", running on a $10,000 Sun workstation. Probably on the right back street in Hong Kong you can pick up a pirate copy for $25, but which street is it?
The airbox inlet tubes, or "horns", are specifically designed to provide a resonance that can increase the total airflow by up to 10-15%. Removing these can cause the engine to lose power and increase the intake noise.
We're so used to the idea that problems have to be solved with silicon logic that we forget about steel and aluminum solutions. "Wave" is great if you have a tricky fuel mixture glitch with #7 cylinder in your Ford NASCAR engine. But with a simple formula that tells us which variables push the airbox frequency which way, and by approximately how much, we can devise dyno experiments that will get us the answers we need - without those expensive Cathay-Pacific coach tickets.
Here is the formula.
(Airbox Frequency), squared, is proportional to inlet pipe area/(airbox volume X inlet length)
This is useful because it shows us that if we want to raise airbox resonant frequency, we must increase inlet pipe area or decrease airbox volume or inlet pipe length.
AN EXAMPLE
If our present engine is a twin, giving peak torque at 8200 rpm, that is 8200/60 = 137 revolutions per second, or 137 X 2 = 273 suction pulses per second. Unless there is some special problem, the airbox will be designed to resonate near that frequency.
If we now want to raise peak torque revs by 10%, to 9020 rpm, we must also raise airbox frequency by a similar amount. If we raise airbox frequency by 10%, its square will increase by 1.1 X 1.1 = 1.21 times, or 21%. That means that whatever is on the right-hand side of the equation must also increase by a factor of 1.21. Take your pick.
You can:
(a) increase inlet pipe area 21% (that is, increase its diameter by 10%) or,
(b) decrease airbox volume by 21% or,
© decrease inlet pipe length by 21%
Because these systems generally work better the bigger you make the airbox, we won't try (b). Since we are raising revs and power, increasing inlet area looks pretty good, so we could choose option (a), increasing inlet pipe area. However, option (c) would appear to be the easiest. Before we go to the dyno, we'll make up a few airbox inlet pipes to give us some test choices. Then we can run through our tests quickly and zero in on the sweet spot. Each end of the box inlet pipe should have a smooth bellmouth.
Likewise, go carefully before removing internal airbox "furniture". Assume nothing, but test with each change to understand its effect. Airbox designs are sophisticated now, so their internal features often have functions.
Any resonant system always has anti-resonances. In the case of an airbox, that is an rpm at which the engine breathes from the box when pressure is at the low part of its cycle. What if there's an anti-resonance right where you want your clutch to engage? Of course you could imagine a system with a variable-length inlet pipe to deal with this, but the easy way is just to kill the anti-resonance by opening a big hole in the airbox. Systems of this type are in use on certain sports motorcycles. When the engine runs near the rpm of the anti-resonance, the engine control computer tells a little motor to open the airbox port. When it revs up, the motor closes the port.
I'm aware of the fact that this is like building a clock for someone who just asked what time it is, but the tech involved is pretty easy to understand. It also explains why radical high RPM motors don't have to deal with it. I hope it was worth the read, and again, the author is Kevin Cameron.
