Building an airbox?

[R6_kid:Me]

My TRX has aftermarket Keihin FCR 41mm Flatslide Carbs fitted, currently running velocity stacks with only stockings over them to stop any foreign material getting into the engine. I've bought an airbox in the hope of getting more stable airflow to help with around town riding, but upon lifting the tank have found that an alluminium catchtank has been fitted which is going to prevent me from using the modified stock airbox in its current form.

My plan at the moment is to modify the bottom of the airbox so that it will mount on to the FCR's with the current velocity stacks in place, and get some measurements of space available so that I can hopefully make an airbox to approximately stock dimensions out of fibreglass to hold an airfilter in place.

What is the best way to go about this sort of thing? I've repaired fibreglass fairings before, but have never made something effectively from scratch.

I was thinking of mocking it up out of carboard first to get the dimensions and check fit, then transfer the stencils to some 3mm MDF to make a two part mold out of before laying the fibreglass (or maybe carbon fibre) up on the inside of the two halves.

Any help or suggestions would be appreciated

[Morcs]

My TRX has aftermarket Keihin FCR 41mm Flatslide Carbs fitted, currently running velocity stacks with only stockings over them to stop any foreign material getting into the engine. I've bought an airbox in the hope of getting more stable airflow to help with around town riding, but upon lifting the tank have found that an alluminium catchtank has been fitted which is going to prevent me from using the modified stock airbox in its current form.

My plan at the moment is to modify the bottom of the airbox so that it will mount on to the FCR's with the current velocity stacks in place, and get some measurements of space available so that I can hopefully make an airbox to approximately stock dimensions out of fibreglass to hold an airfilter in place.

What is the best way to go about this sort of thing? I've repaired fibreglass fairings before, but have never made something effectively from scratch.

I was thinking of mocking it up out of carboard first to get the dimensions and check fit, then transfer the stencils to some 3mm MDF to make a two part mold out of before laying the fibreglass (or maybe carbon fibre) up on the inside of the two halves.

Any help or suggestions would be appreciated

Mate give Fynn a buzz, he did this sort of stuff with his Zxr.

Also you could use the opportunity to change the velocity stacks?

[R6_kid:Me]

Mate give Fynn a buzz, he did this sort of stuff with his Zxr.

Also you could use the opportunity to change the velocity stacks?

The velocity stacks are already aftermarket - and I'm liking how the power is being delivered at the moment, it's just that the carbs get tempermental at low RPM (under 3,000) which makes it a bitch for moving away from traffic lights in anything other than a 'race start' fashion so figured fitting an airbox might help to improve that situation a little bit.

[imdying]

Yeah, cardboard is a good way to start out the design.

[Brian d marge]

one thing with Air boxes , they aren't just a big empty no reason box

seen a few bikes carburettion stuffed up by modding the air box

Stephen

[FROSTY:Me]

Hear what you're thinking dude and it makes sense BUT I think a start would be a couple of pod filters rather than an air box.
From there you might find that createing a still air box around them will be the answer rather than trying to build a complete air box.

[Sensei]

Some info about airbox theory

Modern intakes use the Hemholtz theorem to assist the incoming charge.

See the example equation.

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.

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.

[mossy1200]

God thats a read and a half.
Are you sure your not running rich on the mixture screws since you problems in the low revs?

[R6_kid:Me]

Hear what you're thinking dude and it makes sense BUT I think a start would be a couple of pod filters rather than an air box.
From there you might find that createing a still air box around them will be the answer rather than trying to build a complete air box.

I've got a stock airbox to begin with as my template, but only the lower half. I'm trying to source pictures of a full one to see what the standard bike runs. Will try and find some pod filters that will do the job.

Some info about airbox theory

Cheers for that, will re-read it when I start cutting up cardboard.

God thats a read and a half.
Are you sure your not running rich on the mixture screws since you problems in the low revs?

This could be the case, if I crouch down while riding I can smell gas fumes, but assumed this was due to the fact that there was no airbox stopping the fumes from spreading out - the bike needs to hit the dyno anyway so will get it 'tuned up' before I do the airbox thing.

[Brian d marge]

God thats a read and a half.

see told you so ......big can of worms opening in 3,2,1............


Stephen

been there ..........

[mossy1200]

This could be the case, if I crouch down while riding I can smell gas fumes, but assumed this was due to the fact that there was no airbox stopping the fumes from spreading out - the bike needs to hit the dyno anyway so will get it 'tuned up' before I do the airbox thing.

the fcrs with the correct v stacks should run well if jetted correctly.
CV carbs would be more reliant on airbox.Pods on the fcrs would help .
If your fcrs came in from US then the jets will be incorrect.My CR carbs on the triumph came pre jetted to the mods I did prior and were still rich when installed.

[Brian d marge]

design the box , then tune the bike ....

because the box will change the tune of the bike and dyno time isn't cheap

Stephen

[motorbyclist]

(Airbox Frequency), squared, is proportional to inlet pipe area/(airbox volume X inlet length)

it might pay to recognise that the resonant frequency is in radians/second, not hertz, and they have omitted the speed of sound from the equation...

but their method of changing things proportionally is still a valid approach to the problem

[CookMySock]

Faux an airbox up in PU foam so it all fits nicely. Fill, sand, release agent, and glass it over.

Slice the whole thing up the center with the bandsaw and clean all the foam out. Glue back together and bobs ur auntie.

Steve