dcstrom said:
My own experience on the Vee is that the K&N filters fine once it clogs up a bit.
If so, this is completely counter to the way a foam filter typically works.
An oiled-foam filter uses oil-capture impingement as the filtering mechanism. As such the mechanism for particle capture is like a maze where the gaps between the fibers are much bigger than the particle you want to catch, but the depth of the filter means sooner or later particles are likely to hit a fiber. (Think trying to shoot a golf ball through a tree - the tree volume may be 98% air, but that doesn't mean the golf ball has a 98% chance it will make it through.) When a particle hits a fiber, it is caught and retained, but ONLY if the fiber has oil on it.
This is a completely different mechanism from a paper filter where particles are caught by a sieving mechanism. Particles are retained because the gaps in the filter are too small for the particle to fit through.
The problem with oiled foam is that once the filter loads up and the oil is depleted, the particles don't stick anymore. A loaded filter becomes like a paper filter except that the gaps between fibers are about 100x as wide as a paper filter. If you don't religiously stay up with maintenance on oiled foam, it is like having no filter at all. That's not a big deal on a dirt bike where you might clean/re-oil after every ride.
When I read the literature for K&N (after ignoring all their marketing garbage), it sure sounds like they're using the exact same mechanism as a foam filter for particle capture - they just are using multiple layers of cotton fabric rather than foam. I know K&N may say otherwise, and certainly a loaded filter is going to catch gravel, but my technical judgment is that a loaded K&N is a truly lousy filter from an engine protection standpoint with respect to small particles - particles that would be caught easily by a paper filter. And a paper filter "fails safe" - as the filter loads, the gaps between the fibers in the filter become smaller and the filtering efficiency rises (at the expense of greater pressure loss across the filter). IOW, a paper filter will start to suffer a performance loss, but will always protect your engine. A oiled-foam filter slowly losses efficiency while continuing to let your engine breath fine as it self-destructs.
As I said before, I'm not a K&N fan. And I know others swear by them just like some swear by Amsoil oil/filters or STP oil treatment. When a mfg makes outlandish claims for their product, my radar goes on high alert and K&N certainly makes some outlandish claims that are simply not supported by the available objective data or a rigorous technical analysis.
I'm not completely anti- oiled foam or even K&N if you prefer. On a dirt bike, they're the way to go. Of course, I clean the oiled-foam filter on my DRZ almost after every ride int he dirt. You can save some money with oiled filters over paper. But IMO, you better keep up with filter maintenance if you value your motor.
- Mark
