Honestly, if I was like "yo...my car is lightning fast now that I put my K&N in", it would be psychological. I did not feel any increase in power by adding it. However, I did see a legitimate increase in my gas mileage. I keep careful track of this, and the tank after I put it in, my mileage increased 3-4 mpg and it has consistantly stayed that high. I have the records to prove it, so I don't think its psychological. Oh, and before someone says "well, your old filter was probably dirty, so you were just getting back what you lost from a dirty filter", my car had the original air filter in it from the factory, and when I changed it, the car had 10k on it...far less than the suggested mileage for changing your air filter. When I took the stock one out, it was relatively clean and new.

There has to be something to K&Ns. No matter what type of car you are talking about....Civics, Vettes, Probes (I used to own one), Mustangs, truck guys, etc. , they all swear by K&N. I can't think of another product that is so openly accepted and sworn by by all forms of car enthusiast. Come to think of it, I know motorcycle people swear by them to. With this being the case, I really doubt they are all hype. There has to be something to them for them to be so used and accepted.