All the K&N ones I know of are a wet element filter. They use oil to help with the filtration process. Not enough oil and it won't filter properly, too much and it will coat the hot wire in your MAF (in the Z anyway, probably won't effect the bikes as much) and increase the restriction throught the filter. The Apexi filters are a dry element, like a normal paper filter. Not too sure what they are made of, but flow well. There was an article floating around on the 'net which tested the HKS, Apexi, K&N and another brand and found the Apexi to flow and filter the best. Zoom magazine did a large test with nearly 20 different filters, they where testing power only. Back to back power runs on there FWD drag car (turbo mitsu cyborg), found that the cheap Autobarn metal pod made the most power, but you could see through the element Would only stop large birds and boulders from being sucked up into your engine.
yeah thats the problem these tests are on power only , not on how effective the filter is at filtering which the main purpose of the filtier afer all . Ineffective filter could mean the life span of the enging could be greatly reduced but they dont mention or show test results on that for obvioiuse reasons. As you say it seems the better at allowing air in the worse they are at filtering that air. However if there is a large difference in the effectivness of pod types i might have to look into it a bit more , as i would go down this path and install one myself if i thought it wouldnt greatly reduce the life span of my engine.
On the Zoom test, there was only 2.3kW difference between the top performer and the bottom I've never seen a test properly done with the filtration of performance filters for sports cars. I assume that there is 4WD air filters out there that they would have doen proper tests on. Could be something for ZX2NV to do when he gets some more spare time
I got one of these too, and it removed my flat spot too. But, when I took the old filter out, I found a foam filter that was 30% covered with paint from when the car was resprayed (before my time). The rest was covered by a greasy substance.
