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Thread: Overdrive vs underdrive pulley's

  1. #1
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    Overdrive vs underdrive pulley's

    Is there any difference in power gain between an underdrive crank pulley and an overdrive supercharger pulley. Whitch is best?

  2. #2
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    Gains/losses are subjective, depending on what percentage they amount to. It may not be so simple to just say under/over

    What's the goal, budget, what other if any mods have been done?

    I've done changes with aftermarket/different sized supercharger and/or jackshaft pulleys that sped up the supercharger, which of course allowed it to build more boost. I have yet to change the crank pulley from stock, or use underdrives in any location.

    Gains can also come from changing a pulley from steel to aluminum while keeping the same diameter. Some gains come from reducing driven weight and some from spinning the accessories slower. You essentially 'recover' horsepower, vs. simply adding more. The end result is the same, just arrived at it by different schemes.

    Point is there are different reasons to change sizes up/down~under/over, and/or from steel to aluminum, resulting in different recipes/combinations, from those that are safe for and strictly used on the street, to others that are just for the strip/racetrack. What I'm happy with and what works for me, may be entirely different from your choices, budget etc.

  3. #3
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    From what I understand, the cheapest and easiest way to gain a couple of pounds of boost is to
    change to an overdrive pulley on the supercharger but since the are non available anywhere I thought
    an underdrive crank pulley would work about the same. Any thoughts?

  4. #4
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    Quote Originally Posted by RS2600 View Post
    From what I understand, the cheapest and easiest way to gain a couple of pounds of boost is to
    change to an overdrive pulley on the supercharger but since the are non available anywhere I thought
    an underdrive crank pulley would work about the same. Any thoughts?
    There are more gains to be had from overdriving the supercharger assuming your injectors can supply the extra fuel to compensate and the intercooler can also compensate for the added heat and not heat soak and effectively pull timing killing power.

    Underdriving is slowing the water pump and alternator so that there is less parasitic loss from those two. But the cooling system needs to be optimal cause doing on a corroded water pump and clogged up radiator. Can lead to cooling problems, same applies to the alternator.

    You can overdrive the supercharger from the jackshaft pulley instead of the supercharger. Which helps with belt slip.

  5. #5
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    Quote Originally Posted by RS2600 View Post
    non available anywhere I thought
    What year is your SC? Later model '94~'95 SC can use an early model supercharger pulley to increase boost. I've still got one for sale.

    Early/'89~'93 SC stock supercharger pulley diameter is 3.135"

    If yours is early model and you want safe for street, I think a 7% would be 2.935"

    Did you try Pulley Boys online? They show more than 10 sizes in stock: https://pulleyboys.com/cart/eaton-m9...ed-pulley.html - including a 2.9" for USD$99

  6. #6
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    Thanks for the tip, I'll order one right away

  7. #7
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    Quote Originally Posted by RS2600 View Post
    Is there any difference in power gain between an underdrive crank pulley and an overdrive supercharger pulley. Whitch is best?
    You can do both. Underdrive pulley set (crank, WP and Alt is good for about 15-20 HP from reduced drag on motor) it should be noted that the UD pulley set does not impact the supercharger drive portion of crank pulley. A 5% OD jackshaft pulley or supercharger pulley is good for about 20-25 HP and neither of these mods will require tuning. BTW It's much easier to install a jackshaft pulley than a supercharger pulley.

    David
    Last edited by David Neibert; 01-29-2020 at 02:32 PM.
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  8. #8
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    Now I'm getting a little confused. What's the difference between the jackshaft pulley and supercharger pulley?

  9. #9
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    Quote Originally Posted by RS2600 View Post
    Now I'm getting a little confused. What's the difference between the jackshaft pulley and supercharger pulley?
    I literally just bought both (haven't installed either yet). But heres what I learned:

    The jackshaft pulley is the larger pulley that connects to the supercharger pulley with a belt. An overdrive supercharger pulley is slightly smaller than stock whereas the overdrive jackshaft pulley is slightly larger than stock.
    Getting an overdrive jackshaft puller or SC pulley accomplishes the same thing.
    You could get a 5% overdrive jackshaft pulley and a 5% overdrive supercharger pulley and your supercharger will be overdriven 10%.
    By getting a lightweight aluminum jackshaft pulley, there will be less rotational mass and could free up a couple horsepower to go to the rear wheels. Also, because an overdriven supercharger pulley is smaller than stock, belt slippage can be an issue; which is why you will need a special smaller belt for a supercharger overdrive pulley. However, according to Spinning Wheels SC, you dont need a special belt (stick with stock size) if you get an overdrive jackshaft pulley.

    From what I understand; getting an overdrive Jackshaft is preferred over getting an overdrive SC pulley. But both accomplish basically the same thing. However, overdrive SC pullies are generally cheaper and easier to find, from what Ive observed.

  10. #10
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    So the jackshaft pulley is the one down at the crank, am I right?

  11. #11
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    Quote Originally Posted by RS2600 View Post
    So the jackshaft pulley is the one down at the crank, am I right?
    It's the double pulley below the ac compressor.
    SCCOA Member #2406
    '94 SC AUTO - 2.3 Whipple
    http://www.sccoa.com/forums/garage_v..._vehicle&id=19

  12. #12
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    Ok, thanks

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