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Supercharger Pulley sizing

  1. Jun 29, 2010 #1
    Ok. So my crank pulley is 127mm o.d. which drives a supercharger pulley 73.27 mm o.d. to make 9 psi of boost.

    The problem is, the supercharger puller is too small and causes the belt to rub. So i increased the supercharger pulley to 101.6 mm o.d. and now the it only makes 4.5 psi of boost (not enough)

    I have someone who is going to custom make a new crank pulley, larger than the current one to compensate and make 9psi of boost, by keeping the 101.6 mm o.d. supercharger pulley

    The question is, what size (o.d.) does the new crank pulley need to be, to drive the 101.6 mm o.d. supercharger pulley to make 9 psi of boost?

    I took an engineering class 10 years ago and used my distant memory of gear train ratio calculations. But i got two different answers, 160.2 mm o.d. and 175.77 mm o.d. This of course made me second guess my calculations and it didn't help that my boy called me an idiot for even using the this formula:

    Since- 127 : 73.27 = 9 psi [127/73.27 = 1.73] or 57.71%
    Ratio- 1.73 : 1 = 9 psi
    Therefore- 1.73 x 101.6 = 175.77 mm o.d.

    Please advise, thx.
  2. jcsd
  3. Jun 29, 2010 #2
    You are right, you just want something with the same gearing ratio. So as the orig is 1.73:1 you just want to size the crank pulley up accordingly.
  4. Jun 29, 2010 #3

    jack action

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    I agree.
  5. Jun 29, 2010 #4
    Just make sure you can get a new, longer belt that fits.
  6. Jun 30, 2010 #5
    Thanks for the prompt response and confirmation.

    My friend still thinks i'm wrong though...lol. He says that i should take the percentage difference between the two supercharger pulleys and then increase the crank pulley by that percentage. That doesn't seem to settle right with me since the numbers/results are different. A percentage value of a smaller number results in a different value of that same percentage of a larger number...no?
  7. Jun 30, 2010 #6
    Your friend is wrong, for precisely the reason you state. Unless he means that you increase both both by the same percentage (which is eactly what you were doing). In this case each is 73% bigger.
  8. Jun 30, 2010 #7
    Thank you
  9. Jul 1, 2010 #8
    not to beat a dead horse, but he punched some numbers on a calculator while murmuring something about distance traveled by the pulley in the mile vs RPM and came up with 142 mm for the new crank pulley. He won't tell me what formula he used, but swears he's correct.

    I am concerned because the custom machining of this crank pulley is expensive and takes 8 weeks.
  10. Jul 1, 2010 #9
    Just totally ignore him, he's talking bollocks.

    Pulleys are just like gears if you want one to turn at a set speed with a set input you need a set ratio.

    Say with a drive of 3000rpm (engine speed) this gives a compressor RPM of 5190RPM (ish) with the 127:73 pulleys.

    With a drive of 127:101 you get a compressor speed of 3772 rpm.
    With the ratio of 142:101 you get a compressor speed of 4217 rpm.

    If you use 175.77:101 (as you originally said) you get a compressor speed of 5190rpm . Which is the same as the original sized pulley system.

    You'd better tell your mate it's time to go to the library and get a basic book on gearing out.
  11. Jul 1, 2010 #10
    thx again. I rest my case.
  12. Jul 2, 2010 #11
    It sounds like he's confusing it with transmission or final-drive gearing. I built superchargers for several years (machinist/fabricator), and the advice here is spot-on.

    What type of blower are you running? What is he charging for the pulley?
  13. Jul 3, 2010 #12

    jack action

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    We can't be all wrong:

    From http://en.wikipedia.org/wiki/Gear_ratio" [Broken]:

    Last edited by a moderator: May 4, 2017
  14. Jul 3, 2010 #13
    thanks but he stills we are wrong.. lol
    problem is, we may not be able to physically fit a 175.77mm pulley in there so we'll have to see
  15. Jul 4, 2010 #14
    You would be better to go to a belt manufacture like Browning (Emersion-ept) or Gates. They have engineers that do this type of thing 8 hrs/day five days a week. Also they have engineering information c/w formulas to help you.

    Besides ratios you need to be concerned with small pulley size otherwise there is not enough wrap on pulley, speed max 6000 ft/min, center distance and much more.

    You can go to Emersions site but need to log on, or if you live in a NA city try yellow pages under Power Transmission Equipment and talk with one of their Reps.
  16. Jul 7, 2010 #15
    Your friend's logic is correct but probably did the math wrong...

    if you have a 100mm:60mm pulley

    upsize both by 50% you get 150mm:90mm

    the ratio of 100:60 = 1.6666666
    the ratio of 150:90 = 1.6666666

    So changing both pullies by the same % is correct

    In your case you go from



    x : 101.6

    101.6/73.27 = 38.66% larger

    apply the same factor of 1.3866 to your crank pulley and you get

    127 * 1.3866 = 176mm

    Check the ratio:

    127/73.27 = 1.73333:1
    176/101.6 = 1.73228:1

    So it's maintained (minus whatever rounding errors i made in that)

    Your friend's logic is correct, his numbers are not.

    The other way to look at this (which you mentioned him doing) is looking at circumference (which is what you meant by rotations / distance ...etc)

    circumference = 2.pi.d
    so if you scale your diamter by 50%
    the circumference scales by 50%

    You can translate that same change into a 50% increase in the other pulley's circumference
    which translates into a 50% change in the other pulley's diameter...

    Same thing

    Your friend is a smart man that doesn't know how to use a calculator :) ... the concept is right, the numbers he gave you are wrong.

    PS: If you need a professional pulley calculation (and other engine modifications) check out my software :

    http://www.superchargerperformance.com/the-power-calculator" [Broken]
    Last edited by a moderator: May 4, 2017
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