Air cleaner CFM question...

Discussion in 'General Motoring' started by rndthought, Oct 4, 2005.

  1. rndthought

    rndthought Guest

    First, this is born out of a "mental exercise" between the parts boys
    and myself...about the merits of K&N air filter...filtering efficiency
    withstanding, they are being marketed on the sole concept of CFM at the
    local parts store.

    How would one figure the CFM required at some RPM for an "X" liter

    Would it consume "X" Liters every 2 revolutions? (4 stroke,
    up/down/up/down - so one time it goes down it will be due to the
    combustion stroke not intake...right?)

    1 liter = 0.0353 ft^3 (man, can't imagine cramming 30, 2 liter
    bottles in a 1 foot cube box!)

    So would this be correct? (Liter) * (RPM)/2 *(0.0353) = CFM

    For a 2.3 liter motor with a Red Line of 8000 RPM:
    (2.3)*(8000/2)*(0.0353) = 325 CFM

    Just trying to determine if anyone really needs a K&N air filter that
    claims 450 CFM (in this case) - especially since most people don't
    generally drive around at the Red Line!!!
    rndthought, Oct 4, 2005
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  2. rndthought

    rndthought Guest

    If this is correct, it will only be valid for a normally aspirated
    motor. Anyone know the math to come up with the equation for CFM at a
    given RPM for a given boost PSI????
    rndthought, Oct 4, 2005
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  3. rndthought

    jg Guest

    It's still meaningless without a comparative resistance at any particular
    cfm. Generally the only way to decrease resistance is by decreasing the
    capacity to catch particles, or increasing the area of the filter media
    (either by making it bigger or having pleats etc). And I guess engines are
    designed to operate against a certain resistance, as they are with exhaust
    jg, Oct 4, 2005
  4. rndthought

    James Sweet Guest

    At least stock Volvo filters are generally very good, MUCH larger than those
    used in many cars with similarly sized engines. I don't think there's much
    to gain with an aftermarket filter.
    James Sweet, Oct 5, 2005
  5. rndthought

    jg Guest

    I've never noticed a difference after replacing dirty filters, makes me
    think less resistance wouldn't be noticed either. Except if filtration is
    worse, dust could grind the motor out faster.
    jg, Oct 5, 2005
  6. rndthought

    User Guest

    Unless a filter is severely restricted the only time flow is affected by
    anything other than the restriction forced on the system by the throttle
    plate is at wide open throttle. If you have determined that the intake
    air flow is insufficient to provide satisfactory performance then you
    will have to do some serious elargement of the throttle housing diameter
    coupled with intake runner improvements (both length and diameter,
    interior surface preparation-streamlining and proper surface finish to
    enhance turbulence for rapid fuel vaporization and so on). If you just
    want eye candy K&N is the way to go--the kits are pretty.

    K&N filters have captured the fancy of many enthusiasts through clever
    and effective advertising. The one caveat I've found is that cleaning
    and oiling the filter must be done very carefully. Any excess oil that
    is drawn into the inlet piping will eventually deposit on the hot film
    or hot wire of the mass airflow sensor. Even though the burn off cycle
    for the sensor occurs after every shut down when the motor has exceeded
    about 2000 rpm it is inadequate for vaporizing the oil that lands on it,
    unlike the dust particles that it was designed to eliminate. This
    ongoing accumulation of varnish on the hot film or hot wire will cause
    premature, expensive failure of the part.

    User, Oct 6, 2005
  7. rndthought

    Randy G. Guest

    The K+N's do have the ability to flow more air, but that is not always
    a good thing. A LOT of years ago (over two decades, actually) I bought
    one for my BMW motorcycle. This bike has dual constant velocity carbs.
    The air filter flowed so mch air that the carbs would have had to be
    rejetted to run correctly. I went back to the stock filter and no

    Many of the K+Ns I have seen look smaller than the stock filter, but
    they pass more air, more freely? At what cost? How much more dirt do
    they pass as well? If you want a freer breathing motor, have it ported
    and polished and have a custom exhaust system made to match. If you
    are looking for simple, quick fixes that make more horsepower, I have
    a fuel line magnet that I removed from my 240 you can buy from me.
    Don't you think that if the manufacturers of the vehicles could
    squeeze more power from their vehicles with a different air filter
    that they would do it?

    The problems you outlined are compelling enough reason to not use one
    on a car with an AMM.

    __ __
    Randy & \ \/ /alerie's
    '90 245 Estate - '93 965 Estate
    "Shelby" & "Kate"
    Randy G., Oct 6, 2005
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