Neil, When testing the paper filters, at the very outset of the test, the particles of dust clogged some of the pores of the filters. For a very short time, the efficiency of the filters was better than when new, without measurably increasing the restriction of the air cleaner. In some cases we measured over 99% efficiency. Thereafter, as sure as death and taxes, the efficiency didn't increase, (That we could measure.), but overall restriction increased with added dust load. I haven't done any study of the K&N filter systems except to read advertising, but for a given size of filter element, it would seem to me to be a natural law that size, filtering efficiency, and restriction must be tied together in some natural formula. Without access to some meaningful test data, it's anybody's guess which is the better filter. Joe Savard Lake Orion, MI [Non-text portions of this message have been removed]