Found the link again, I cut and paste the entire article below for furute reference,
in part its good info on airbox's
http://www.snowtechmagazine.com/article ... airbox.php

Another must read I didnt cut and past here is this one.
http://www.snowtechmagazine.com/article ... petemp.php

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What Your Mother Never Told You About Airbox Modifications

By Lonn Peterson

The airbox is without a doubt the most misunderstood piece of equipment on any ATV, Motorcycle or Snowmobile. The factories sometimes make them too restrictive often to reduce decibel levels. Are there gains to be achieved by opening up the airbox thereby making it freer flowing? Yes, there usually are. Are we to understand that the less restriction an air box has, the better? No, absolutely not! There is a point where you can go too far. Here is where it gets interesting.

With your standard airbox in place there is a greater pressure differential between the cylinder and the atmospheric pressure. In other words, the fuel and air want to go into your cylinder due to the pressure drop (lower pressure). As your airbox is opened up more and more, the pressure differential becomes less and less. The fuel and air is less inclined to flow into your cylinder.

Here is an explanation that may be easier to understand. Let’s say that you are drinking juice through a straw. Now someone pokes a hole in the side of your straw. You must now suck harder to get the juice while also getting quite a bit of air. Let’s put a much larger hole in the side of the straw. Now, you must suck like crazy to get any juice while getting liberal amounts of air. This is exactly the problem your Engine has when you open up the airbox too much, eliminating nearly all of the restriction.

When your intake flows too freely you will have an Engine that is fussy about jetting for temperature, altitude, etc. The general rideability is often lacking. Throttle response is sluggish in the lower to midrange rpms. Fuel mileage will also suffer. Extensive recalibration may be able to improve some of these conditions, but most will remain. The engineers knew exactly what they were doing when they left a given amount of restriction in the airbox.

We stated earlier that there may be gains to be achieved by opening up the airbox. Then we turn around and tell you that an airbox that flows too freely is a bad deal. So what is the answer? Some of you may have already guessed that the answer is in “the proper balance” between free flowing and overly restrictive intake. How do you achieve “the proper balance” or the sweet spot? One way is to begin opening your airbox a small amount at a time until you hit the sweet spot for your area. Of course, if you go too far you can correct this with the old stand-by, duct tape, or you can glue pieces back in.

Notice that we talk about the sweet spot for your area. The sweet spot would be different for various parts of the country. For example, a person that lived in Denver, CO would need a larger opening than a person that lived near sea level.
The easiest, quickest, most accurate way to find the sweet spot is with a TPI Valve (TPI stands for Total Performance Intake). The TPI Valve mounts on any airbox and allows you to easily adjust the size of the opening to find the sweet spot for your area. All air entering through the TPI Valve passes through a foam filter in the center of the Valve. The main idea is that the TPI’s are adjustable for temperature or altitude. Simply adjust them to where the Engine runs the best. The TPI’s are only $19.95 each (most big bikes take two) and can be seen on www.thunderproducts.com web site.

We are not against high flow air filters or even the pod type filters that completely replace the airbox. We are not against modifying an airbox. Each of these has their time and place. However, our motivation for writing this article comes from talking with people on a daily basis that are having problems relating to carburetor jetting and air intake systems that are lacking adequate pressure drop differential. We hope that this article will help some of you that have been having related problems as well as preventing others from going down the wrong path.
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Thanks. That's kinda what I've been thinkin' - everything that I've read has said to open up that airbox, just give it more air. Then I started to think about upsetting the ballance with too much air. Then I got my Dial-a-jet which strongly recomends to not modify the airbox.

I guess it's similar to jetting in this regard: trial and error, practice makes perfect.

-mike

airbox pumping

I just got a good idea (at least I think it's a good idea):

Since I'm installing a Dial-a-jet, and the whole point of that is easy, out-and-about re-jetting, I'm going to just modify and relocate my entire air box to behind my head. The lid will become the base (as it will be fixed) and the base will become the removable portion.

That way I'll actually beable to easily adjust the Dial-a-jet without removing the air box every time.

I guess I'll mount the radiator to the angled part of the frame above where the Airbox norrmally is... Will the thermosyphon work if the rad is on an angle like that?!

hmmm...

-mike

By doing the air box mod on my Pilot and adding the second 2" opening I have
only had benefits and don't see any negative affects.

My 89 pilot had a 4 inch hole cut in the back plus the stock hose intake. Just got through doing a double hose mod and turned the hole into an adjustable air regulator. Finished yesterday, so after a little testing, I will up-date with some pics. Initial test ride revealed an increase in performance, even though the cfms went down from original set up. More to come later. ( this setup is so cool, I might have to pattend it. Works much like a dial a let, its just a dial a air!)

Here is a link to some pics of my air regulator and box mods. There was originally a 3 inch hole cut where the reg. is now to allow enough air in to run with lid on box.
http://www.msnusers.com/Pilotpicsairboxmods
Explain this. After closing up the hole and adding the second hose and scoop, I had to re-jet everything richer even though the air duct size was reduced. Could this be the result of pressures as described in the original topic? Runs much better in all ranges now.

What your refering to is none as RESONANCE EFFECTS. Youvhave installed an exhaust system which has changed the resonate frequency which now needs to be matched on the intake side. this has been done by the intake mod. The formula to do this for the engineerd ones(which I am not) is F=Vs/2msquare rout of A/Vc(L+1/2square rotettA, the formula will show you that the resonace in a flask (intake system,which includes the crankcase volume). The frequency is directly proportional to the square root of of the intake pipe cross sectional area. What you have done is reached a good flask. this can be inhanced by increasing you crank case volume and or pumping. Via reed spacer which will increase area and then increasing intake port area thus increasing crank case pumping. With out trowing in sonic speed and and the relationship on its dependent temperature and the temp of of a strem of gases (fuel air mix) simultaneously being being cooled by evaporating fuel/gasoline and then being heated by the surinding metals( piston,cylinder and exhaust gases) The flask you achieved has a volume that varies continuously with the piston speedand intake tracts them selves. Not only area but shape will vary from the carb intake bell to the port window (Reed stuffer smaller area but better shape increase flow better flask). the tract lrenght will affect the peak power and a given rpm. This may be why you see a longer or short intake tract on some motors that has has an intake port change, in order to get peak power at a given rpm. that's why they change to a different drive clutch in order to get the motor (( Internal Combustion Engine ? )) into it peak power band when they mod there exhaust, intake , cylinder preasures in relationship to port timming. THE MORE FLOW THE MORE GO AND THERES NO REPLACMENT FOR DISPLACMENT. After all your trying to improve effencey a 400/500 will only put out so much before it goes BANG by stessing the part past there mech. limits.

YOUR AIRBOX, FRIEND OR FOE?

Separate the Fact from the Fiction
By Lonn Peterson/Thunder Products Inc

There is a very common belief that getting rid of all the restriction from the air box is about the best thing you can do. Wow! Just think of all the horsepower, and it's practically free. Isn't it strange then, that all the manufacturers build model after model, year after year with those restrictive air boxes? Don't they read the same magazines we do? If they did, they would certainly learn about all of the ways to get rid of all that nasty old restriction, right?

Maybe not. Let's look at the physics of how fuel enters your intake system. The atmospheric pressure exerts force on the fuel in your float bowl, forcing it through your jetting circuits, into the air stream passing through your carburetor. In order for this process to work, the internal pressure in your carburetor must be lower than atmospheric. Pretty simple. If you have removed your air box, cut a number of holes in it or are running without a lid, you may be in for a surprise. Your internal pressure is now nearly the same as atmospheric, making fuel delivery difficult.

Engineers call the difference in pressure a pressure drop differential. A particular amount of restriction (some call it vacuum) provided by your air box keeps the internal pressure lower than atmospheric, insuring consistent fuel delivery over a wide range of temperatures and altitude. Without an acceptable pressure drop differential, the Engine will be fussy about changes in jetting to maintain performance.

Another aspect that affects pressure drop is air speed (velocity). Simply put, the higher the air velocity in the carburetor, the lower the internal pressure. When you attempt to operate with a wide open air box or with the lid off, the air velocity is actually slower (yes slower) due to the enlargement of the intake opening. You now have plenty of air; however you also have poor air velocity. Velocity is needed to atomize the fuel and produce quick throttle response. This slower air speed promotes higher internal pressures, closer to atmospheric, hindering fuel delivery.

Yes, it is possible to get an increase in performance by running with an open air box or even removing the air box. This may be OK if you always ride at the same temperature and altitude, say Southern California for example. However, if you are dealing with temperature and altitude changes, expect to lose horsepower and performance unless you are a jetting whiz. The same jetting set up is not right for everyone or everywhere. Temperature, altitude, air density, atmospheric pressure, fuel density, Engine pressure drop and humidity are all part of the total equation for air/fuel delivery. This is why it is important to have adjustability of air as well as fuel.

Is the factory air box restrictive? Yes, it is usually somewhat restrictive. To insure optimum performance, it is necessary to arrive at the correct amount of pressure drop (restriction) that works for your riding requirements. This always requires some experimentation. Rather than drilling holes, cutting holes, and taping or gluing holes shut, there is an easier and more scientific method. There is a product called a TPI Valve (Total Performance Intake) costing $19.95. The TPI Valve looks like a small picture frame with a slide door that opens or closes the frame. There is an air filter built into the TPI Valve. You cut approximately a 2" x 2" hole in your air box, usually in the lid. Install the TPI Valve. Now you can regulate the size of the opening, controlling the pressure drop and air speed of your air box. This allows you to quickly and easily determine the right amount of opening by trying a number of different openings in a matter of minutes. You may find your machine runs best at 1/4, 1/2, 3/4 or maybe full open. The amount of opening in your TPI Valve will give you the equivalent of one to three main jet sizes depending on how restrictive your air box is. We call this "air jetting". The correct amount of opening can be judged by the machine's performance. Good performance from zero to top end indicates the proper amount of opening. You can also return to a totally stock air box by closing the TPI Valve. This is handy for very wet, muddy or sandy conditions.

Before you run out and spend all of your hard earned dollars on some super intake system that gets rid of all the restriction and claims to make tons of horsepower, you may want to ask a few questions.

After discussing the air portion of the intake system, we feel compelled to at least mention the fuel side of the system. The ability to tune an Engine to perfection depends on being able to regulate fuel and air. The Dial-A-Jet automatic fuel induction system is a completely adjustable external jetting system that delivers maximum horsepower to any Engine at any temperature or altitude. Dial-A-Jet improves throttle response and fuel efficiency. It works well with stock or modified engines, standard or high-performance air boxes and exhaust systems. Easy to install kits are available for snowmobiles, ATV's, dirt bikes and motorcycles.



http://www.thunderproducts.com/AirboxArticle.htm

The straw thing with the hole doesnt really seem to relate. In that case your trying to draw in a liquid and getting some air with the mix. If you cut a hole in the air box its pulling air, the same thing coming in the inlet. And since the hole is opening up the cross sectional area of the opening, you have to suck less to get it in.

But, there is some tuning involved in the box volume itself and the linlet size and location, so more inlets does change that.

Straw thing is trying to relate to the loss of signal in the carb or its ability to efficiently draw fuel from the float bowl up through the metering gets and then to emulsify the mixture.

When you lose carb signal (hole in the straw affect) you cant pull the fuel into the air stream properly you end up with a lean mixture the result is the owner installing huge over sized jets so it can suck the gas up out of the bowl easier through the bigger hole than trying to suck it through the smaller hole.

I have seen this many times over the years on Pilots, guys running super big main jets, I will be running a 160 genuine Keihin main jet while others are running 180 and bigger, my Pilot would be faster and out performing these machines fact is my full modified Pilot running the 160 main was as fast as 4 different ATV Racing full modified CR500 converted Pilots, we drag raced on hard pack dirt, soft sand and even going up hill and my full mod 400cc Pilot with the little 160 main jet was right along side them all the way.

Until people can wrap their minds around the carb signal thing they are doomed to run a Engine that gags pukes and sputters and they can never seem to get it jetted correctly they also suffer more seized pistons with their bigger is better more flow is more go theories they can never prove to me in the real world.

I learned about the carb signal thing at others expense watching them blunder their way through life thinking they were better and smarter engineers than the factory, these guys lean by reading magazines for advice where you cant get the whole truth and ask no questions, written by hacks and quacks that cant even get the basic terminology correct, simply put its just not creditable info and requires lots of detailed testing, lots of psychological gains found in magazines seems the average person likes the idea he is smarter than the factory designers and has one upped them idiots at least until they line up against my Pilot for a comparison run.

Quick example how I learned, I have a friend that has been drag racing motorcycles for about 35 years, back in the mid 80's his dad owned a used car lot and one of them 6 cylinder Honda motorcycles came on the lot so naturally he took it with him to the strip so he could run in two classes, double the fun for the cost of entry fees and fuel for the bike.

He took it and raced it about 5 weekends made countless runs trying different jetting and different tire pressures established a good baseline, then by reading the popular mods for that bike in the day in magazines he removed the whole air box system and ran it with no box or filter, just bare carbs, he spent another 5 weekends trying to recover the full second he lost in the 1/4 mile, tried every jetting combination he could think of the Engine would just not produce, he reinstalled the air box set the jetting back to his fastest runs then spent the rest of the season targeting the bikes with owners who removed their air boxes, he admitted that was not the only bike he had owned that more flow = more slow but it sounded much faster,
the increased vibration from the intake noise sent messages to the brain he was faster until he made it to the drag strip or radar gun.

He learned this lesson with snowmobiles too, every winter he dad went to the auctions and bought about 6 sleds then all his buddies run the piss out of them most the winter then he sold them on the lot, ride sleds all winter and still make money