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Old 10-22-2003, 01:39 AM   #1
Scooby Specialist
Member#: 45735
Join Date: Oct 2003
Location: ANYWHERE BUT Newport News
2002 Impreza WRX

Question Underdrive Pulleys/Lightweight Pulleys

I was wondering about how and why pulley kits work.

I've noticed 2 different kinds of kits, underdrive and lightweight. From what I understand, underdrive kits use SMALLER pulleys to reduce the amount of power needed to spin the accessory belts, but don't power them to their normal levels. Lightweight pulleys are the same diameter, but milled out and made out of lighter materials??

I run quite a bit of stereo in my trunk, so I'm worried that an underdrive pulley system might be a really bad idea. My biggest question is: Do lightweight pulleys have the same "underdrive" effects, and, are they worth the money?

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Old 10-22-2003, 08:34 AM   #2
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Join Date: Feb 2003
Chapter/Region: MWSOC
Location: St. Louis
98 Supra TT


If I'm not mistaken, the terms "Underdrive pulley" and "Lightweight pulley" are two terms for the same thing. The term "Underdrive pulley" from my understanding, refers to the crank, alternator, and a/c compressor pullies, sometimes the power steering pump pulley as well.

Almost any aftermarket pulley kit you get will come with smaller and lighter pullies than the factory units.

For the WRX, most pulley kits only come with the crank pulley and the alternator pulley. I just picked up a GFB set for myself yesterday.

Please, someone correct me if I'm wrong.
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Old 10-22-2003, 08:54 AM   #3
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Join Date: Aug 2003
Location: Toronto
2003 WRX


Underdrive pulleys are smaller in diameter (and usually lighter) than the stock pulley, lightweight pulleys are the same size as stock but are beneficial because they are much much lighter.

I have the Perrin pulley and it is the same diameter but weighs <20% of the OEM pulley. Great bang for the buck mod, not for power but for throttle response.
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Old 10-22-2003, 10:32 AM   #4
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Join Date: Aug 2001
Chapter/Region: MAIC
Location: Manchester, PA
F#ck U & Your Stance

Default Light Weight Pullies....

I am running Airborne Motorsports Lightened pullies on my Stg 2 WRX with no problems. It makes throttle response alot better and make rev matching a little quicker.

I have a extra set of silver Lightened Pullies that I need to unload, let me know if you are interested.

More info on the pullies can be found here:


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