The air filter is the unsung hero of every engine on the road. Without it, dust and debris in the air would wear down your piston rings and cylinder walls like a storm erodes a coastline. As the filter does its job it gets loaded up with dirt, and a dirty filter is going to rob your engine of power and put a dent in your fuel mileage. That’s why you’re supposed to replace the filter every 10,000 to 15,000 miles. And when it comes to replacement air filters, they come in three flavors: OEM paper, oiled cotton gauze and oiled foam.
Most OEM filters are made of cellulose, or paper, like a coffee filter. Tiny pores in the paper let air through but keep the vast majority of dirt out. Many modern OEM filters even have a viscous coating to help trap fine dust. The problem is, paper filters are restrictive and those tiny pores plug up quickly. So to get more flow and increase the filter’s dirt-holding capacity, the element is pleated, like an accordian. It only takes up a little more space than a flat filter, but has vastly more surface area and thus more pores, more airflow and a longer service life before it gets stopped up.
OEM filters are very effective, but their priority is filtering particulates and protecting the engine, not maximizing air flow. So a big downside is that they’re restrictive, at least compared to “high flow” performance filters. They’re also disposable, as in once it’s dirty, you throw it out and replace it with a new one that may cost $20 to $40.
Aftermarket filters are appealing for two reasons. First, they can offer increased airflow, which could make more power — more on that in a minute — and second, they’re almost always reusable.
K&N is probably the most well-known aftermarket filter on the market. It resembles an OEM paper filter, but the pleats are composed of layers of oiled cotton gauze, not a single sheet of paper. The gauze is more porous, so it flows more air. In fact, if you hold it up to the light you can actually see through it, which doesn’t seem like a great characteristic for something that’s supposed to keep dust out of your engine. Luckily, the filtering material is oiled, so as dirty air swirls through and around the fiber filaments, the oil grabs and holds the grime. And a K&N style filter can hold a lot of dirt, which often means more miles before you need to wash it.
You’ll see all the crud the filter collects when you wash it out. And that’s a key benefit to oiled-gauze filters — you can reuse them over and over. A K&N does cost more than an OEM paper filter, usually about 50% to 100% more, and you’ll need to buy the $20 cleaning and re-oiling kit, but in the long run it can save you money, not to mention save trash from going to a landfill somewhere.
Next up is oiled foam. You see this style of filter stock on off-road vehicles because oiled foam can hold a lot of dirt while still flowing well, so they’re well-suited to super dusty environments where a paper filter would clog up quickly. They also function when wet since the foam and oil aren’t absorbent. Get an OEM paper filter or gauze filter wet and the fibers will swell, strangle airflow and possibly stall your engine.
Oiled foam operates on the same dirt-capturing principle as the K&N, but it’s thicker. In fact, it’s what’s known as a “depth” filter and is typically an inch thick. Air has to wiggle and wind its way through the foam, which is not only layered coarse to fine, but saturated in a super-tacky oil that grabs and holds grime. It’s a very effective means of filtration as long as there’s still oil available to capture dirt.
When a foam filter gets dirty enough that all the oil dries out, it’s possible for dusty air to make its way through the foam and into your engine. That’s obviously bad news, which is why oiled-foam filters need to be washed and re-oiled as often as every ride, and it’s a messy procedure. Servicing the air filter that frequently may be OK for a dirtbike or quad with an easy-to-access airbox, but any sort of accelerated maintenance schedule is going to be hard for street riders to swallow, no matter how good the filter is.
Another, often predominant reason riders install an aftermarket filter is to get more power out of their engine. However, a noticeable power increase is highly unlikely without complementary modifications. An engine is an air pump and it can only inhale so much air, so unless you’ve done something to take advantage of a freer-flowing air filter — like installing cams or a race exhaust, and definitely tuning your fueling — you’re not going to gain any perceptible performance from dropping in an air filter. In fact, if you neglect to tune your bike’s fuel, either with a re-jet on carbureted models or a remap on EFI bikes, you’ll likely experience running issues.
So, which filter is best for your bike? It really depends on your application. If you’re a street rider who logs lots of miles and wants maximum protection, you can’t go wrong with an OEM paper filter. The extended service interval alone is appealing, particularly when the airbox is hard to get to. If, however, you ride in especially dusty conditions, have a modified engine, or simply don’t like the idea of a throw-away air filter, an oiled-cotton or oiled-gauze element is a great option. Each style of filter has its pros and cons, and now that you know what they are you can decide which type is right for you.