I know this subject has been talked about a lot, and I also know that there are, or appear to be, two distinct schools of thought when it comes down to K&N (or replacement) filters vs. OEM- and I'm really not trying to start a discussion with regards to that (you either do or don't). But- I'm interested in this, partially because my particular bike has holes drilled in the backside of my airbox, so there is a direct exposure there to the atmosphere and the elements- rain, dirt, etc... In reading about this subject (and I've done a bunch) I came across a post in one of the threads (can't even recall who the author was) and he (or she) stated that if modified, the K&N outperformed the factory (paper) element. There was no follow-up given though, as to what that mod was. I'm hoping, whoever that was that wrote that (or anyone else who might know) could give me a clue. I do know that supposedly they filter best when they're left a little dirty. Lastly, along this same subject, I did read that if I did the switchover to the K&N, that rejetting isn't needed. The bike's a '75 R90s.