By on November 11, 2013

Dirty-and-Clean-Air-Filter

The contrarian in me loves it when conventional wisdom is proven to be not so wise. For decades, even before the first oil crisis of the 1970s, motorists have been told that making sure that your air filter is clean is one of the ways that you can improve  your fuel economy. It’s intuitive to think that a clogged air filter will affect the way an engine “breathes”, how efficiently it can get gases in and out of the combustion chamber and how that might decrease fuel economy. That may have made sense decades ago, however it turns out that two different studies, one on gasoline engines and the other on diesels, performed by a team at the Oak Ridge National Laboratory, show that the fuel economy of modern, digitally controlled fuel injected engines isn’t significantly affected by the state of their air cleaners’ cleanliness. What made sense in the era or carburetors may no longer be applicable today. Apparently the engines’ ECUs working to keep emissions in spec are capable of leaning out the fuel mixture to account for a dirty air filter restricting airflow into the engine, resulting in insignificant drops in fuel economy. Though dirty air filters didn’t materially affect fuel economy in the modern cars, they did experience a decrease in acceleration performance so it’s still a good idea to replace a dirty air filter.

Abstracts of the studies after the jump.

Thomas, J., West, B., Huff, S., and Norman, K., “Effect of Intake Air Filter Condition on Light-Duty Gasoline Vehicles,” SAE Technical Paper 2012-01-1717, 2012, doi:10.4271/2012-01-1717. (full study available for purchase from the SAE here):

Proper maintenance can help vehicles perform as designed, positively affecting fuel economy, emissions, and the overall drivability. This effort investigates the effect of one maintenance factor, intake air filter replacement, with primary focus on vehicle fuel economy, but also examining emissions and performance. Older studies, dealing with carbureted gasoline vehicles, have indicated that replacing a clogged or dirty air filter can improve vehicle fuel economy and conversely that a dirty air filter can be significantly detrimental to fuel economy. The effect of clogged air filters on the fuel economy, acceleration and emissions of five gasoline fueled vehicles is examined. Four of these were modern vehicles, featuring closed-loop control and ranging in model year from 2003 to 2007. Three vehicles were powered by naturally aspirated, port fuel injection (PFI) engines of differing size and cylinder configuration: an inline 4, a V6 and a V8. A turbocharged inline 4-cylinder gasoline direct injection (GDI) engine powered vehicle was the fourth modern gasoline vehicle tested. A vintage 1972 vehicle equipped with a carburetor (open-loop control) was also examined.

Results reveal insignificant fuel economy and emissions sensitivity of modern vehicles to air filter condition, but measureable effects on the 1972 vehicle. All vehicles experienced a measured acceleration performance penalty with clogged intake air filters.

John Thomas, Brian West and Shean Huff, “Effect of Air Filter Condition on Diesel Vehicle Fuel Economy”, 2013-01-0311 Published 04/08/2013, Oak Ridge National Laboratory (full text here).

Proper maintenance can help vehicles perform as designed, positively affecting fuel economy, emissions, and overall driveability. This paper addresses the issue of whether air filter replacement improves fuel economy. Described are measured results for increasing air filter pressure drop in turbocharged diesel-engine-powered vehicles, with primary focus on changes in vehicle fuel economy but also including emissions and performance. Older studies of carbureted gasoline vehicles have indicated that replacing a clogged or dirty air filter can improve vehicle fuel economy and, conversely, that a dirty air filter can be significantly detrimental to fuel economy. In contrast, a recent study showed that the fuel economy of modern gasoline vehicles is virtually unaffected by filter clogging due to the closed loop control and throttled operation of these engines. Because modern diesel engines operate without throttling (or with minimal throttling), a different result could be anticipated. The effects of clogged air filters on the fuel economy, acceleration, and emissions of three late model turbocharged diesel-powered vehicles were examined. The vehicles were powered by turbocharged diesel engines with different displacements and engine designs. The results reveal rather low sensitivity of these modern diesel vehicles to air filter condition.

Ronnie Schreiber edits Cars In Depth, a realistic perspective on cars & car culture and the original 3D car site. If you found this post worthwhile, you can get a parallax view at Cars In Depth. If you think 3D is a plot to get you to buy yet another new TV set, don’t worry, all the photo and video players in use at the site have mono options. Thanks for reading – RJS

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59 Comments on “Replacing Clogged Air Filters Won’t Really Improve Fuel Economy On Modern Cars, Though It Will Improve Performance...”


  • avatar
    Shane Rimmer

    So, for the average motorist, it will affect fuel economy. It stands to reason that the typical driver will try to overcome the hits to acceleration through more liberal application of the throttle.

    • 0 avatar
      jz78817

      only if you’re accelerating at or near wide-open-throttle (WOT) which the average motorist rarely does, if ever.

      • 0 avatar
        highdesertcat

        I replaced the paper air filter in my wife’s 2012 Grand Cherokee V6 with a washable K&N air filter after about 8000 miles and the results are noticeable in better mpg and quicker acceleration.

        Coupled with the use of 91-Octane Premium E85 gas, the GC is much more responsive to throttle input.

        Then again, I’m a K&N fan and have been using them on all my vehicles starting in the early 1970s. They last forever and are real easy to maintain, clean and oil.

        I just wash them in gasoline, and spray a fine mist of Marvel Mystery oil on them to catch the airborne crud. Works for me!

        • 0 avatar
          krhodes1

          Its been pretty well proven that K&N filters flow better because they don’t filter as well. But they probably filter well enough. I have them on my Triumph, and the OEM performance intake system for my BMW came with one straight from BMW. They even cleaned and oiled it as part of my “free” maintenance.

          • 0 avatar
            highdesertcat

            I haven’t had any complaints and I’ve been using K&N since the 1970s in all my bought-new vehicles.

        • 0 avatar
          BigOlds

          Most likely it’s a self-fulfilling prophecy. When you tell people you’ve done something to improve fuel economy (even though you haven’t) they actually drive more carefully, thus improving fuel economy.

          It’s why purchasers of things like fuel line magnets and intake vortexers report better fuel economy, even though the independent research shows no benefit.

          Hint for the next time you see something like that advertised: If a $1.00 improvement helped fuel economy without any drawbacks, it would be standard equipment on all cars

          • 0 avatar
            highdesertcat

            I got started with K&N because getting paper filters for my vehicles was an iffy prospect when I was stationed in Germany, and I liked the fact that I could just wash out the filter, oil it and re-use it, just like the air filter in the 220D I had bought there.

        • 0 avatar
          jz78817

          *sigh*

          unless you’re always flooring it (and probably not even then) you aren’t going to legitimately discern any difference in performance. Your “butt dyno” is not evidence of anything.

          • 0 avatar
            redmondjp

            ^ This.

            And they don’t filter as well. This has been shown repeatedly by increased silica levels in the oil (Blackstone Labs) when compared to the OEM paper filter.

            Most people don’t keep their cars long enough for the increased wear caused by dirtier oil to show itself.

            It’s disappointing that so many people believe the hype, even when presented with scientific evidence to the contrary.

          • 0 avatar
            highdesertcat

            We all make choices. My choice was K&N filters. I like the flexibility it allows me to clean and oil them any time I want to, and after three cleanings I reach the break even point.

            Considering that I live in a pretty dusty environment in the High Desert of NM, it beats having to run down to Wal-Mart, K-Mart, O’Reilly’s, or Autozone to spring for a new paper air filter after every dust devil I drive through.

            I’m not selling K&N, but it works for me.

            Ultimately, we each have to be happy with our choices. I’m happy with mine because it works for me.

      • 0 avatar

        My first question is why do we need Oak Ridge to do a study like this?
        I guess that since they no longer can really justify working on Nukes, they are bored. But they probably should be looking for new jobs not fooling with car air cleaners.

        Interestingly, even carbureted cars can be made to run with more power and mpg with fuel even cut with the air shut off 95%… we actually have a patent on doing that.

        Does Oak Ridge know about it? No. They still think that the cutting edge is with nuclear technology. That is the government, fifty years behind and still spending.

        That aside, I would think that degree of air shut-off, conventionally, is the key. At some point, the ECU would not be able to compensate. If not running at all counts as poor mileage, that is.

        • 0 avatar
          Sigivald

          How many diesel vehicles does the US Government own?

          (Hint: Department of Defense.)

          Enough that having people who are already on salary investigate something that might (or might not!) save half a mile per gallon vs. waste a fair amount of money on replacement filters *might actually make economic sense*…

        • 0 avatar
          jz78817

          “My first question is why do we need Oak Ridge to do a study like this?
          I guess that since they no longer can really justify working on Nukes, they are bored. But they probably should be looking for new jobs not fooling with car air cleaners.”

          you honestly thing a National Laboratory can only work on one thing at a time?

          • 0 avatar
            BigOlds

            Besides, the purpose of any government organization is (or should be) the public good, which can be the support of the military, but can also be educating consumers.

  • avatar
    Hummer

    But if your performance suffers, aren’t you going to get more into the throttle, effectively increasing fuel usage?

    • 0 avatar
      jz78817

      no, not unless you’re at or near WOT. fuel consumption is all about airflow through the engine. opening the throttle a bit more is offset by the additional restriction of the dirty air filter.

    • 0 avatar
      TR4

      No, the closed loop fuel injection (with O2 sensors) will trim the fuel back to maintain stoichiometric ratio. The driver would open the throttle more than he is used to, but the clogged air cleaner reduces the airflow, and the fuel injection system reduces the fuel so there is no net change.

      @Ronnie: air filter salesmen worldwide are cursing you!

      • 0 avatar
        highdesertcat

        There is no net change but performance is affected. At my altitude, knocking because of 87-octane fuel affects performance more than a clogged paper air filter.

        I overcome that by burning Premium 91-octane E85 in the engines and keeping airflow as unrestricted as possible. K&N does that for me.

        It truly is remarkable, the difference with a K&N filter. AND the ECO-light comes on a lot sooner than it ever did with a paper filter.

        • 0 avatar
          PonchoIndian

          E85 use will cause a decrease in mileage, and lower octane can be tolerated at higher altitude.

          That K&N is doing a placebo effect on your brain.

          • 0 avatar
            highdesertcat

            No it’s not. I’ve been using K&N since the 1970s in all my bought-new vehicles and if it didn’t work or didn’t save me money, I would have dropped them like a hot potato.

            In NM we can no longer buy Pure gas and everything here is now either 10% Ethanol or ranging all the way to E85 if you live in the sticks.

            My guess is that the mixture in my wife’s GC gas tank is anywhere from 10% ethanol to E85 and everything in between because it depends on where my wife buys her gas when she’s on the road.

            At home I use 10% ethanol Premium gasoline on all my other cars and AC generators. Only our GC is tuned to run on E85.

          • 0 avatar
            Sigivald

            “No it’s not. I’ve been using K&N since the 1970s in all my bought-new vehicles and if it didn’t work or didn’t save me money, I would have dropped them like a hot potato.”

            Have you done long-term blind (ABX) testing?

            If not, then you don’t know they really work vs. you adjusting your driving.

            Indeed, if you’ve always used them in all your new vehicles, you *don’t have a comparison point* on those vehicles, do you?

            That’s not science.

          • 0 avatar
            highdesertcat

            Sigivald, is there something wrong in me choosing to use a K&N filter over a paper filter?

            I don’t need long-term blind (ABX) testing if it works for me.

            I’m not selling K&N, but I made a conscious choice to use it in all my vehicles as long ago as the 1970s.

            If you’re happy with paper, go head on! We each have to decide for ourselves what works for us.

            K&N works for me and it’s immaterial to me if there is scientific evidence to substantiate its performance in my vehicles.

            $39 bucks at amazon, with free shipping and no tax, that’s three paper filters and I can wash, oil and reuse the K&N as often as I want to. That’s what’s important to me.

          • 0 avatar
            BigOlds

            If I may,

            He’s not trying to tell you that you can’t use K&N, and to a post above, there may well be benefits (such as the certain availability of a reuseable filter in a place where replacements can be difficult). He’s trying (like I was above) to get you to think critically about the decision rather than making assumptions and then allowing placebo effect and confirmation bias drive you to waste money.

            Marketers are, in general, much more effective at making money than scientists.

          • 0 avatar
            highdesertcat

            BigOlds, I completely understand everyone’s comments and intent to share and educate.

            I respect other peoples’ opinion. However, they’re not paying my bills nor are they paying for the upkeep on my vehicles.

            Ultimately, we are all faced with the dilemma of making the decision about which road to travel, in this case paper air filters or something else. I choose something else, in my case K&N.

            I’m staying with K&N because they are the ideal for me. And I say this after more than 40 years of having used them in all of the vehicles I have owned that K&N made a filter for.

            I was also an early adopter of the Tiger Capacitive Discharge Ignition system way back in the early 1970s. Today’s ignition systems closely resemble that bulky Tiger CDI in design and architecture, miniaturized. Technology improved the breed.

            Maybe at some point in the future, the air filters in cars will be washable and reusable, like the air filter in my 1972 M-B 220D was. For all the years I owned that car I reused the same OEM air filter!

            Fram, Purolator and Wix would lose a huge chunk of income, were that to happen. Just because marketers advocate the use of paper filters does not mean that there isn’t a better way.

        • 0 avatar
          Scoutdude

          Play around with the blend of E85 and E10 to try and get it in the E40-E50 range and check your fuel economy. Many studies have shown that there is a sweet spot in that range for most FFVs that returns the best MPG. I know I played with it with the Taurus FFV we had and it did get the best MPG with a blend in that range, about 10% better than E10. It also made more power with that blend but not as much as with E85.

          • 0 avatar
            highdesertcat

            Yeah, I’ve heard about that but we don’t play with the blends. I buy E10 in town and my wife buys E85 in some of the outlying farming areas where she travels to.

            My wife and I rarely let any gas tank get below 1/2 before we top it off along the way, anywhere we travel, either alone or together.

            Although there recently was an exception when we nearly ran the gas tank on the GC dry when we towed a trailer for my wife’s niece from WY to Zephyr Cove, NV, by way of SLC and I-80.

            When traveling on I-80 along the Great Salt Lake, there was nothing out there. By nothing I mean nothing except some kind of a structure holding giant colored balls on the North side of I-80.

            I did keep and extra 5-gal jug on the yoke of the trailer, just to be on the safe side, but found a gas station before all the fumes in the GC gas tank were used up.

          • 0 avatar
            Scoutdude

            Well if you fill it with E10 a couple of times and then fill it with E85 when it hits about 1/2 a tank then you are creating a mid range blend. Of course E85 is only E85 in the summer and can be as low as E70 in the winter depending on the location of the source of the fuel.

          • 0 avatar
            highdesertcat

            Yes, you are right on all counts but we just put in whatever gas we find along the way and don’t monitor how well the vehicle performs.

            I don’t ever want to test the advertised 500+ miles range of our GC. That time along the Great Salt Lake was scary enough for us.

            The great distances in my part of the country are accompanied by desolation so keeping the gas tank filled reduces worry and stress of a break down.

            Imagine, there was a time before cell phones and texting. How did we manage to git along without them?

          • 0 avatar
            Scoutdude

            So you never check the MPG? I’ve heard you state many times that you’ll buy gas to get where you where you want or need to go but aren’t you a bit curious as to how your particular vehicle is performing MPG wise? I too will buy the gas that I need but I still do the quick calculation of what MPG I got on the last tank of fuel even on my Scouts which get relatively poor MPG.

            How about the seat of the pants feel with the different fuels? I know the first time I took our Taurus on a trip where I could buy E85 and brought it home my wife asked me “what did you do to my car?” Fearing the worst I sheepishly responded “nothing”. She replied “So you didn’t tune it up or modify something?” I said no and she responded that it seemed to have more power. I did tell her well that is because of Corn Power.

          • 0 avatar
            Sigivald

            Also: http://www.weirdus.com/states/utah/stories/tennis_ball_tree/

            The world is weird.

          • 0 avatar
            highdesertcat

            No, I don’t care about mpg or fuel economy.

            The only businesses suffering from me having to pay more for gas are Starbucks, McDonalds, Burger King, Taco Bell, Chili’s, Applebees and 7/11 as I swap my money for gas instead of their goodies.

            In the GC there is an ECO mode indicator in the top left portion of the eVIC and I noticed that it comes on a lot quicker when using the K&N than with a paper air filter.

            Before the K&N the ECO indicator came on whenever my wife went off-throttle or had set the Cruise Control on a flat road.

            Because of the high-altitude nature of US54 and US70, with long, gradual slopes, the ECO light rarely stayed on very long as the Cruise Control accelerates to maintain speed.

            With the K&N filter the ECO light comes on more often and stays on longer. It was noticeable. But we still fill up the cars before we hit the half-mark on the fuel indicator.

      • 0 avatar
        Scoutdude

        It is actually the Mass Air Flow sensor (which most cars now have) that is doing most of the work of compensating for a clogged air filter. The O2 sensor is mainly making sure that the AF ratio is NOT stoichiometric and actually switches back and forth between slightly lean and slightly rich. The catalytic converter needs the extra oxygen and the unburnt fuel to be able to do its job properly.

        • 0 avatar
          danio3834

          Interestingly, leading brand reusable oiled air filters have a tendency to dirty the MAF sensor and cause fuel economy and drivability issues. Lucky for HDC, his JGC doesn’t have a MAF, but I strongly suspect his claimed gains are not measurable.

          • 0 avatar
            highdesertcat

            danio3834, what you wrote is true. Neglect will usually cause all sorts of problems downstream.

            I have always made it a practice to wash my K&Ns at the time of each oil and oil filter change, usually 3K – 5K on the clock but never less than at least once every six months, if no 3K is reached.

            We recently took a 3200-miles trip in the GC and I did the oil change thing before leaving and then again right after we got back home.

            I was surprised to find salt crystals from the Great Salt Lake embedded in the K&N filter from a wind storm we passed through near Tooele.

            The M-B 220D Euro-spec I bought in 1972 with my dad’s money so he would have a car to travel in on his two-year excursion throughout Europe with my mom, had an oil-bath re-usable air filter. Greatest thing since toilet paper!

            That got me interested in K&N air filters and I’ve been a devoted and loyal user ever since, using them in all my cars and motorcycles that K&N made a filter for.

            At least we have a choice to use what we want. Were it up to Fram, Purolator, Wix, et al, we wouldn’t have that choice and they would fight tooth and nail if any car maker would include washable, re-usable lifetime air filters on any of their vehicles.

            K&N makes a great product IMO and the re-usability saves me money after only three filter washes.

            Spread that over a 3-5 year ownership period, and that is a lot of Moolatte’s and Burgers and Fries.

          • 0 avatar
            krhodes1

            @danio3834

            I agree with you strongly – ordinarily I would not use a K&N on a car with a MAF. On my BMW, there is a secondary charcoal filter downstream from the air filter, upstream from the MAF sensor. I figure that protects the MAF to some extent. From BMW, the K&N had the right amount of oil on it, and I checked after the dealer techs cleaned and oiled it – they did it right too. Still, I will probably put the paper filter back at some point. Call me conflicted… :-)

            If warranty had not been a concern, I would have gone for the European air intake setup for my car – MUCH bigger paper filter, no charcoal filter, MUCH cheaper. But I want NO possibility of warranty quibbles from a marque that is fairly famous for them. The Performance Intake may not add much in the way of power (BMW says 3hp) but it certainly works in conjunction with the Performance Exhaust to sing sweet, sweet music!

  • avatar
    Hank

    So…what are the implications (and what was the rationale for the automakers) for cars like the Ford Focuses that came with “lifetime air filters”?

    • 0 avatar
      jz78817

      the “rationale” is that they’re required to attain PZEV certification; they have vapor traps in the intake to capture residual fuel vapors, and the air inlet system has to be designed to not need to be opened up for 80 or 100,000 miles (can’t remember which.)

      • 0 avatar
        28-Cars-Later

        Thank you for the explanation, but I still have to label a “lifetime” filter a fail.

        • 0 avatar
          raph

          I guess that depends on your definition of “lifetime” – the best response I’ve heard and my attitude toward anything OE and lifetime is until the powertrain warranty runs out. That’s what you should consider lifetime.

          • 0 avatar
            28-Cars-Later

            The industry is going to keep 90s models on the road much longer then they would care to if the attitude is going to be “X is disposable buy another car”.

  • avatar
    Scoutdude

    In news related to the 3D printed gun last week and ORNL, ORNL is planning on donating thousands of 3D printers to high schools around the US who have teams that compete in FIRST Robotics Competition. in the coming years. No not the kind that print metal but the kind that prints in ABS. http://www.aviationweek.com/Article.aspx?id=/article-xml/awx_11_08_2013_p0-635147.xml At the FIRST World Championship last season the team they directly support with mentors and use of equipment had a robot that was mainly 3D printed, a combo of metal and plastic and plastic reinforced with carbon fiber wraps.

  • avatar
    SCE to AUX

    This is because the time-weighted average output of a car’s engine is only 10-30 HP, depending on the vehicle and driving conditions, and most of the time is spent substantially below redline.

    So if the air cleaner is rated to deliver airflow for the full power rating, with a restricted air cleaner you won’t notice an efficiency change or power change until you approach extreme conditions.

    What about cabin air filters? If it’s 20% clogged, will one of my passengers be gasping for air?

  • avatar
    Ian Jordan

    This didn’t take a study to understand. It’s physics.

    A clogged filter is just like having your throttle plate closed a bit. Imagine the air filter as a second throttle plate in front of your current one. It’s open WOT when it’s new, it’s open 98% when it’s clogged.

    It does restrict air flow, but so does your standard throttle plate when you aren’t WOT, that’s the point of a throttle. So you lose some power, but you make it up by adding a bit of throttle. This does NOT lead to more fuel being injected since the same amount of air gets into the engine. In the end, at cruise, exactly the same amount of air gets into the engine, which means the same fuel, which means the same MPG. Your pumping losses don’t change since you added some in the air filter but reduced the pumping losses at your throttle plate.

    • 0 avatar
      Greg Locock

      hurrah. A voice of common sense, and an understanding of engines.

      If you are not at full throttle it doesn’t matter one iota

      If you are below say half the redline rpm it doesn’t matter.

      if you are aiming for good fuel economy and you are at full throttle for much of the time then you are doing something odd.

      if you are aiming for good fuel economy and you are over 50% rpm for much of the time you are doing something odd.

  • avatar
    areader

    ORNL is part of DOE. Over the years they’ve done a lot of work to improve vehicle efficiency. They do more than nuke stuff. They came up with a way to substantially reduce drag on semis by attaching a box like structure to the rear. I see somebody has a product out now, but I’ve seen only a few on the road, but I’m not on the road much myself.

    • 0 avatar
      rodface

      I didn’t know they did aerodynamics research. It’s too bad that there’s so few of these government- or industry-funded research labs around any more (at least, that’s my impression),

      The aerodynamic device that appears to have gained most widespread acceptance are trailer skirts, thanks no doubt to their being mandated by California.

  • avatar
    jacob_coulter

    I would love to see a study regarding the optimal time to replace an air filter in order to keep particulates out of your engine.

    As more dust accumulates, does more dust make it’s way into the engine? Or does it just keep filtering, the side effect being less airflow?

    BTW, I’m not a fan of K&N filters, at least for newer cars. The gains are minimal, the oils in the gauze can sometimes damage MAF sensors by coating them, and it’s universally agree that it lets in more dirt particles vs paper filters.

    Not sure if that’s really worth all those negatives for a 3hp gain on a good day.

    I remember in the 90′s, it was just accepted that simply swapping a K&N panel air filter automatically gave you 15-20hp and a few more miles per gallon, which is laughable. If those types gains were true, every OEM would have them standard.

    • 0 avatar
      Scoutdude

      On the one hand the more dirt on the filter the better it filters. On the other hand in the case of fine dust it does absorb the oil or adhesive on the paper filter reducing the ability of the filter to trap a small particle passing through one of the holes. Yes many paper filters are treated with a light coat of oil or an adhesive to attract the dust and make it stick to the paper. They can also contaminate a MAF, I’ve seen it on many cars that exclusively used paper filters. The big problem with the K&N filter is the fact that many people over service them and then over oil them. I’ve got a couple of cars that I have put over 100K on the K&N filter in them w/o an issue with the MAF. However I’ve followed directions and have not serviced them in that time.

      The problem in determining an optimum replacement interval is that it varies greatly depending on the operating conditions. Which is why many diesel trucks, which don’t have a throttle and therefor ingest a much greater volume of air, have restriction gauges in the intake tract. When the gauge indicates that the filter is causing a restriction then it is time to replace the filter.

  • avatar
    indi500fan

    A lot of the dirt equation depends on your locale.
    I know a guy who ran a 60s era 283 Chevy 80,000 miles with no air filter element at all. (Back then 80,000 miles was a long time).
    This was in Cleveland where it’s damp a lot, rarely dusty.

    Now I’ve seen diesel pickups using K&Ns in desert parts of Arizona with a lot of fines downstream in the intake tract. No hard data on effect on engine life, but it appeared troubling to me.

    • 0 avatar
      pragmatist

      A good rule is that if you see ANY dust inside the filter, it’s a bad thing.

      In truth, nothing beats a good paper filter for effective cleaning (this has been demonstrated by repeated scientific testing.

      The paper filter on my Wrangler sees plenty of mud splash offroad, spending a few bucks for new paper is well worth it.

  • avatar
    ex-x-fire

    These results will make someone I know happy, I service his union supplied car. Just the other day I told him he needed a new air filter, his reply was “No, I don’t care. The union pays for my gas.”
    I’ve been wondering why the new cars don’t have hot air directed into the air filter box during steady cruise. I’ve noticed on the hot days that my yaris gets the best mpg out of the whole year.

    • 0 avatar
      kscott

      The reason for the hot air was to prevent carburetor icing; the hot air actually causes a slight reduction in efficiency, or at least a reduction in the amount of power produced, since the air going in will be less dense. On an aircraft engine, carburetor heat is manually controlled, for use when needed, and you can feel the reduction in power when you turn it on. On a carbureted car engine, the intake hot air may be controlled thermostatically. On cars with fuel injection, the commonly-done thing to prevent throttle-plate icing is by running coolant through a passage in the throttle body, which is also thermostatically controlled.

  • avatar
    johnny ringo

    I installed a K & N air filter in my 2008 Odyssey about a year ago; fuel economy and performance gains, if any, are very modest. My primary reason was the fact that the filter is reusable. I clean and re-oil the filter every 20k or so and don’t need to buy any more air filters. Works for me.

    • 0 avatar
      highdesertcat

      My sentiments exactly!

      However, as someone who has been using K&N filters for more than 40 years, I wash and re-oil my K&N filters every time I change the engine oil and engine oil filter. (Usually every 3K to 5K miles, or before I go on any 3K-5K long-distance trip, and right after I come back home from that trip)

      That’s my routine, and it works for me. Your environment may be a lot less dusty than mine is.

  • avatar
    manbridge

    One thing missing is dirt migration. Around here, once a filter is clogged the air seeks the easy way around the sides of the filter. This is easy to see if you smear some axle grease on intake tract down stream of filter and check it for contaminants at oil changes.

    I notice this especially on Fords as their air boxes warp somewhat with age. The dirt usually wreaks havoc with hot wire MAFs in the same way as over oiling K&Ns.

  • avatar
    LJD

    Not entirely scientific, but I found this to be an interesting read…
    http://www.bobistheoilguy.com/airfilter/airtest1.htm

  • avatar
    danio3834

    Good post Ronnie. In this day and age, if there were fuel economy gains to be had in simply switching to a different air filter, you bet the OEMs would be using them.

    Along the same lines as this, bolt on performance exhaust components are won’t help your fuel economy either. You could delete the cats and flash in a leaner tune which would definitely help fuel economy, but a “freer flowing” system will only help you burn more fuel, not less.


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