Randy Norian's thoughts on
tuned inlet design for Motorcycles

thanks to Randy for letting me go public with this series of E-mails


    Oddly enough, I am pursuing this very topic (design of a resonant airbox) on another forum. I'll let you know if I get any good responses!

    Here is from a previous post on this topic

    OK, this is regarding my diesel, but just so you know I'm still pondering airboxes and this one is a good example.

    Typical helmholtz resonators are a volume (airbox), fed by a duct of certain length and cross section. these 3 things determine the resonant frequency of the airbox. look at an RGV250 and you see a pair of inlet ducts, a few inches in length, that feed the airbox. shortening those ducts raises airbox rez freq. anyhow--

    My TLS has a little flapper in the inlet duct, it closes at low rpm. to provide restriction? nooope- read on!

    OK, I gave the TLS airbox my best analysis. (sat and stared at it) just how does the flapper affect things? The airbox is a CLASSIC helmholtz resonator, complete with entry duct (that is the tube coming up from the floor) . stock, I figure (flapper open) the duct length is approx 2.4" and area is approx 5 sq in. with a box volume of 550 cu in it resonates at approx 120 Hz.

    flapper closed, sectional open area of inlet duct approx 1 sq inch, airbox now resonates at 53 Hz (low end boost)

    remove flapper, you lose that variable resonance

    shorten the inlet duct to 1" in length, resonance goes up to 184 Hz. cut it off completely and I'm not sure what happens?? This is a very clever bit of work. The TL has a weird double-gulp intake event every other revolution. I'll treat it as a single for simplicity- with flapper closed, the airbox resonates approx 3200 cycles/min, so you'd have a weaker resonance at 1600 rpm and then a stronger resonance at 3200 rpm. once the flapper opens, you have airbox resonance at 7200 cycles/minute -- so you get a weak resonance at 3600 rpm --and a stronger resonance at 7200 rpm.

    All these resonators use a box with inlet duct of certain length. it's never just an open hole. There has to be a better way to improve things than cutting everything out of there.

    One could apply this to an RG500 airbox- one on each side of the engine- sufficiently large... the use of a flapper could seriously aid low rpm power

    Say we want a boost at 3000 rpm, where we have 6000 intake events/min we can shoot for a helping wave every other revolution (3000 cycles/min D 50Hz) treat each side separately- I think one could make an airbox of 400 cu in (0.23 cu ft) on each side. feed this through an inlet duct of sufficient size, - say - 4 sq in (.0278 sq ft) , , with a flapper that blocks it down to 1 sq in (.007 sq ft) . and length of... 3.6" that gives a resonance at 52 Hz. with flapper open, resonance occurs at 104 Hz (6240 cycles/min) and you get a boost in the midrange at 6200. or juggle the inlet tube length to counteract the pre-pipe flat spot anyhow, I'm still working on stuff... but my next effort will have airboxes that work!

So long


    and more from Kevin Cameron

    The airbox used to be just an intake silencer and a place to put the air filter. Now it's much more than that, so read on before you gut or toss your box. Just as is being done on new cars and motorcycles, snowmobile airboxes and their intakes are being built as resonant systems. When the airbox is resonating strongly, driven by the engine's suction pulses, its rapid internal pressure fluctuation covers a range of plus and minus 10-15%. This is just like the resonance of a bottle when you hum into it. If your engine's intake events run in step with the positive side of this resonance, it's just like getting a 10-15% supercharge boost for free. That's worth having. And what if you modify your engine, raising its peak-power rpm beyond the range of the airbox resonant frequency? There is a simple relationship you can use to alter airbox frequency by changing the length and/or diameter of the airbox intake pipe(s). That's worth having.

snipped- post was over size limit


    Any hi-fi enthusiast knows that woofer enclosures work best when the resonant frequency of the enclosure is nicely centred 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 carburettors. 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. Carburettors 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 analysers 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 93horns 94, 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 loose 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 aluminium solutions. 93Wave 94 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


If our present engine is a twin, giving peak torque at 8200 rpm, that is 8200/60 D 137 revolutions per second, or 137 X 2 D 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 D 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.(a) increase inlet pipe area 21% (that is, increase its diameter by 10%) or,(b) decrease airbox volume by 21% or,(c) 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-resonance. 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.

So long


    Randy Norian


        May 2006

For further details E-Mail:- KGB-Racing at Saltmine dot Org dot UK

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