This is a bit like an oil thread. The RAMAIR is very good for those who wish to retain the bearclaw. So are the unifilters pictured above, but they are a bit fiddly and require regular cleaning, which folk are not inclined to do often. I have tested the ramair over a set of RU1770 K&Ns and the bike did 203hp with 1770's and 199hp with ramair. It is not just the filter medium, but the open filters allow cool air to get to the filter. The 4040 filters are good but expensive. SN2510 is another good number K&N. I have been using the 1770's for at least ten or twelve years, but most folk wish to keep the bearclaw. On a completely stock engine with an exhaust only, pretty much any filters will do the same, ramair, 2780 , uni's, whatever fits under the claw.