The K&N product. It's made exceptionally well. Good quality. Surprisingly durable. Easy to install and to service. Most of all, in the individual ("pod") form it frees up a lot of creative potential (i.e., real estate) in cut and weld brat and bobber building. And its basic premise is laudable: a cleanable, reusable, almost life-of-the-vehicle air filter. Sounds good.
However, that last is not reality. K&Ns are not very cleanable and they don't really last all that long. And worst of all they really don't filter too well. Major powersports shops some time ago discovered that some engines' low cylinder compression was due to the failure of aftermarket air filters in keeping dirt out of the engine. K&N air filters are among these, and unfortunately can be viewed as the bulk of the problem. But here's the unintuitive part. The low compression is due not to worn rings or cylinders, as one would assume, but to fully deployed cam chain tensioners. Yup, severe cam chain wear. Excess dirt enters the crankcase. The highest friction part in there is the cam chain. The elongated cam chain retards cam timing, resulting in significantly lower than normal compression. I have seen this firsthand in numerous machines, and while cam lobe wear is also typical in Japanese bikes and produces the double whammy of both reduced valve action and retarded valve timing, that scenario takes dozens of years to develop. I look for both of these issues when starting engine work, and I find significantly worn cam lobes on nearly every 70s and 80s engine I see. But that's a different situation. Here we're talking a year or less of use, and then it's always the cam chain.
I won't insist that K&N air filters were responsible for the excessively worn cam chains either in every reported instance or in all my own observed ones, but I strongly suspect it. With over 46 years in powersports, I have of course used K&N filters. My observation is that even properly serviced, the K&N filter inherently passes more dirt into the engine than do the other filter types.
So you will appreciate my surprise when several months ago I received a notice in my email inbox from the K&N company encouraging me to read their October 22 blog, "What Happens If Dirt Gets In Your Engine?"
Really?! The irony! In my view the K&N product *causes* excessive dirt to get into the engine. And oddly, while piston and cylinder wear is recognized in the blog as critical, crankcase contamination resulting in cam chain wear is completely ignored. Too bad.
Your mileage may vary. But it's something to think about. And no, I am not on a crusade against the K&N product. But neither do I like it particularly. And, noting the blog's declaration regarding "fifty percent more air flow" available using their air filter over the stock one, isn't it a little too much to assume, no matter what the rhetoric, that this is possible at the same filtering efficiency? Hasn't anyone ever considered where that extra flow comes from? Mechanics have known for a long time, even if many consumers continue to choose to ignore it.