What does an air filter do, and which one is right for my car?

This page will briefly try to answer some questions about automotive air filtration, how it works, and what it does and does not do. The pie chart on the right gives a general idea of the makeup of the air around us. Of course this can vary quite a bit, if you are driving down a dirt road in an area that has had little recent rain, the conditions under your hood would be somewhat different. The red area of the chart would be greater than 1/3 of 1% of the air mass.

The bar chart on the left gives a general idea of the size of the particulates that you find in the atmosphere. Most, but not all, air borne engine wear is due to particle sizes in the 10 to 20 micron range, anything much smaller (less than 2 microns) should not do too much damage. Of course larger than 20 micron particles can cause huge damage to an engine, but any modern air filter should catch these.

Automotive air filters use one or more of 3 principles to do their 'filtering' work.

Interception - As the name implies, this works by intercepting particles as they try to flow into areas that are too small to let them pass through. This is the primary method employed by the paper element filter.

Impaction - Larger particles that do not follow the flow path of the incoming air and impact directly on the filter material there to be captured or simply bounce off. While this happens with the paper element filter, the outer layer of a dual foam type filter is designed to use this principle to great effect, prolonging the periods between maintenance cleanings.

Diffusion - This principle operates only in foam or cotton element filters and deals with the very small particles subject to 'brownian' or nearly random motion in still air, dust motes swirling in a shaft of sunlight are a good example. These small particles are more affected by the turbulence in the air stream causing chaotic motion that causes them to adhere to the filter surface. The inner 'fine' foam filter is designed to take advantage of this principle.

Another aspect of an air filter's job is to reduce turbulence around the SU's air intake, this is why an air horn is built into all our filters. An air horn or velocity stack, if perpendicular to air flow (as most SU applications are) will induce turbulence at speed. Our filters provide a plenum effect so that the air horn has relatively calm air to draw from. Momentary reversals of the air flow, present to some degree in all internal combustion engines, are also dampened by this design.

And speaking of air flow ...

Our 2.6" filter has a usable filtering area of 32.5 Square Inches (210 Cm2). The 1.6" filter has a usable filtering area of 21.1 Square Inches (136 Cm2). A 120 cubic inch (2000cc) engine @ 6000 rpm needs about 35 Square Inches of usable filter area - 180 cu.in. (3000cc) at the same rpm about 50 square inches. Two of our filters on dual SU carburetors provide 65 (2.6") or 42 (1.6") respectively. I am specifying the filters by usable filter area because (and this is a secret!) ... all modern (and unused) filter types flow about the SAME! Some ARE better than others but you have to look very carefully to tell the difference. How much air they can flow when they begin to load up on dirt and how well they clean the air are what sets the various types of filters apart!

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