Aftermarket replacements for the stock pleated paper air filter are not good choices. Obviously, if your bike has been customized and you can’t use the stock type air filter, you do what you have to. But it is a compromise.

The general public is not aware how restrictive Honda’s intake systems are. This is due to the federal certification process Honda has to go through to bring a new model into the country. During this process Honda has to, among other things, ride the prototype bike past a sound meter. They therefore engineer many things into the bike to make it as quiet as possible. One of these is the air filter housing. In fact Honda’s intake systems are so restrictive, that on a Honda CBX1000, removing the airbox requires the carburetor’s main jets to be increased ten sizes. Yes, ten. The K&N company has capitalized on this characteristic of Japanese sound control and has been very effective at selling their product as a power-increaser (the CBX gains a minimum of 25hp and can with a few additional changes exhibit an increase of up to 40hp). They have also promoted the K&N product as a wash-it-don’t-replace-it, and "lifetime" alternative to the stock air filter, which is very misleading.

The K&N gauze type air filter flows more air because it flows more dirt, a fact confirmed by users around the globe. Whatever your preferences, you cannot escape this simple fact. And you can prove it to yourself. The industry standard procedure for testing the contaminant condition of a pleated paper air filter is to hold the filter up to a naked 60 watt incandescant light bulb. All the light should get through. On a good paper air filter it does, a bad paper air filter blocks some of the light. And a K&N? Not only can you see the light, you can actually see the light bulb. It’s more pasta colander than filter. To further understand how much more more air the K&N air filter will pass, consider that the carb rejetting for individual K&N filters and that required for simple air horns (“velocity stacks”) is exactly the same on the aforementioned CBX. That is, the engine “sees” the two as identical. It’s as if there is no filter.

And that is my complaint with the filter. The K&N (representing "Ken" Johnson and "Norm" McDonald) company started in 1969 as a flat track racing team, run out of a Kawasaki dealership in Riverside, California. Malcom Smith later bought the dealership and gave it his name, while the air filter business changed location. But the filter should have stayed a race-day item and not have gone prime-time. Do what you want with your race bike but telling people the K&N is simply an alternative to the stock filter is deceptive on many levels. It is a rock guard. It is not a filter. And far from being a “lifetime” air filter, at some point the gauze filter becomes un-cleanable; it reaches a point where the dirt does not come out. Of course, there is even less filtering ability if you don’t oil the filter in the first place. The trend to not oil (I was surprised anyone would do this but apparently it is “a thing”) may have begun when folks had problems with their fuel injection MAP sensors which would fail when the filter's oil hit them. But of course not oiling the K&N just makes things worse. There are reports of folks who have found silt clods built up inside their bike’s FI throttle bodies. Look it up. It's real.

I don't mind if folks use them, I have used them in my own machines and I have built bikes with them—always making sure the customer knows the result will be less expected long-term miles from the engine. In vintage perhaps this is less a concern. Very few maintenance concerns take on much of a critical nature when you are riding only 500-1000 miles in a year.

Aftermarket foam air filters are also a problem, they have their own issues. Many of them are actually more restrictive than the stock paper air filter. And of course the oil the filter requires always gravitates to the bottom, even when a special racing filter oil is used. The foam filter also eventually disintegrates, but not until the carburetor has been fouled with the oil. And the foam air filter is a lot of work to clean correctly (see my video), requiring as it does a handful of cleaning stages including solvent, then detergent, then clean water and either hot or moving air (not compressed air). And as it nears the end of its useful life, cleaning efforts actually help destroy the filter—its glued-togther seams start to separate. Foam air filters are also hugely over-represented in air filter fires on powersports vehicles.