Airbox design.

[Martinh]

Hello everyone.

I'm looking at designing my own airbox at some point. I have to overcome the first hurdle of not knowing anything about airbox design.

So if anyone knows anything or can point me in the direction of some good info that'd be great.

A few questions:

Why are larger airboxes more desirable? Shirley the engine only needs so much air? Can you have too big an air box?

Would a smaller airbox with a big intake work just as well if not better?

Does ram air actually work by increasing pressure or just get air into the engine quicker i.e.blowing air into the intakes?

What about using valves to trap air?

Maybe taking this a little too seriously but i am genuinely confused.

Any light shed greatly appreciated.

cheers martin.

[fran9r]

I'd like to do the same to accomodate the FCRs onto the 9R...so I'd like the answers too!

I do know race bikes use all the available space to increase airbox size, and dump the resonator boxes on ram-air bikes.

Ram runs at a fraction of PSI, so it will be pressurised. A bit like a turbo, but a really, really low gain.

Valves to trap air? Not sure what you mean, but the flapper valves in TLs and SPs are usually dumped to help performance.

[foo]

I asked this same question on the old .com forum, as I was thinking about building an airbox for an R1. As it turned out, I eventually got hold of a race airbox and fitted that, but anyway:

Millemille was very knowledgeable and helpful on the subject. If I remember correctly, what he told me, and I learned from other sources, is that:

• Airbox design is a complicated subject. Manufacturers put a lot of effort into it, and still get it wrong sometimes. It's probably not possible for an individual to copy their design processes, due to lack of resources, knowledge, etc.
  
• However, that doesn't matter as much as you'd think, because a lot of what concerns a manufacturer is not of interest to an individual builder. They have to worry about issues like noise testing, cost, manufacturability, and so on, and you don't (to anything like the same degree). Some rough and ready rules for an individual design are:
  
• The pressure effect (Ram air) is minimal. The airbox will still be below atmospheric pressure when the bike is running, just not as much as without a decent airbox design. Forget about pressure as a source of power.
  
• Cool air is good, warm air is bad. The density difference between warm and cool air can have a measurable effect on power. This is one of the reasons why air intakes on modern bikes are high up in the nose of the bike. It's where the coolest air is. If the intake's under the tank, it'll be picking up warm air from around the engine.
  
• Smooth airflow is good, turbulent airflow is bad. What this means is that the inside of your airbox (and the pipes leading to it) should avoid sudden changes in direction or cross section. They should also avoid seams, ledges or discontinuities that will cause the airflow to become turbulent. This is the other reason why intakes are at the leading edge of the bike --it's where the bike hasn't yet disturbed the air.
  
• Within reason, bigger airboxes are better than smaller ones. The rule of thumb seems to be aim for at least 1.5 times the swept volume of the engine. So, for a litre bike, you'd want a 1500cc airbox volume as a minimum.
  
• Internal bellmouths, velocity stacks, whatever you prefer to call them, help. This is because they exploit the venturi effect which speeds up the airflow through the carburettor or throttle body, helping with atomisation of fuel. Also, the smooth transition of a bellmouth is better for airflow than an abrupt step.
  
• The carburettor/throttle body area is a mimimum section in your intake design. If you work out what it is, your intake runners should be no smaller than that. They can be bigger, but there's little value to having huge, tapering, intakes as you fall foul of the need not to change the cross section too drastically (the fifth point, above).
  
• If you go on the internet, you'll find a lot of stuff about tuned (resonant) intake design. This wasn't a huge amount of help to me because the dimensions of the airbox and intake tubes on an existing bike are pretty much constrained by what you already have to work with.
  

That's about all I can remember from the original thread. I'm sure Millemille will jump in if he has anything to add. It may be worth sending him a PM.

[David W]

The bottom line is that a bike engine want to breath cool, still air from the biggest airbox volume that it practical to fit in. Most race setups run the throttle bodies or carbs inside the airbox to allow for maximum volume.

If you want some inspiration, have a look at Thorsten Durbahn's site as there are some good photos of the RC30 and RC51 stuff he's done. www.durbahn.de

As for manufacture, make a model/prototype from cardboard first. Then you can make a mould from that, and then a production version in fibreglass from the mould. Make sure you reinforce the edges and holes and try to make any corners or edges radiused if you can. Don't underestimate how hard a life an airbox has because if you make it too light/weak it will break up.

I've looked into this a lot because the airbox on the SP1/SP2 would appear to be the single biggest restrictor to performance and it basically too small for the needs of a 10,000rpm, 1000cc v-twin.

[foo]

<snip>

I've looked into this a lot because the airbox on the SP1/SP2 would appear to be the single biggest restrictor to performance and it basically too small for the needs of a 10,000rpm, 1000cc v-twin.

Interesting. What did you do in the end, David?

edited to add:

There's also another benefit to putting the carbs in the airbox, and that's you don't have to worry about pressure differentials. By this I mean that carbs work on the pressure difference between the ambient air and the vacuum developed by the inlet. If your airbox is at a different pressure to ambient, it will affect the way the fuelling works. Putting the carbs inside the airbox removes this as a concern.

end edit

[David W]

Interesting. What did you do in the end, David?

I bottled making my own and bought a secondhand Durbahn airbox from Rainer, which is still languishing in my garage awaiting a few little repairs and me coming up with a suitable airfilter for it!

[foo]

... and me coming up with a suitable airfilter for it!

Good one. I had the same problem with the Graves airbox I got --and there was also no fitment internally for a plate type air filter. In the end I got some foam and a can of filter oil and made my own. You'll need some wire mesh as well.

DK and Motorcycle Products do the stuff. It's cheap as chips as well.

[David W]

I got a big sheet of quite posh stuff from Pipercross (they are quite local to me in Northampton). Basically the raw material they use for their filters for cars and bikes. Just need to come up with something clever involving a bit of chicken wire, cable ties and rivets.

[Martinh]

Cheers for the info guys very useful.

In terms of volume, is the size that actually makes the difference or is more to do with shape? I was wondering if putting a wall in the airbox would be a worthwhile idea, so that i could concentrate on directing air towards the carbs but also have a larger pocket of air for when there is less air going into the intake. Maybe helping throttle response? Or would the air in the other half of the airbox just sit there?

(Does that ^ make sense?)

I did do some sums on working out intake length, throuble is its for my zxr250 so the numbers for revs may have skewed things a bit i think i came up with 3 foot long inlet trumpets. (18000 plus).

Is the optimal solution enormous individual ram air funnels that are connected to the intakes on the carbs? Practicality aside of course.

I was looking into using the existing fake ram air tubes but was wondering about water getting into the airbox if the air is coming in from above the airbox. I've seen a picture of the srad 750 intake path and it goes almost around a u bend before it gets to the airbox presumably to aid air filtering.

My mention of valves relates to the possibilty of trapping more air by stopping it escaping, but i guess its negated by the fact that air doesnt go back up the intake does it?

cheers

martin

[foo]

Cheers for the info guys very useful.

In terms of volume, is the size that actually makes the difference or is more to do with shape? I was wondering if putting a wall in the airbox would be a worthwhile idea, so that i could concentrate on directing air towards the carbs but also have a larger pocket of air for when there is less air going into the intake. Maybe helping throttle response? Or would the air in the other half of the airbox just sit there?

(Does that ^ make sense?)

Here's a quick sketch of the Graves airbox:

As you can see, it is more or less a box --most of the shape is due to it having to fit under the tank, and in between the frame spars at the headstock. The inlet on this airbox enters from under the headstock, which isn't ideal.

I did do some sums on working out intake length, throuble is its for my zxr250 so the numbers for revs may have skewed things a bit i think i came up with 3 foot long inlet trumpets. (18000 plus).

Hence why I said not to get too driven by calculations --they give you answers that are technically correct, but not helpful from the point of view of anything you could make.

Is the optimal solution enormous individual ram air funnels that are connected to the intakes on the carbs? Practicality aside of course.

No. Having a bellmouth/velocity stack on each carb intake, inside a single airbox is what everybody does.

I was looking into using the existing fake ram air tubes but was wondering about water getting into the airbox if the air is coming in from above the airbox. I've seen a picture of the srad 750 intake path and it goes almost around a u bend before it gets to the airbox presumably to aid air filtering.

Just put a couple of small drain holes into the bottom of the intake tube. Ideally your intake tubes should be straight and of constant section. Any bends you do have should be as gentle as possible.

My mention of valves relates to the possibilty of trapping more air by stopping it escaping, but i guess its negated by the fact that air doesnt go back up the intake does it?

That's right. Air moves from higher to lower pressure. The airbox will always be at lower pressure due to the vacuum generated by the engine intakes. So the air will want to move from outside into the airbox. Putting obstructions in the way will just impede the airflow, and make the airbox work worse.

[David W]

Martinh - I think you might be thinking about this a bit too hard? Like Foo's Graves airbox, my Durbahn box is pretty much just a bigger box that is fed by the ramair.

Don't be confused with the manufacturer claims about airbox inernal flapper valves doing anything clever. They are simply there to make the airbox quieter at the revs that bikes are tested.

My suggestion is getting the carbs off and then making something that gives you the biggest volume you can practically manage. Using the underside of the tank as the 'lid' is another way of getting a bit more volume, but you obviously need to make sure you have a reasonable seal.

[thundercat rider]

I'm really interested in airbox design as well; still doing some research about a thundercat/fzr 400 hybrid and how to mod my airbox to fit.

Over at af1 forum I read this (I skulk around that forum cuz some very knowledgeable people frequent it):

http://www.apriliaforum.com/forums/showthr...rbox&page=2

I did a very extensive test recently on all possibly intake mods in a 2007 Tuono. Most powerful was stock filter in EVO airkit, 2nd most powerful was stock airbox, choker removed, stock filter, then I tested a pod filter style kit developed for use in the cramped confines of the can am spider chassis....down 13 bhp, odd since it definitely makes more power in the intended application and since it sounded nothing short of amazing.

I have tested renegade kits and did not find there to be much loss anywhere and they look trick.

About resonant tuning of airboxes, or hemlotz frequuency....the volume of the intake box in calculated as that of the area between the closed intake valves and the rear edge of the air filter material....think on this one for a bit, and remember the important volume is what comes after the filter and before the intake valve heads.

I believe the standard mille engine airbox is only 0,8L for a 1000cc twin. The evo airbox is a mod to use inner part of the tank as the upper part of the airbox.

This guy did some research about an airbox for a yzf750/ fzr 1000 hybrid. Don't know if his stuff is good though:

http://www.d-murray.co.uk/convert.htm

I was also wondering why many 600cc airboxes all share the same basic design for the ram air tubes entering the airbox. They don't go straight into the airbox:

http://cgi.ebay.co.uk/Kawasaki-ZX6R-F3-Nin...bayphotohosting

[foo]

<snip>

About resonant tuning of airboxes, or hemlotz frequuency....the volume of the intake box in calculated as that of the area between the closed intake valves and the rear edge of the air filter material....think on this one for a bit, and remember the important volume is what comes after the filter and before the intake valve heads.

Helmholtz resonance is named after Hermann von Helmholtz who discovered the phenomenon in the 1850's (Wikipedia link here: Helmholtz Resonance). It's the same principle that makes organ pipes work, and why blowing across the top of a bottle makes a noise.

The thing to remember about it is that every cavity has a natural harmonic frequency at which the air in it will resonate. It's set by the shape and volume of the space, which as you rightly point out, in an airbox could be considered to be between the air filter and the throttle blades.

The important point about resonant frequency is that it's a single frequency, and the effect will only be of help to the engine when the resonant pressure waves happen to coincide with the engine intake valve opening cycle --in other words at a specific, narrow, RPM range.

The EVO kit result seems to bear out the basic principle of making the airbox as large as is feasible, and fitting a free-flowing filter. These seem to me to be the most relevant two ideas for a home builder, as it's usually easy to see how to put them into practice.

I was also wondering why many 600cc airboxes all share the same basic design for the ram air tubes entering the airbox. They don't go straight into the airbox:

It's pure speculation, but I'm guessing that the transition from the intake runners to the airbox is used to turn the airstreams so that they enter the airbox more parallel with each other and don't collide in the middle of the airbox. It's probably to try and reduce turbulence.

The bend will also slow the air stream down, as it'll lose energy turning the corner.

[David W]

About resonant tuning of airboxes, or hemlotz frequuency....the volume of the intake box in calculated as that of the area between the closed intake valves and the rear edge of the air filter material....think on this one for a bit, and remember the important volume is what comes after the filter and before the intake valve heads.

I did some Googling and found a couple of threads about this, on Ducatis. Standard they have small filters in the intake tubes but some people (JHP in Coventry I think was one name dropped) were using Pipercross 'sock' type filters that fit directly over the velocity stacks. One of the reasons for this setup is that the fuel tank pops off a crashed Ducati quite easily allegedly and the sock stops the motor swallowing half a graveltrap and scrapping the motor as well.

Anyhow, it was claimed by people that seemed to know what they were on about that the difference in performance between the filter setups was fairly negligible, but the increased engine protection tipped the balance in favour of the sock design.