Join Date: Dec 2009
Location: Fort Collins, CO
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This is not directed at any person, it's just to explain my thought process....
My concern about K&N isn't air flow - I'm willing to grant that they may flow more air than the stock filter.
The question is: WHY do they do that??
My reasoning is that to let more air through, unless you greatly increase the surface area of the filter, you must be providing bigger holes for air to pass through, thereby letting more tiny particulate matter through the filter.
More air flow is not a good thing if it also means letting more grit and particulates into the engine. In fact, if that is the case, it's a bad thing.
The K&N filters I have seen have much less surface area then the stock paper filters, which increases my motivation to ask the question in the paragraph above. Just coating a thinner filter with oil doesn't make it more effective at filtering out small particles.
In the short run, I doubt that this matters much. Over the course of 60K or even as much as 100K miles, maybe you won't notice increased engine wear from the additional particulate matter passed into the combustion chambers. But it seems to me that somewhere around the 100K mile mark (or maybe earlier), you probably will see increased oil consumption and decreased compression because of accelerated wear in the cylinders and rings.
If you trade every 3 or 4 years, that's not a big deal - you're passing the problem to the next owner and it's nothing that you need to deal with.
But if you try to drive cars to 150K or 200K miles, then it seems reasonable to me to think twice about running a K&N or other non-stock filter which has less surface area.
I haven't seen scientific tests quantifying the difference in particulates admitted by stock filters vs. K&N. If someone knows of good studies (NOT provided by K&N) I'd be interested in reading them. My reasoning may not be accurate...it's just the best I can do with the information I have.
2008 BMW E91 328xi Sportwagon
2009 BMW E92 328ix Coupe
2000 Jeep Grand Cherokee
1983 280ZX Turbo