Engineering Explained examines high-performance air filters to see if there are any benefits

Jason Fenske wants to know if a high-performance air filter makes any difference on the performance of your car. Because he likes to explain the engineering behind this sort of thing, Fenske sets out to see what sort of difference a variety of filters make on his car and he does so through science.
We’re looking at Jason’s own Subaru Crosstrek. He’s purchased a clean OEM-equivalent filter, a cheap “performance” filter, and a high-performance K&N air filter. He also has his stock, dirty filter as a baseline against the manufacturer-listed specs for the Crosstrek.
Jason secured time on a dyno so he could register horsepower and torque readings with his Crosstrek for each of the four filters being tested. Going from dirty to clean stock saw a minimal gain. The less expensive performance filter added further gains over the clean stock filter. Finally, the K&N air filter netted the largest gain as it provided a 4.3-horsepower and 5.1 pound-feet of torque boost over the clean stock filter.
Now, we don’t know how much more stuff might be getting by those performance filters. But there’s definitely more air moving into the engine, and it’s enough to see a slight bump in the power output.

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12 responses to “Engineering Explained examines high-performance air filters to see if there are any benefits”

  1. Fred Avatar

    That has always been the rap against high performance filters. How do they get more air thru without bringing more dirt.

  2. Ol' Shel' Avatar
    Ol’ Shel’

    Remember that “designed to” is a way of suggesting a benefit without claiming one. Of course they intended for it to be better, but they’re not about to make any guarantees about performance gains. You see similar phrasing in the supplement industry.

  3. Lokki Avatar

    A quick check suggests that the Crosstrek has 152 horsepower (SAE Net). So, is a 4.3 horsepower increase noticeable?
    A side note: the dirtier a conventional air filter gets, the better it filters. This is not to say that’s a good thing for mileage but the engine itself remains protected.
    However, as the oil dries out or wicks away on a K&N, the filtering diminishes, and it may even be suspected that accumulated dirt previously trapped by the oil might come loose from the filter element and enter the engine.
    Finally, a question: have you ever known any of your friends who purchased a K&N and then actually washed and reoiled the filter after a year?
    FWIW and that’s not much, really

    1. Scoutdude Avatar

      It is not supposed to be cleaned and re-oiled every year, the recomendation is every 50K at the earliest and yes I’ve done that a number of times now as I’ve had one K&N for ~20 years and another for 15.

  4. outback_ute Avatar

    It would have been an interesting comparison to have tested with no filter too, that is how I ran my ute at the drags – worth it for the noise alone really.
    Also testing the power gain without filtering performance seems like doing half the job to me. I’m picturing a test rig using a vacuum cleaner, and weighing the filters to see how much they trapped. But the this is YouTube so that should be a separate video for the extra views!

  5. Harry Callahan Avatar
    Harry Callahan

    This reminds me of a discussion shared in my college Philosophy class:
    Q: What makes something Good?
    A: When it fulfills its design purpose.
    In this situation, what makes an air filter “good” is its ability to filter particles from air. As such, the proper way to measure this is not to compare power levels, but to compare filtration effectiveness.

  6. je zalanka Avatar
    je zalanka

    yeah, tried that. K&N filter passes more of everything including dirt. even passes an oil mist that contaminates the hot wire airflow sensor. the question on my mind is if your filter is not passing enough air what is stopping you from using a larger air filter and housing? filtering the air to the engine intake systems is foremost in design criteria. I would much rather protect my engine than get that 5.1 lb/ft of torque. we already have 455lb/ft stock to work with. now, should you be running a carb, a K&N may work really well for you but so would a much larger surface area filter media which would no doubt stop more dirt than the K&Ns open weave cotton gauze, oiled or not. somewhat less messy to service, also.

    1. Scoutdude Avatar

      If your K&N is passing so much oil that it contaminates the MAF it has waaaay too much oil. I’m up to 153k miles and 15 years on the K&N in my Mountaineer that was purchased new and had it installed on day 2. I’ve cleaned and recharged it 3 times now and have never touched the MAF. Meanwhile I’ve had to clean the MAF on a lot of cars that had paper filters which also contain oils and/or adhesives that contaminate MAFs.

      1. je zalanka Avatar
        je zalanka

        you only cleaned and oiled you filter every 50,000 miles??? hmmm.

  7. ptschett Avatar

    The most comprehensive comparison I’ve seen is still this:
    The more 9’s of filtration efficiency, the merrier. Here’s a photo of a set of vials the Donaldson company sales reps used to give out, that illustrate how much difference it makes…

    1. Harry Callahan Avatar
      Harry Callahan

      I enjoyed reading that very much. Thank you.

  8. Alff Avatar

    Wonder what hp gains would have been with no filter.

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