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Thread: Air Filter Sizing

  1. #1
    boostaholic
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    Question Air Filter Sizing

    I've been digging into this topic lately and I think it may be an issue on my current build. I did some worst case calculations that I believe have merit, I have made some simple formulas to help calculate proper air filter sizing. To simplify this you can essentially treat the motor as an air pump. So based on its displacement, you can figure out how much air the motor requires. When you add a turbocharger into the mix, things get a little more complicated, but essentially as you build boost, you need more air, pretty simple I believe. You need the increase in air to increase the pressure in the piping. My calculations really simplify things, but they do hold merit. Also another note is that as the RPM increases, the air requirements go up as well.
    On my TIII Holset HE341 powered R/T I am running an AEM 21-203DK. I have back calculated this air filter to supply 550CFM. Just to lay out a few more details, I am running a 3" intake from the turbo to the filter location between the IC and the radiator to get cooler air.
    Using these worst case scenario calculations I have calculated a required CFM on my car of 1100 CFM, this is base on a RPM of 7000 and a boost level of 35 PSI. So looking at these numbers, it looks like my filter is undersized. I can tell you that my car has no issue hitting 35 PSI, but it does feel like as the RPM's climb the car does not pull as hard, it feel sort of restricted to me.
    So my theory is the turbo can supply the required pressure that I am commanding, but I believe that it is spinning faster that it really needs to, to meet this boost level. I think you can treat the air filter as a restrictor and I would not be surprised if there was measurable vacuum in my intake pipe. I also believe that since I am spinning the turbo shaft faster than really required for this boost level that there is more back pressure in the exhaust manifold than necessary.

    Does anyone have any real world testing of these theories, did you swap filters and pick up power or remove the filter altogether? Has anyone measured the vacuum of their intake? Has any one measured turbo exhuast drive pressure? I really think this is some low hanging fruit that cold make a measurable difference. I have a fitting tapped into my ported manifold and I plan to measure my exhaust back pressure and my intake vacuum level. I can supply my formulas or spreadsheet if anyone else wants to dig into this. I may be a rare case with the boost levels I am pushing on my car, but this data could be useful on any turbo builds.

  2. #2
    turbo addict
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    Re: Air Filter Sizing


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    Supporting Member II Turbo Mopar Contributor zin's Avatar
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    Re: Air Filter Sizing

    Air cleaners, fuel pumps and ignitions systems... Just three things you really can't have "too much" of when it comes to engines/cars.

    When you look at what even a small depression/pressure drop on the inlet of a turbo does to its pressure ratio, it's amazing our cars run as well as they do when the air filter gets dirty!

    While one can do the equations and figure out the CFM, we are more often than not limited by the available real estate as to the area/size of the filter/piping we can get under the hood, so while I'll never discourage one from "doing the math" as it's eye-opening most of the time, "big as you can" is usually what I'm going for...

    Since it sounds like you have the ports available, it would be enlightening to see what the pressure differentials are on either side of the air filter as well as the boost/exhaust differential, both will show if a change was beneficial or a bust...

    Mike
    "The Constitution is not an instrument for the government to restrain the people, it is an instrument for the people to restrain the government - lest it come to dominate our lives and interests." - Patrick Henry

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  4. #4
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    Re: Air Filter Sizing

    fyi, the very best air filter that fits the original log turbo or T1/TII air cleaner box is the discontinued accell power filter from back in the 90's

    still findable with a genuine google search - which you may have to repeat a lot until you find one - took me a couple of years to come up with the new one I have now

    it's two stage foam , uses filter oil and offers 0 resistance to airflow and give a feelable improvement to throttle response when used in a L body

    link to pic with part number from my album on the other site
    http://www.turbododge.com/forums/mem...ir-filter.html

    good luck to any who try hunting one down - it should be pretty cheep as they were under 30 bucks

  5. #5
    Moderator Turbo Mopar Staff Force Fed Mopar's Avatar
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    Re: Air Filter Sizing

    Well, you can keep it pretty simple on a turbo car and find out how much your turbo will flow in perfect conditions at it's peak efficiency point and find a filter that flows at least that.

    On an NA car, some calculations would be in order, but I would assume if you had flow numbers for the head, doubling or quadrupling the intake flow number would work fine. Only one cyl is actually pulling air in at a time, but I'm sure at high rpm it needs more air filter to keep a volume of air big enough for it to refill the plenum quickly enough.
    Rob M.
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    2.5 TIII swap is here!

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    Supporting Member II Turbo Mopar Contributor 83scamp's Avatar
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    Re: Air Filter Sizing

    It's not completely relevant, but I had this issue on the original carbed motor on my Scamp. After I rebuilt it, with the ported head, and re-jetted carb, I had a wicked hesitation just off idle. I scratched my head over it for a week. Finally out of desperation, I just pulled the air filter out of the housing. Voila, my stumble was gone! I replaced the original paper filter with a K&N, and never had another issue.

    So airflow through the filter can be a huge restriction to an engine...
    83 Scamp GT, 2.5 TII/A-520
    89 Dodge Daytona ES turbo, Daily driver

  7. #7
    Super Moderator Turbo Mopar Staff contraption22's Avatar
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    Re: Air Filter Sizing

    EngineMasters just did a comparison of air filters as part of a pump gas real-world reliable turbo LS episode. It's a new episode so it is not available for free streaming yet. At 600+ hp, they tested it without a filter, and then with several types of filters by Airaid, and found minimal loss in power with any type. Of course, that assumes the filters are of an appropriate size.

    Just use the largest filter you can practically fit. If you're still worried about restriction at that point, if you have logging ability, log the air pressure before the compressor inlet. Or just add a vacuum gauge there.

    Other things you might want to consider getting data on are intercooler pressure drop, charge temp and exhaust backpressure. They can all get worse as the revs climb.
    Mike Marra
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  8. #8
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    Re: Air Filter Sizing

    I'm concerned about this with the Ramerati, will be using a K&N in a stock T1 airbox, feeding turbo through 3" pipe and fed through the power module. Thinking of confabulating a secondary cold air duct direct to the airbox.
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    John Laing "The sole condition which is required in order to succeed in centralizing the supreme power in a democratic community, is to love equality, or to get men to believe you love it. Thus the science of despotism, which was once so complex is simplified, and reduced . . . . to a single principle." -- Alexis de Tocqueville "One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand "To evolve, you don't need a Constitution. All you need is a legislature and a ballot box . . . . things will evolve as much as you want. All of these changes can come about democratically; you don't need a Constitution to do that and it's not the function of a Constitution to do that." -- Justice Antonin Scalia

  9. #9
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    Re: Air Filter Sizing

    I had this very conversation years ago with a K+N tech. I had a thread call airflow requirements, and it proved to make a real difference without measuring any pressure ratios or restrictions.

    And here it is. http://www.turbo-mopar.com/forums/sh...w+requirements

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    Moderator Turbo Mopar Staff Force Fed Mopar's Avatar
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    Re: Air Filter Sizing

    Quote Originally Posted by johnl View Post
    I'm concerned about this with the Ramerati, will be using a K&N in a stock T1 airbox, feeding turbo through 3" pipe and fed through the power module. Thinking of confabulating a secondary cold air duct direct to the airbox.
    That is definitely going to restrict it, I think. I would make something better for it to draw through. Wonder if you can hole saw the inner fender and pull from there? I see you overflow sitting next to the airbox, not sure if a larger intake hose/pipe is going to push that up and require relocation or not.
    Rob M.
    '89 Turbo GTC
    2.5 TIII swap is here!

    Project LookOwt
    '91 Daytona ES, 61k original miles, Rick Lozier's old 3.0 nitrous car
    Back to basics, then the mods go back on....

  11. #11
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    Re: Air Filter Sizing

    Thanks Shadow 88 good stuff. By that formula I get (outer limits 20 psi) 645 cfm, and as you say, 800cfm is suggested.

    So I went to the K&N site and found part number 33-2006 - the standard 5" x 8" panel filter for L bodies - but I could find nothing there about its capacity in cfm.

    Rob, I can see space and a path for an additional cold air intake tube that would 90* out the bottom of the box and forward. First though, is that 5x8 panel big enough.

    Edit -
    Found this - http://bobsmuffler.com/filter.htm

    Says K&N filters flow 6 cfm per square inch, so 5x8 only flows 240 cfm.
    Last edited by johnl; 05-08-2018 at 04:34 PM.
    John Laing "The sole condition which is required in order to succeed in centralizing the supreme power in a democratic community, is to love equality, or to get men to believe you love it. Thus the science of despotism, which was once so complex is simplified, and reduced . . . . to a single principle." -- Alexis de Tocqueville "One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand "To evolve, you don't need a Constitution. All you need is a legislature and a ballot box . . . . things will evolve as much as you want. All of these changes can come about democratically; you don't need a Constitution to do that and it's not the function of a Constitution to do that." -- Justice Antonin Scalia

  12. #12
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    Re: Air Filter Sizing

    With a little research you can find the numbers you are looking for. K&N states a known value of 6.03 CFM per square inch.
    The 33-206 is 8" x 5.125". So a quick calculation says 8 x 5.125 x 6.03= 247.23 CFM.

    You need to use calculations based off of a boosted application. The formulas I used to get my numbers is from this site:
    http://www.modularfords.com/threads/...o-Applications
    I made a simple google doc spreadsheet to input numbers in to get square inches and proper filter sizes.

  13. #13
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    Re: Air Filter Sizing

    Thanks DoubleD. very helpful.

    Now I'm wondering, is 3" diameter from turbo inlet to filter enough? Depends on length too, of course.
    John Laing "The sole condition which is required in order to succeed in centralizing the supreme power in a democratic community, is to love equality, or to get men to believe you love it. Thus the science of despotism, which was once so complex is simplified, and reduced . . . . to a single principle." -- Alexis de Tocqueville "One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand "To evolve, you don't need a Constitution. All you need is a legislature and a ballot box . . . . things will evolve as much as you want. All of these changes can come about democratically; you don't need a Constitution to do that and it's not the function of a Constitution to do that." -- Justice Antonin Scalia

  14. #14
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    Re: Air Filter Sizing

    I run a 3" intake on the Holset on my RT at 30+ PSI, I would think that should be large enough. My motor is requiring 1050 CFM of air.

  15. #15
    Super Moderator Turbo Mopar Staff contraption22's Avatar
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    Re: Air Filter Sizing

    I did some research on this a while back. The consensus was the plumbing wouldn’t need to be much bigger than your inducer diameter.


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    Mike Marra
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    1983 Dodge Rampage FOR SALE 37K original Miles. Needs work. But solid body and chassis. Trying to avoid a partout. Make me an offer.

  16. #16
    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Air Filter Sizing

    Quote Originally Posted by contraption22 View Post
    I did some research on this a while back. The consensus was the plumbing wouldn’t need to be much bigger than your inducer diameter.


    Sent from my iPhone using Tapatalk
    I always look at what the F1 guys do to see what the optimum would be. I doubt any of us would notice a difference between piping the size of the inlet to the cover, and something much larger. Without a 7* taper down to the inlet, most systems with an increased size would not be beneficial IMO.

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    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Air Filter Sizing

    The question is whether or not they were restricted at the inlet like they are in some series.




  18. #18
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    Re: Air Filter Sizing

    Good point, Cordes.

    Mike's comment inspired me to poke around engineering formulas that are above my head but in reading I arrived at the insight that the inducer creates low pressure in its inlet pipe and that low pressure results in the expansion of gasses within the pipe such that air is encouraged to rush into the other end of the inlet pipe. That is, a sort of symbiotic relationship develops at the two ends of the inlet pipe, at one end - the low side - there's vacuum and at the other end there's a mass of air rushing in, with inertia, while it's expanding, losing mass/volume/heat. That's why Mike's point is correct. It's the opposite of what's happening on the high side of the compressor. Of course, we are dealing with relatively low boost and short/fat pipes with minimal internal drag or inertia as compared to the huge volume/mass of F1 or the old Offenhausers.
    John Laing "The sole condition which is required in order to succeed in centralizing the supreme power in a democratic community, is to love equality, or to get men to believe you love it. Thus the science of despotism, which was once so complex is simplified, and reduced . . . . to a single principle." -- Alexis de Tocqueville "One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand "To evolve, you don't need a Constitution. All you need is a legislature and a ballot box . . . . things will evolve as much as you want. All of these changes can come about democratically; you don't need a Constitution to do that and it's not the function of a Constitution to do that." -- Justice Antonin Scalia

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    Supporting Member Turbo Mopar Contributor
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    Re: Air Filter Sizing

    Quote Originally Posted by contraption22 View Post
    EngineMasters just did a comparison of air filters as part of a pump gas real-world reliable turbo LS episode.
    Salad bowl for the win!!!!!

  20. #20
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    Re: Air Filter Sizing

    I went from the Stock T1(blowthrough) airbox to turbo hose with no filter at the track sucking hot underhood air, to a 3" with the biggest K&N Xtreme top cone filter in 3" still sucking hot underhood air on my S60 turboed 2.5 and picked up a solid mph and a tenth in the 1/4 with no other changes. I then added a 45 degree bend, to mount it where the battery used to be and built a cold air box around it and the SMEC. I then new my intake temps via the sensor in the SMEC. Not all the air was coming through the SMEC but if the SMEC was ambient then I new the air inside the box was ambient. On a holeset with a 16v you better be running 4" at least. I was going to run 4" on my GT30R with a ported 8v. Life and family have gotten in the way over the years and it's taken a long time to get it anywhere near being able to run but I still plan on that 4" setup. The more air with the least amount of vacuum you can run (boosted with ram air would be sweet) the better your turbo works.

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