Underdrive pullys?
Thread Starter
Melting Slicks
Joined: Aug 2010
Posts: 2,487
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From: Palm Beach Florida
St. Jude Donor '12
Underdrive pullys?
Could someone explain how a underdrive pully would give you more H.P. . My friend gave me a underdrive pully set -Billit/March for my B-day today. The water pump and Alt. pullys are about the same size. But much lighter. The crank is REAL small compared to my stock. I started to put them on. I'm waiting till tomm. to get smaller belts. Thanks!
Race Director
Joined: Jul 2004
Posts: 12,681
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From: Scottsdale Arizona
Pretty simple. All of the accessories take power (HP) to run them. By spinning them slower, they will take less HP. By putting larger pulleys on the accessories or a smaller pulley on the crank, the accessories will take less HP from the motor. They will also run slower and deliver less water flow or electrical power.
Le Mans Master
Joined: Oct 2000
Posts: 8,897
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From: Santa Ana CA
Could someone explain how a underdrive pully would give you more H.P. . My friend gave me a underdrive pully set -Billit/March for my B-day today. The water pump and Alt. pullys are about the same size. But much lighter. The crank is REAL small compared to my stock. I started to put them on. I'm waiting till tomm. to get smaller belts. Thanks!
The possible downside is that your alternator and water pump turn slower and therefore produce less amps and move less water to keep your engine happy.
I use modest underdrive pulleys in my MillerSpeed Gilmer system and have no problem with either amps or water temps, but I do use a 100 amp alternator and a high performance aluminum water pump.
Race Director
Joined: Jul 2001
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The flaw in the logic of these pulleys is that the accessories really don't consume much power to start, with so slowing them down slightly won't make much, if any, difference.
In most cases, having a alternator, PS pump, water pump or cooling fan turning too slow at idle is not a good thing.
In most cases, having a alternator, PS pump, water pump or cooling fan turning too slow at idle is not a good thing.
Drifting
Joined: Apr 2008
Posts: 1,305
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From: Lewisburg WV
thegazman
I think if you look at the water pump pulley, it is slightly larger than stock giving you about the same water flow. The only place that you don't need as many rpm's to function properly is the power steering pump and the A/C compressor. What the heck, put it on it looks good.
Thread Starter
Melting Slicks
Joined: Aug 2010
Posts: 2,487
Likes: 10
From: Palm Beach Florida
St. Jude Donor '12
Im not worried about the Alt. I replaced it a month ago with a 100 or 120 amp. As for cooling, I already have a new Big block rad, new fan and clutch. The car runs at 180 all day. So even IF it does go up a little, I think the car can handle it. Thanks everybody....Will post pics once it's all done with the new hoodliner in place.
Tech Contributor
Joined: Jun 2004
Posts: 20,834
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
My opinion is that the gains are mostly found during acceleration. Think of all the accessories as distributed flywheels (such as the alternator rotor, w/p impeller, and the driven pulleys). Reducing the drive ratio slows down the individual acceleration rates. Using less of the engine's power to accelerate the accessories leaves more to propel the car. (The number is small, but often it's free for the taking.)
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Team Owner
Joined: Jul 2004
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From: Redondo Beach, California
The pulleys on my GM Performance Parts catalog Serpentine kit were intended to be used in the 1991 and 1992 Camaro and Firebird (and probably some other cars.) I'm pretty sure the crank pulley was sized to spin the water pump and alternator at speeds to care for the car going in stop and go traffic in the very hot weather of the Southern US.
My 68 isn't going to be driven under those conditions. I installed an underdrive March pulley to reduce alternator and water pump speeds. No problem. The car runs a cool 180 degrees F, but at idle, with the electric fans blowing, the alternator cann't keep up at engine idle adn I see a sllight battery discharge. No problem when the car is moving.
My 68 isn't going to be driven under those conditions. I installed an underdrive March pulley to reduce alternator and water pump speeds. No problem. The car runs a cool 180 degrees F, but at idle, with the electric fans blowing, the alternator cann't keep up at engine idle adn I see a sllight battery discharge. No problem when the car is moving.