Airboxes R vs SP

[Warwick]

With the 38s I need to run some holes in both airbox lids (SP upper) if running the airboxes. As noted above though, the only sensible way to approach this stuff is to empirically test things as you go, so, as mentioned previously:  it is probably '...best to get a base reading with the boxes completely stock and then try mods... so that you can properly assess their effectiveness?' On the specific bike in question...

Have you tried some holes in the lids, Dan? I know your carbs are slightly smaller, but with the 38s mine is significantly choked if running unmodded boxes I have to say. I use extremely free flowing 'filters'. I don't think I'll be running the oe airboxes at all on the track 3XV this year myself though in fact. More clutter gone. Nice! Just got to stop the carbs eating things...

[maccas]

Not yet Warwick, I'll see how we get on next time on the dyno. It might be what's needed to get it revving, however I will investigate all other avenues to see if something else is holding the game up first 

Dan

[Warwick]

Yup. I love the smell of systematic testing in the morning...

[Kingy38]

my airbox pics as promised,i'm running martin 77 pipes and (almost) sp spec ported r cylinders,stock 28mm carbs,rejetted to 220 upper & 270 lower,making good power ( last dynojet run was JUST under 60 bhp on the stock cdi) revs like a good 'un, no flat spots or anything 

[jcsnook]

Ian,

Was all that modding "necessary" to get your bike to rev or run properly?  Just curious mind you, not critical, as I don't think Warwick went that far with his airbox mods and obtained similar HP output with stock 28mm carbs.

To each his own state of tune I suppose.  I'll be doing my own experimenting soon I guess once I get my SP spec ported barrels back from having the o-ring grooves cut in them, and get the one original piston back from Wossner

[Kingy38]

prob not totally necessary TBH,just my own take on trying to get as much air into the airboxes as poss really.
lower box does look like it's designed for a bigger intake though don't you think? hence my mod....

[Crank]

Is it not just a whole lot easier to run a set of pod filters with 38's, or am I missing something here??

[Warwick]

When first developing it it was a road bike so I preferred to retain the airboxes on balance. There can also be issues with clearance and the position of the filter on the lower cyl of course.

[mbsteve]

There is a tech artical by 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 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.

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 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 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,
(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-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.

Isn't life getting complicated?

[yorkee]

Just to stirr up this thread:   

[Warwick]

Eagle-eyed viewers will note that the lid on the far left is an SP lid. Useful to be able to see the SP intake trumpet like that.

Also of possible interest for those who like to ponder such things is the little 'boost bottle' that is oe fitment on the SP models' square lower cyl airbox intake pipe... Some kind of aid to bottom-end carburation I imagine?

[jcsnook]

Eagle-eyed viewers will note that the lid on the far left is an SP lid. Useful to be able to see the SP intake trumpet like that.

Also of possible interest for those who like to ponder such things is the little 'boost bottle' that is oe fitment on the SP models' square lower cyl airbox intake pipe... Some kind of aid to bottom-end carburation I imagine?

I never noticed that "boost bottle" added to the lower cylinder airbox intake pipe!    Interesting...

[mbsteve]

Eagle-eyed viewers will note that the lid on the far left is an SP lid. Useful to be able to see the SP intake trumpet like that.

Also of possible interest for those who like to ponder such things is the little 'boost bottle' that is oe fitment on the SP models' square lower cyl airbox intake pipe... Some kind of aid to bottom-end carburation I imagine?

It can also be used to heat up the intake air as you can see this done on cars intakes.

[Jakes]

I also only recently noticed the little extra bottle on the lower airbox on my SPR. Never really understood what it was for, cant imagine that it makes a big difference

[EEKNOWS]

So it was worse ?

I don't want to start an argument , but will the 3xv behave the same way if the airboxes are changed / modified etc ?

was a lot of work to get back to where it was with the airbox snorkel fitted, sounded great but noise doesn't equal HP. So snorkel went back in and jets size went down. Ever since then I have run engines with an airbox. I'm not overly sure of the method of tuning a airbox to the engine but bigger seems to be better, smaller airboxes seem to be sensitive to changes. Only way to check is on  a drag strip or dyno.