Google "Spicer Air Filter Test" and you fill find a very good test done in '04 that is much more comprehensive than the one mentioned above. Unfortunately, it's very dated now. You will even find some discussion of it here on BITOG. The good thing is that the testing was done to SAE test standards. The bad thing is that it was a narrow range of filter applications.. actually a single one... Duramax trucks. Still, it's a good indicator of efficiency, dirt holding, etc. It was once on an "official" website but that is long shut down. Many sites glommed onto it, however, and you can find it permanently posted all over. Arlen Spicer spent a good amount of money on it... where are you now, Arlen?