The A/C conundrum

Discussion in 'General Harley Davidson Topic' started by Flashback, Mar 6, 2008.

  1. Flashback

    Flashback Active Member

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    For those who are looking for best performance rather than cool (sometimes you can have both) The easiest add-ons deal with air flow (A/C, Pipes, Heads). But before you read an ad or take a recommendation you should have a good idea of what your riding style and specific comfiguration needs.

    The formula to calculate required flow rates for different cubic in displacements at specified RPMs at 10" is:

    rpm x cubic inches per cylinder x number of cylinders / 1925 = cfm

    For example if we want to know the ideal cfm for a Twin Cam 88 at 2500 rpm we plug into the formula this way:

    2500 x 44 x 2 / 1925 = 114.29 cfm at 10"

    A stock Harley filter flows at 130 cfm so would be perfectly adequet if you drove no more than 2500 rpm. But lets say you wanted something for 4000 rpm you would need a rated filter of at least 200 cfm. A SE A/C flows 160 cfm which is better but in this example still restrctive.

    When reading ads that espouse a certain product, obstensibly proving it's assertions with cfm flow rates, it's at least necessary to know the manometer test pressure. if the manometer test pressure is diffeent you need to use a fomula to equalize the figure so you can begin making a fair comparison. First divide wanted test pressure by known test pressure. then get the square root of that result and multiply it by the rated cfm. for wxample if we have a known test pressure of 300 cfm at 18" for product X, and we want to compare it to product Y rated 250 cfm at 12" what would the air flow of product X be at 12"? To find out divide 12 by 18. The answer is 0.667. The square root of this is 0.817. This number is then mutiplied by 300 cfm which gives 245 cfm. So 300 cfm @ 18" = 245 cfm @ 12".

    This may all seem confusing but if you are loking for real performance from your scoot knowing what you need and how to compare whats out there will give you the result you are looking for.
     
    Last edited: Mar 6, 2008
  2. glider

    glider Veteran Member

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    If we are talking about the HD 0800 K&N filter that replaces the stock HD paper filter in the SE stage 1, that CFM flow is 231 and not 160, more than enough for any stock or lightly modified engine. The larger stage 2 filters that are available from places like zippers for one, flow much more than needed and net no true results over the HD 0800 filter regardless of the hype attached to them when used on a stock or slightly modified engine..
     
    Last edited: Mar 6, 2008
  3. Flashback

    Flashback Active Member

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    I took the stock and SE numbers from a tech article. If they are wrong sorry. But it really doesn't make a difference since i was only using it as an example. The point was if you are running a 1930's 45 FH or a 500 ci boss hoss theres an easy formula to see what your needs truly are. Also a formula to compare products on an equal footing. The numbers may have been wrong but the formulas are still valid.

    thanks
     
  4. glider

    glider Veteran Member

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    It will flow a bit more than stock ( HD stock = 171 CFM ) because of the design but the HD 0800 K&N will be much better flow than the stock filter that is used in the stage 1 kit ( HD-0800 = 231 CFM).