Another way that is not as common, but is almost as effective, for rating filters and takes 1/20 to 1/100 the time of the beta ratio tests, is the mean flow pore (MFP) rating.
You need a porometer (measures the size of the of the pores in filter media), which aren't that common and pretty expensive
. Barring those restrictions, a porometer is a great way to measure the effectiveness of a filter
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Here's some of the mathematics behind it. Think of a bell curve. This bell curve represents the different size pores in your filter, and is called a pore distribution curve. The very top of the bell curve is the Mean Flow Pore. This is the average pore size that will be allowed to pass by a filter.
If you were to then take the CFM (cubic feet/minute) flow rate of that filter you were working with, get the square root of the CFM and divide the MFP by the result, you get whats called permeability per pore ratio, perm/pore for short. This is able to be compared to other filters and the higher the perm/pore ratio, the better the filter.
That probably is a lot to absorb, so I'm sorry if I confused you guys, it took me a while to get it too
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I'll try and get that info for you gents (and gals) about the K&N filters, and if I can get a sheet of the specs/data, I will post them later.