® More on Air Filters

Elsewhere on my site I have said a lot about air filters, but the subject bears more consideration. The truth is K&N air filters (and all of their type made of wire mesh over fabric gauze, for example BMC and others) are a liability to the engine. They pass more air by passing more dirt, much more. The gauze "filter" is a race-day design that should never have become as popular as it has. Racers don't care if they get more ring and cylinder wear, or if they have to service the filter often. Their engines are getting rebuilt anyway. But for every other use, the gauze type filter makes no sense. Most folks choose it I suppose on the basis of the maker's 100,000 mile promise, that is, that with care the filter lasts that long. To a smaller group perhaps the lure of performance advantage is the reason. But these are both doubtable claims. First, most folks find that despite care the gauze filter will reach a point at which it cannot be cleaned effectively without using processes that will harm the body of the filter. So they are not lifetime filters. Second, performance gains are possible, but they are neglible and are only realized if requisite changes to the fueling system are also undertaken, changes that become necessary because the filters pass more air, especially on certain models of vehicles, and changes that most users of the product do not know how to perform and don't end up doing so. Even many professional techs lack the ability to make the necessary changes to carburetor jetting or fuel injection controls.

But it gets worse. For some time, better shops have been on the alert for a certain symptom that accompanies gauze air filter use. The symptom is low cylinder compreasion combined with excellent cylinder leadown. Say what? Yes. Experienced techs know that a compression test and a leakdown test work together to ferret out conditions that cannot be determined by either test by itself. Such is the case with engines that have been run on gauze air filters, particularly in dusty or dirty environments. A low cylinder compression test normally would point toward a worn cylinder and/or rings, and this can often be the case when using an inferior air filter. But when the cylinder at the same time passes a leakdown test, this indicates the cylinder and rings are fine, and for that matter the valves too, so something very different is at hand. Pro techs know to then check valve timing, and it will subsequently be found that the camshaft(s) is (are) significantly retarded, producing the low cylinder compression without affecting actual cylinder sealing ability. The cam chain has worn prematurely, "stretching" until the cam is so late that the buildup of cylinder compression is affected. This conclusion is confirmed by observing that the engine's cam chain tensioner is almost fully deployed. The cam chain is toast.

What happens is the same dirt that the gauze air filter passes into the cylinder also makes its way into the engine's crankcase. This is due to the simple fact that most if not all engines have a link between air filter and crankcase as produced at the factory. Dirt that gets into the oil acts as an abrasive to the moving parts that are bathed in engine oil, with the cam chain being the most vulnerable. Why is the cam chain more vulnerable than say the main bearings or rod bearings? Because of its design. Most cam chains today are of the link plate design. This design, unlike the once common roller chain, is very high friction and has high specific mass, meaning it tends to wear quickly, often within 30,000 miles even under ideal conditions. Adding an accelerant to that wear in the form of a grinding paste made of oil and dirt just makes things happen sooner, often much sooner.

Conjecture? Theory? Pointless rambling? Not at all. I have observed it firsthand. More importantly, it's happening all over. Within the inside of the industry a persistent rumor is that during the late 1990s a class action suit was filed against American Honda for presumably failing to honor its warranty on its FourTrax ATV. I actually managed the service department of a Honda dealership when I first heard of this. Reportedly, it came to light that Honda was asking these inquirers the all-important question, "are you running a K&N air filter," to which so many seeking warranty consideration for needed engine repairs replied "yes" that the class action was established, and was by the way defeated/defended by Honda, again, reportedly. I can tell you this. At Kawasaki, a dealer is alsays asked the K&N question, even today. And although the rumored Honda-related action concerned off-road machines, similar engine wear on road going models has been observed by shops also, in areas in the western U.S. where dry dirt is abundant, such as California, Arizona and New Mexico. And even those engines not having cam chains are subject to premature oil pump and crankshaft wear.

Do you need more reasons to not use a K&N air filter? I don't. Relative to all this, see my article on the three types of air filters. Oh, and one more thing. As with all my articles, I write this out of the storehouse of experience. Nonetheless, checking around the Internet has shown that a lot of other folks question the quality of gauze type air filters also. Until this week I had no knowlege of the raging wave of comment on this subject. I have merely been speaking from my own experience. Consequently, I hesitate to associate myself with these folks because so much of what is on the net is unsubstantial, vitrolic and partisan, and I loathe that kind of thing and have no axe to grind, no angst to express. However, look beyond the obvious rhetoric in much of what is said about gauze filters and pick up on the debate's objective factors. They're real enough. Amidst all the heat there is in fact more than a little light...

Last updated September 2021
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