Breathe Easy With The Best Air Filter For Your Car

As my audiologist can confirm, I’ve spent too much of my career standing next to loud cars running on chassis dynamometers. Air intake modifications are a perennial favorite of the tuning industry, so I have tested every variety you can imagine; from simple drop-in filters to polished aluminum cold-air induction kits, and all the way to custom-made carbon fiber airboxes. So, I have a good idea of what works and what doesn’t for performance.

I have never done any filtration testing — there, I said it. I feel better getting it out in the open. To supplement that particular hole in my knowledge base, data has been gathered from several academic research sources. Lastly, reviews from other media outlets and customers were stirred into the usual product rating gumbo. We like our gumbo spicy, so we also throw in some opinions. Bon appetit!

As with any automotive product, there is far more opinion on air filters than actual data from scientific testing — or as Facebook continually proves, there is far more lavatory than laboratory data. Every home mechanic and dealer tech has a story about one filter that ruined a car in a hundred miles and another allowed the engine to run for 300,000. 

My pick for the best overall filter is the K&N Air Filter. Having used and tested countless different brands in everything from daily drivers to race cars, I am confident oiled filters have no ill effects on vehicle components. And, while I haven’t done any filtration testing myself, and honestly wouldn’t be equipped to do so, data has convinced me the filtration efficiency is worth the extra cost. 

Air filter testing is an extremely complex process that has challenged vehicle manufacturers and the Society of Automotive Engineers for decades. One measure is filter efficiency, which refers to the amount of particulate matter which does not make it past the filter. Even that isn’t as simple as it sounds. You first have to determine what size of particulates you want to measure. It is widely believed most engine damage is caused by particles above 5 microns in size — a human hair is at least 50 microns in diameter. Most filters are rated between 95 to 99 percent efficient on particles greater than 5 microns, but most paper filters come with the caveat that they don’t meet that rating when brand new and require time to “load up” first. 

If everything is working as it should, there is no metal-to-metal contact during normal operation. Bearing surfaces have a thin layer of oil between them, the minimum expected, as you might guess, is just over 5 microns. When this is true, particulates below 5 microns can’t get sandwiched between two surfaces causing surface damage. But with oil viscosities constantly getting thinner, tolerances getting tighter, and engines spinning faster or using higher cylinder pressure than ever, the latest data says many oil-film thicknesses are regularly in the 2- to 3-micron range.

Air filters stop particulates in three different ways. The first and most obvious is impaction; this is when the particle’s inertia carries it into the individual elements of the filter medium and stops it like pasta in a strainer. The second is interception, here you have a particle that is small enough to be directed with the air stream and would normally pass by the medium, but it passes close enough to stick to it. Obviously, this captures smaller particulates than what holes in the filter are designed for, but only a certain amount will get close enough to stick. Lastly is diffusion. This is for particles so small they move in zig-zagging patterns known as Brownian Motion. Similar to interception, the particles which are so small they would never be captured by impaction, hit the filter medium, and stick. If there is no mechanism to make the particle stick to the medium, like say in an oiled filter, particle motion interrupted by interception and diffusion may get slowed but not stopped.

The second measure of an air filter, which is mostly of interest to aftermarket tuners, is flow. Vehicle manufacturers design airboxes to flow more than enough air to supply your engine in any circumstance. Tuners will have you believe that engineers are either incompetent or have some nefarious plan to limit your car’s power, but neither is true. I have seen some amazing claims for power gains from air filters and modified induction systems; while you may gain a horsepower or two at one very specific RPM, they never give power across the entire range. 

I have seen small power gains for naturally aspirated cars on a dyno, which rarely if ever, translated to actual performance gains when tested on a track. Especially in the case of modern turbocharged engines where the ECU is monitoring boost pressure in the intake manifold. If your car wants 15 pounds of boost, it will keep the wastegate closed until it gets it. If an intake suddenly allowed you to move the same air easier or faster or whatever, it would still open the wastegate when it got to 15 pounds of boost in the intake manifold, making the same power.

To sum up, take filtration claims with a grain of salt — a grain under 5 microns in diameter. What companies advertise might be the absolute best scenario with the filter loaded up and only tested with ideal particulate size. Next, take flow rates and horsepower gains with the entire salt shaker. Flow rates are rarely given with pressure and density, which contribute to those numbers. Data shows that oiled filters likely catch more particulates of smaller size than dry filters, but are those particles so small that they won’t cause damage anyway? 

Oil has been used to trap dust in air filters for almost as long as internal combustion engines have existed. K&N was the first to popularize using oiled cotton in the 1960s, as opposed to the oil bath method invented decades earlier. K&N and other brands’ cotton medium is the most popular, but oiled foam is also common.

Experimental data has shown these types of filters offer higher efficiency for smaller particles since they are for lack of a better term, sticky. That filtering efficiency is there from the installation until servicing, making them at least predictable. The vast majority of oiled cotton filters are considered lifetime parts, which is always good.

By far the most common type of filter you will find in the automotive world. They are cheap, good at filtering particulates, and you can change them quickly. The last point is particularly important to service centers where washing a filter and waiting for it to dry would leave a car sitting on a lift for more time than needed. 

Maximum efficiency of the filter is obtained at some point after initial installation, meaning the first bit of service might include the inhalation of particulates considered harmful. Depending on the filter, this load-up phase may be only a few hundred miles, so it might be irrelevant. This is the best choice for the average owner who never wants to think about an air filter.

Using anything from woven polymers to expanded foam these work in much the same way as paper. The medium is made with various sized holes that correspond to the size of particles they are meant to stop. Like paper filters, these often require a load-up phase to reach full efficiency.

Good quality dry filters are considered lifetime parts. At the very least, they have lives in the tens of thousands of miles. The reusable models will require washing and drying, so they are not a drop-in and go like a paper filter. They will save owners the sometimes messy task of re-oiling. 

The main structure of an air filter is called the frame. It not only holds the media in place but also acts as a seal between the upstream and downstream portions of the airbox. These can be made from soft foam with very little stiffness and relies on the airbox to maintain its shape. They can also be made from a stiffer rubber or polyurethane material which holds the filter’s shape and is much more durable, important for long-life filters.

Higher quality filter manufacturers will go to great lengths to maximize the design of the filter frame. The stronger the material, the less overlap it needs to mechanically hold onto the medium, meaning more of that medium is available for filtering. The size of the frame is also built to a higher tolerance, meaning it will clamp into the airbox with less effort while still providing a tight seal.

Most lifetime filters require you to buy specialized products to clean the filter. Oiled filters obviously require oil, as well. While a reusable filter may cost you anywhere from two to five times the cost of a paper filter, plan on spending another 15 to 20 bucks on the service kit. It sounds like a considerable expense, they generally provide enough products for several years of servicing.

It’s worth noting that not only are some oils aren’t compatible between different brands of filters for optimum performance, but some will dissolve a competitor’s filters. I’m sure it’s completely accidental. For this reason, it is worth being brand loyal when buying air filters.

Like many maintenance products, air filter pricing is all over the board. I have heard of air filters from dealership service departments costing hundreds of dollars, while a cheapo paper filter at your local auto parts store may cost less than $10. For most part numbers, something like a K&N panel filter is around $40 to $50. As mentioned above, the service kits for reusable air filters are going to be in the $10 to $20 range. 

If you plan on keeping your car long-term, a reusable filter is a good investment that will pay for itself. If however, you are leasing your car for just a few years, you won’t get that money back in terms of paying for disposable filters or a longer engine life. Lease a car, stick with the paper.

You’ve got questions. The Drive has answers.