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Underdrive pulleys?

This is a discussion on Underdrive pulleys? within the External Engine forums, part of the LSx Technical Help Section category; anyone have underdrive pulleys (crank and alt.) On their ls1? If so, any gains? And does the car discharge at ...

  1. #1
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    Underdrive pulleys?

    anyone have underdrive pulleys (crank and alt.) On their ls1? If so, any gains? And does the car discharge at idle? Does it matter what pulleys you buy? Worth it? Have em on my s10 and the lights stay dim till I hit the gas. Dont think its a big issue though. Thanks

  2. #2
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    IMO they are a waste of money.
    To charge properly the alt needs a smaller diameter otherwise dimming can occur.
    Waste:
    Alt no matter what spinning faster or slower demand remains the same and so will the drag.
    Power Steering same thing if vehicles have it.
    Water pump is where you save some but if and it usually does, the engine runs at a higher temp negating any gain.
    AC will at lower rpms have a harder time cooling since its spinning at a lower rpm, btu output drops.

    If someone was looking for every ounce of power at a track for a short burst, then and only then the spinning of these accessories being slower gies some back. Daily driven again IMO all gains are negated!!

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    Former Mopar Man 2002ssslp's Avatar
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    Put your money into other mods.

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    Senior Member INMY01TA's Avatar
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    I have the ASP %20 underdrive crank pulley. No ill effects at all and will help keep the belt from walking at high rpms.

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    You've Seen The Butcher GoldenFlame's Avatar
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    I have a ATI 10% UD on my C5.. No noticeable gains, but that's not why I did it. My stock balancer was wobbling like a son of a bitch.

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    Electrical Engineer KMdef9's Avatar
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    If the underdrive wasn't a harmonic balancer as well, I would recommend it, as it would be a $30 upgrade. But ~$200 is a waste and a lot more beneficial mod can be done with that money.

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    Electric Water Pump, Power Steering Delete , and A/C Delete are much more worth it.

    Cleans up the engine bay real nice

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    Member Dzel's Avatar
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    Anyone know the size of belt I need for a PS and a/c delete?
    10secWS6 If you can't swing it, then don't bring it

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    Member Dzel's Avatar
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    Sorry to bring up an old thread

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    Sold: LS1 '85 El Camino ls1camino's Avatar
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    AC is it's own belt, so just remove it.

    As for the other belt, take a string and measure around the pulleys. I bought the size I measured as well as two sizes smaller. You can always return what you don't use, but constantly going back to the parts store can get tiring.
    Dzel likes this.

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    Member Dzel's Avatar
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    Thanks I'll do that

  12. #12
    Junior Member Forgiven's Avatar
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    Quote Originally Posted by 2ManySS's View Post
    IMO they are a waste of money.
    To charge properly the alt needs a smaller diameter otherwise dimming can occur.
    Waste:
    Alt no matter what spinning faster or slower demand remains the same and so will the drag.
    Power Steering same thing if vehicles have it.
    Water pump is where you save some but if and it usually does, the engine runs at a higher temp negating any gain.
    AC will at lower rpms have a harder time cooling since its spinning at a lower rpm, btu output drops.

    If someone was looking for every ounce of power at a track for a short burst, then and only then the spinning of these accessories being slower gies some back. Daily driven again IMO all gains are negated!!
    Some of what you say may be correct. One thing you did not think of is that the motor will not be working as hard to turn everything, there is a mechanical advantage gain going 25% smaller. The engine will not run at a higher temp....unless sitting idling for a long time perhaps....the bigger pulley is there because the car has to be engineered for all drivers, even those who may let their cars idle a lot. As soon as you go off idle underdriven there will be plenty of water flow to cool the motor... Once off of idle an underdriven car will charge and cool just fine. All gains are not negated IMO.

  13. #13
    I have an slp 25% pulley with no side effects. Didnt notice any difference. I doubt it added more than 5-6 hp but ill hit the dyno soon and find out.

  14. #14
    The name says it all !!!FAST!!!'s Avatar
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    Quote Originally Posted by 2ManySS's View Post
    IMO they are a waste of money.
    To charge properly the alt needs a smaller diameter otherwise dimming can occur.
    Waste:
    Alt no matter what spinning faster or slower demand remains the same and so will the drag.
    Power Steering same thing if vehicles have it.
    Water pump is where you save some but if and it usually does, the engine runs at a higher temp negating any gain.
    AC will at lower rpms have a harder time cooling since its spinning at a lower rpm, btu output drops.

    If someone was looking for every ounce of power at a track for a short burst, then and only then the spinning of these accessories being slower gies some back. Daily driven again IMO all gains are negated!!
    If you are overheating at idle you have more problems than under drive pulley's. I've had March 25% UDP's for almost a decade now and lots of stop and go traffic in LA, Chicago, Las Vegas, etc. Never had an instance of overheating. No upgrades in my cooling system yet, not even 165* thermo. The purpose of under drive pulley's isn't to gain horsepower. It's to eliminate more parasitic loss. Same reason for electric water pumps and a/c and p/s deletes. It also reduces the moment of inertia. Plus, a good UD system comes with a fluid damper which eliminates much more engine harmonics (bad stuff) than does the stock rubber. Also most will come with a smaller alt pulley. Just make sure you get the correct belt for it.


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