There are three main types of air filter used on motorcycles - paper, oiled foam, and oiled gauze (K&N).
I have always been an fan of paper air filters. You know - the type that come from the factory on most street vehicles. These types of filters have a large surface area due to being pleated, yet also have a very fine filtering ability since their construction is a very tight paper fiber construction. I think these are the best type of air filter for keeping a clean engine. The downside is that you cannot service this type of filter. When it gets clogged, it's time to replace it.
Next we have K&N filters which are a similar pleated construction, but use a thin later of gauze held in place by wire mesh. These supposedly give better airflow, but at the huge sacrifice of air filtration ability! Have you ever held one of these up to the light? You can see right through it. Run one on your motorcycle, and I guarantee you will be able to wipe a layer of dark dirt from the *clean* side of your airbox. These filters have no place on a street bike you intend to rack up lots of miles on. Supposedly, the big benefit of these filters is that you can clean and re-use it, but with the initial cost, and reduced filtering, it doesn't seem worth it to me. It has been proven time and time again that the power increase is negligible from using one of these filters.
Lastly, we have foam air filters. A properly constructed foam filter will have two densities to progressively trap dirt particles, and will be oiled. These are commonly used on off-road bikes due to the presence of so much dirt and sometimes water. They do an excellent job of filtering due to the thickness of foam the air must pass through, but require frequent maintenance to get the dirt out and replace the oil, since the open foam cells will eventually clog up. These could also be used on a street bike, if you're willing to clean it more often.
What kind of air filter do you run and why?
I have always been an fan of paper air filters. You know - the type that come from the factory on most street vehicles. These types of filters have a large surface area due to being pleated, yet also have a very fine filtering ability since their construction is a very tight paper fiber construction. I think these are the best type of air filter for keeping a clean engine. The downside is that you cannot service this type of filter. When it gets clogged, it's time to replace it.
Next we have K&N filters which are a similar pleated construction, but use a thin later of gauze held in place by wire mesh. These supposedly give better airflow, but at the huge sacrifice of air filtration ability! Have you ever held one of these up to the light? You can see right through it. Run one on your motorcycle, and I guarantee you will be able to wipe a layer of dark dirt from the *clean* side of your airbox. These filters have no place on a street bike you intend to rack up lots of miles on. Supposedly, the big benefit of these filters is that you can clean and re-use it, but with the initial cost, and reduced filtering, it doesn't seem worth it to me. It has been proven time and time again that the power increase is negligible from using one of these filters.
Lastly, we have foam air filters. A properly constructed foam filter will have two densities to progressively trap dirt particles, and will be oiled. These are commonly used on off-road bikes due to the presence of so much dirt and sometimes water. They do an excellent job of filtering due to the thickness of foam the air must pass through, but require frequent maintenance to get the dirt out and replace the oil, since the open foam cells will eventually clog up. These could also be used on a street bike, if you're willing to clean it more often.
What kind of air filter do you run and why?
I'd be paying the same price for a one-use paper filter, as I would for an aftermarket one that I can clean, re-oil and reinstall. 