Everyone claims their air filter flows best, and at the same time removes the most dirt. Think about this. A solid piece of metal will prevent dirt from entering your engine, but it won't flow any air. On the other hand the screen used on your bedroom window will flow a lot of air but won't catch much dirt. There are really three goals involved in air filter selection.
- Provide maximum airflow.
- Protect against airborne dust particles.
- Perform efficiently when dirty.
- Protect against airborne dust particles.
- Perform efficiently when dirty.
You should try to use an air filter that traps particles over 10 microns. A grain of sand is roughly 50 microns. BMC, who builds a lot of the air filters used in F1, claims they have filters that can trap 7-micron debris and still flow efficiently. Most of the air filter manufacturers though are very reluctant to share this information. K&N told me that it was proprietary information and they were not about to tell me what micron levels their filters trapped.
Filter Size
When fitting a conventional round filter on top of an engine, with a carburetor or throttle body fuel injection, a large diameter short filter will flow more air than a small diameter tall filter. A 10-inch diameter filter that’s 2-inches tall will flow more air than a 5-inch diameter filter that is 4-inches tall. A basic rule is that the height of the filter should be between twenty to twenty-five percent of its diameter. Coast Fabrication feels that “Many racers underestimate how large the filter must be to avoid restriction.”
Velocity stacks present a unique problem. If they’re exposed to the outside air velocity stacks may hinder performance at high speed. I'm referring to the stacks and air horns that you see on some Can Am and F5000 cars. These stacks extend into the air stream so the direction of the air rushing over the car is at a perpendicular angle to the intake tube. Air moving rapidly over these stacks creates turbulence inside the opening. At high speed this rushing air tends to create a partial vacuum inside the tube.Installing an air filter on top of each stack, or over a carburetor air horn, eliminates this vacuum by creating a plenum over the opening. The air is smoothed and straightened. ITG feels that if you have ram pipes there must be a gap between the end of the pipe and the underside of the top of the filter. Sock type filters or screens affixed over the intake can help engine life but in most cases they will cause a small loss of power.
Some cars use an airbox that is actually an air intake chamber. If the inlet to this airbox is too small the engine will be starved of air. If the airbox opening is too large the engine will be less efficient because you will have too much air. There is a correct amount of air. K&N points out “In the vast majority of cases increased airflow will increase engine performance.” But not always.Paper, Oiled Gauze and Foam
The disposable paper filter is the most common filter and it isn’t too much different than your coffee filter. The only real difference is the way the pleats are used to gain more surface area. The total surface area on a paper filter is critical.
K&N is an example of an oiled gauze filter. These work similar to a paper filter but two things make them different. The media is much more open (significantly bigger holes), and the media is oiled. This oil creates a sticky layer and as the dirt runs into the media it gets stuck in the oil. That's why you have to clean them on a regular basis. Foam filters work by having millions of small open “cells” through which the air, and anything floating in it, must pass in order to reach the engine. High quality, duel density foam filters manipulate the size of these “cells”. There is often with a fairly large cell size on the outside element to grab the big stuff and a much smaller, more tightly packed cell structure on the inside element designed to catch any of the grit that made it through the first element.
Coast Fabrication points out that while all filters have advantages and disadvantages foam excels if there are space constraints with your car. Companies such as ITG and BMC can develop a custom foam filter for your application. No matter which filtering medium you finally select though it all comes down to using as large a filter as you can fit on your car and cleaning the filter on a regular basis. The other choice is to rebuild your engine more often.
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