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Thread: Something about Pully sizes I don't understand

  1. #31
    World's Fastest Street HD
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    if you've already got the belt and installed and it seems too loose, probably best bet is just get one 1/2" shorter.

  2. #32
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    Quote Originally Posted by Ryan@RhynoTune View Post
    People need to reference their pullies in terms of diameter, not gains in PSI. Bottom line is a 8.5'' pulley will not gain the same on both engines.

    This is a simple calculation, the hardest part is figuring out why people can't grasp this simple concept.

    7.5/2.93= 2.55 blower ratio for a Lightning @ 8 PSI
    7.5/3.25= 2.30 blower ratio for a HD @ 6 PSI
    8/2.93= 2.73 blower ratio w/ 8'' lower (2#) for a Lightning
    8/3.25= 2.46 blower ratio w/ 8'' lower (2#) for a HD
    - Time out, so with a 8'' lower, you still aren't even at the same blower speed as a stock Lightning. So how do you gain two pounds? Do I really need to go on? I guess I must.
    8.5/2.93= 2.90 blower ratio w/ 8.5'' lower (4#) for a Lighting
    8.5/3.25= 2.60 blower ratio w/ 8.5'' lower (4#) for a HD
    - Another time out, so with a 8.5'' lower on your HD, your not even at the same rate as a Lightning with a 2lber. So you start w/ 6PSI, buy a so called 4lber, slap it on with your stock upper, and bam your at 10PSI!!! But the odd thing is, your still spinning your blower slower to get that same 10PSI than a Lightning does. It's magic
    I assume after reading that, someone is saying that isn't much of a difference. These numbers are small and meaningless. This is the part where I multiply the given ratio times 5250 RPM.
    5250x2.73 (L blower ratio for a 8'' lower aka 10 PSI)=14332.5
    5250x2.60 (HD blower ratio for a 8.5'' lower aka 10 PSI)= 13650

    So you have a difference of 900 RPM for the supposed same amount of boost. Bottom line, you do not get the same amount of increase on the two engines with the different uppers.
    Never one to accept things at face value
    8.5/3.25 = 2.615
    5250 x 2.615 = 13723
    Difference is more like 600 rpm. Fair bit less than 900.

  3. #33
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    Not sure what you're saying, the difference between your calculation and mine is 78 RPM, and in the favor of my point.
    2002 HD - 631 RWHP - 757 RWTQ
    Currently fabbing twin-turbos

  4. #34
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    5250x2.73 (L blower ratio for a 8'' lower aka 10 PSI)=14332.5
    5250x2.60 (HD blower ratio for a 8.5'' lower aka 10 PSI)= 13650

    So you have a difference of 900 RPM for the supposed same amount of boost. Bottom line, you do not get the same amount of increase on the two engines with the different uppers.[/QUOTE]

    You wrote this not me.

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