Stan Bradbury's Speedway Training Notes
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Here, the main requirement is cleanliness, providing that the filter design is satisfactory in the first place.
I did a private survey of several different types of air filters to measure the pressure-drop across the various filters using a sensitive inclined-tube manometer and a constant air suction through the filters. This test was intended to indicate which type and condition of filter offered the least resistance to air flow into the carburetor and therefore the engine. There was no attempt made to evaluate the filters ability to keep dust and dirt out of the engine as this was assumed to be satisfactory.
Clean K & N type filters won hands down among an the manufactured filters. Notice I said "clean." A mixture of dust and filter oil forms a clay-like mud on the surface of air filters and considerably reduces the amount of air which can flow through a filter without reducing the filters' ability to prevent dust, etc., from being inhaled into the engine. Manufacturers who claim that their filters work better the dirtier they get, are referring to the amount of dirt (or lack of it) which passes through a dirty filter and are not referring to the reduced volume of air which a dirty filter would cause. Only a light oil or oil mixed with Varsol should be used on filters which require oiling. A heavy oil on a foam filter literally plugs the holes in the fabric or foam and will considerably reduce air-flow. Quite a number of riders fit a protective foam "sock" over their K & N type filters to reduce the tendency of fine dust to mix with the filter oil and plug the filter pores. This is a good idea as long as the foam is cleaned and/or changed frequently, particularly as any addition filter material means less air reaching the engine. Only a thin, open type of foam should be used for such additional filter material. Filters made entirely of oiled foam will certainly prove effective in preventing dirt from reaching the engine but again, only light oil should be used and dense types of foam should be avoided.
Some fit a piece of "panty hose" over the filter and this also helps to keep the dust from mixing with the filter oil. Years ago, a piece of silk or nylon stocking was the only form of filter that was used on speedway engines.
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