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Old 04-28-2009, 03:16 PM   #1
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An Underdrive Pulley Add On

My vehicle is an 07 GMC Sierra with the 4.3L engine and a 5 spd. manual, the truck has 16,700 miles on it. About 10,000 miles ago I added crank and alternator underdrive pulleys. Contemporary advice at the time suggested strongly that my vehicle would run too hot with the water pump underdrive pulley. Subsequently, I have been told that the radiator in my truck is the same size as used for V8 engines. If this is the case, overheating would not pose a problem because of the extra capapcity even with a slower turning water pump. I would like to add a water pump underedrive pulley to increase my power (i.e. economy). I'd apprecaite any suggestion and especially real time experiences. Thanks in advance!
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Old 04-28-2009, 03:31 PM   #2
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So you've already under driven your crank then you want to further under drive the water pump?

Did you double under drive your alternator?
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Old 04-28-2009, 05:00 PM   #3
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My thoughts might be messed up but this sounds right to me…

If the crank and the alternator are both 10% smaller by ratio then the alternator is spinning at the same speed as it was in the beginning. The ratio to the crank and water pump have changed to being under drive (I think). Adding a water pump pulley that was the same ratio as the others would put everything back to spinning at the same speeds but should have dropped the amount of rotational mass.
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Old 04-28-2009, 08:37 PM   #4
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Most of the places that make underdrive pulleys will undersize the crank and oversize the accessories so that it actually works. if they didn't, they wouldn't be in business long.

how much of a gain have the current pullies provided? they make a difference on high revving imports, even slightly lower revving domestic performance engines. I woudln't think they'd make much difference on a domestic daily low revver. especially not in a barely revved hypermiled domestic low revver.
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Old 04-30-2009, 02:11 PM   #5
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Thanks for the information! It has been all worthwhile. My pulleys are from Marche and, althought I'm not sure how much undersize they are, my gauges show the engine to be charging normally and the oil pressure is where it is supposed to be. The extra power meant a lighter throttle and did help performance and mileage. I picked up 1.5-2 mpg.
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Old 05-12-2009, 06:20 AM   #6
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of course oil pressures the same. the pumps driven from the internal rotaing assembly.

Glad to hear I was wrong.
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