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If you are trying to get the last few HP to the rearwheels (like for racing), then you will get it. If all you are running is water pump and alternator (no power steering), then do you really want to turn those slower under race conditions? Probably not. Most manufacturers of these claim from 5-15 HP gains. Reality is probably closer to half of that at peak RPM.
 

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Underdrive pulleys reduce the alternator's charging ability and the water pump's pumping ability at idle. If you are marginal on your radiator's cooling capacity, an underdriven water pump could cause overheating.

Tanner
 

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I have the aluminum v-belt underdrive pulleys that are from Ford Motorsport. I needed pulleys anyway, plus wanted v-belt since it looks correct and I was only using waterpump and alt., with no other accessories. Never had a single issue with this setup, plus the pulleys are polished aluminum, so they look cool too.

If you are just trying to upgrade, it's probably not worth the money, but if you don't have a full set (like the position I was in), then I'd say it's worthwhile since stock replacements would probably cost more anyway...
 

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I have the aluminum March set like the guys above. I used to have ASP underdrives in steel. Both sets would only charge my my system at idle if I had an overdrive pulley on the stock alt, or what I have now, a 3G alt with standard pulley.

What I'd like to try is staying stock diameter and if possible, get an aluminum crank pulley from these guys.
http://www.aspracing.com/frame.htm

The lighter weight metal will free up some rotational losses if you have something like a big old truck pulley with dual serpentine drives on it. Otherwise it won't do much.
 

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I used a crate motor so I didn't have any pulleys, otherwise I probably would have stayed with the stockers. Since I had to get something though, I finally settled on the March Power and Amp series pulleys. They're intended for auto trans cars and keep the alternator turning at higher RPM's to keep the charging rate up at idle. Seems to work good on my 351 with 3G alternator. Shows a steady 13.7 - 14 volts all the time and 88 - 91 Degrees C with a 180* thermostat. They now have two versions in aluminum, which can save you about $70 if you can live without the high shine. HTH
Frank
 
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