K&N has a formula one can use to determine the square surface area of filtration needed with thier product (oiled cloth) for your particular application. It is explained as follows:
Pleated K&N filter material will flow 6.03 cfm of air per square inch. By comparison, a single square inch of the highest flowing paper will allow 4.95 cfm of air to pass and the freest flowing foam will flow 4.38 cfm. Use the formula below to compute the minimum size filter required for your particular application. The usable portion of the filter is called the EFFECTIVE FILTERING AREA which is determined by multiplying the diameter of the filter times Pi (3.1416) times the height of the air filter in inches, then subtracting .75-inch. 0.75-inch is deducted to compensate for the rubber seals on each end of the element and the filter material near them since very little air flows through this area.
A=(CID X RPM)/20839
A = effective filtering area
CID = cubic inch displacement
RPM = revolutions per minute at maximum power
Using this approach, one can quickly see that many of the low profile K&N's (with solid top) may not be adequate, and most all of the standard size and shaped "paper or foam" element units are totally restrictive. For example, the large oval "Cobra" style assembly with 2" high paper element provides only 72 square inches of filter area. A 427 at 6000RPM requires 122 square inches of filter area using fresh K&N cloth. For the same flow with new paper, it would require 184 square inches. That's one huge paper filter.
This all now speaks to the foundation and concept behind the K&N X-treem top. It allows one get the needed filter area on an assembly that is reasonable in size.
As for the Stelling & Hellings "period correct" assemblies, they are woefully inadequate in terms of flow. Keep them in the trunk for the car shows. As well, the oval Cobra style assembly (single our dual quad) is also quite restrictive, as has been proven in dyno tests that were done to determine the effectiveness fo the K&N X-treem assemblies. Next to no air cleaner at all, the K&N is so far the second best alternative.
Cheers...
Zderf
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