# Airbox Volume

#### hoosierf

A friend wants to fabricate an aluminum fuel tank to go under the seat of his 200 sx so he can finish a 2 hour harescramble without re-fueling. The problem is that the only place it will fit is in the air box. How detrimental to performance might such a modification be? Just looking at it, I'd say the airbox volume would be reduced 40% at the top.

What say ye experts?

#### Jaybird

##### Apprentice Goon
Total area of the intake oriface's is the constraining factor. The volume in the box is merely a recieving tank of sorts.
You can only get in what air the intake holes outside the box allow.

Invision a big air tank....imagine it full of air....there is only one oriface entering the tank....the oriface is 1/4" in diameter.... the only air you can get into the tank is what the 1/4" oriface will allow at any given pressure. The tank can be of any size and you will still get the same amount of air in the tank becasue your restriction is the 1/4" opening.

If you block any of the openings and make them less of an opening, then you lose volume. Imagine covering up half the 1/4" opening.....it would allow half the volume at that point.

Place anything you want in the big tank, it will still intake the exact same volume empty, or half full of something.
Only when you fill the tank enough to make the area inside the tank less than the area of the 1/4" opening will you hamper airflow.

#### David Trustrum

##### Subscriber
Ok I’ll bite.

Yes you are totally right if your suction source is a constant suck, like if you had a piston with an infinite stroke & it never had to come back down.

Revised theory? :cool:

#### Jaybird

##### Apprentice Goon
The key is intake oriface. As long as the cubic inches of the reciever (air box) isn't made less than the total of the intake, it hinders not.

We could have a box 2 meters square and it cannot provide the motor with more air than the intake orifaces can generate. Now, like an air recieving tank, the volume of air inside the box will still provide the engine with air even if the intakes are sudddenly clogged....but the volume that the box can hold will be eaten up in about three revolutions of the motor.

On the other hand, if the area in the box is less than the intake area, we starve the motor. (imagine a 10mm pipe feeding air to the engine, there could be ten 10mm pipes coming into the feed pipe, but the feed is restricted to the 10mm pipe at intake.

David, intermittant stroke only puts things on hold until next cycle. Theory still applies.

The key for air intake is to have as much intake area as the carb bore can produce. The carb bore is the final dictating restriction.

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#### cujet

Member
As a general rule, 2 stroke airboxes are not high tech tuned volume parts. So the typical dirt bike airbox has the task of keeping excess water, mud and debris off the air filter. I have yet to see a tuned 2 stroke airbox. Street bikes regularly come with tuned airboxes that augment the power output in certain RPM ranges.

Chris

#### jmics19067

The key for air intake is to have as much intake area as the carb bore can produce. The carb bore is the final dictating restriction.

I agree I am guessing the difficult part would be trying to figure out how much air is actually passing thru the carb witha strong engine signal pulling it in compared to a weaker signal trying to pass thru the filter.

Most air boxes I have seen are usually fairly big around the filter but yet are for lack of better words sealed off when you put the seat on. Not really sealed you do have lots of gaps and you also have that air gap at the backof the seat /rear fender area. So what Iam thinking is that you have a small area<carb>, large area <filter>, small area< any way the air can get to the filter>. The filter being large so air can easily pass thru it. Even when partially clogged with dirt and the air box sorrounding it so all air can flow to all around the filter. Now if you fill the airbox up are you going to be blocking off a section of the air filter so you have a high amount of flow aimed at one part of the filter ? with that being the case are you going to be clogging up that one exposed area of the filter with dirt and the air can't flow to the rest of the filter efficiently?

Using Jaybirds barrel with a hole in it scenerio,

if you have a hole in the bottom of the barrel the smallest orifice is going to be the restricting factor to flow , but if you are straining dirty water the strainer or plumbing to the strainer could quickly become the restricting factor

Does that make sense?

#### MrLuckey

##### Fire Marshall Ed
Call me silly but can't he just put a bigger fuel tank on it?

#### jmics19067

Originally posted by MrLuckey
Call me silly but can't he just put a bigger fuel tank on it?

:think: :eek: :worship:

#### Jaybird

##### Apprentice Goon
LOL...good point, Eddy.

Or perhaps one of those number plate tank jobs.

jmics,
You are correct in that the openings that allow for air flow are the real restrictions. That is why folks will see an increase in performace ability when they open things up to allow a greater volume of air to enter.

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#### Studboy

##### Thinks he can ride
Jaybird, you mean like cutting small holes into the sidepanels?

I've been thinking about experimenting with doing that on a set of sidepanels that I have. It probably won't help much if any though, but worth a try.

Has anybody done this to their dirtbike?

#### marcusgunby

On the 94 KXs that was the hot mod at the time, and also removing the splash guard(the clear one)

#### David Trustrum

##### Subscriber
When I got my KDX it had no airbox lid, ok all fair & good, fairly common practise, (although I made a splash guard from an old oil container) but then someone had drilled holes in the side of the airbox.

No.1 small sharp edge holes have lousy flow characteristics & hey - the airbox lid was missing so I think the intake was big enough & No.2 what the heck were some pissy little holes going to do?

More so, my effective stream crossing safe depth was effectively 6” lower & on the side more exposed to the bow wave. On a MX bike you may not care but a trailbike they got filled in pretty quick.

My GasGas sadly has a pretty leaky airbox at the sides & I have had a couple of scares crossing streams. Needs modding.

#### Studboy

##### Thinks he can ride
Marcus,

On my 94 KX250 I had a set of side panels with the grab handles cut on the inside. I was pretty sure that the bike could pull a little faster up hills at the sand dunes.

Does anyone here think that I would benefit from drilling lets say 4 1/8" holes in each of my side panels near the top strictly for dune riding.

#### David Trustrum

##### Subscriber
I had a set of side panels with the grab handles cut on the inside.

It would certainly make it faster to pull out of a bog. :debil:

#### TheJunkMan

Member
I am not a big fan of extra holes in the air box for one simple reason easier for the big sh*t to get in to the filter, Quad guys do it all the time and if they see any mud or water that air filter can plug or be a biotch to clean I just like a little dusting on my filter not the whole pasture/dune/track/rocks in my air box