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.