So what is an underdrive pulley, and what is it for?

Joe


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86 IROC TPI 305
700-R4
K&N Air filters
Next mod = trans-go shift kit (level 1)

 

An underdrive crank pulley is a pulley that has a smaller diameter than the stock pulley. By decreasing the size it will turn the belt driven accessories less (hence underdrive). It's supposed to free up some horsepower and decrease accessory wear. Also you can get underdrive pulleys for the accessories (the pulley sets usually have an alternator and/or water pump pulley to complement the crank pulley) that are larger than stock...again it slows down the belt driven accessories.

-Mark W.
'88 SC Camaro w/ 305 TBI

 

All aftermarket underdrive pulleys are very small compared to the factory ones. I bought a set for my truck but only used the crank pulley. Slowing down the alternater even more with the alternator pulley hurt my charging system when all the accessories were on.

GM pulleys appear to all have the same 3 bolt mounting pattern and are all the same height from the block. ('69 and later). Measure your stock pulley and check how many belts are used. If you only use a single serpentine belt then all the better. Lets say your stock pulley is 7 1/2" in diameter. Go to your local junk yard and start measuring pulleys. I found a factory pulley 6 1/4" in diameter and the underdrive pulley on my truck is 5 7/8". As long as the junk yard pulley has the proper grooves for the belt(s) you are using, you just found a cheap underdrive pulley. The one on my race car cost $5 and the one on my truck cost $120+.

After that you'll just have to figure how short of a belt to use to make it work.

In theory the smaller pulley allows the engine to spin up faster. Tie a weight to a string 2 feet long and spin it around. Try again at 1 foot long. The smaller string will let you spin it up faster. Go to an extreme with string 5 feet long. Once you do spin it up, it takes more energy to keep it spinning also. The underdrive pulleys work the same way.

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Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
461 Big Block installed and ready for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

[This message has been edited by Stephen 87 IROC (edited February 25, 2001).]