underdrive pulleys

**94svtcobra** · 05-15-2008, 10:52 PM

I recently bought a set of summit racing under drive pullies for my 94 cobra. I installed them but my local garage told me that the pulley I put on the water pump was too large and the one I installed on the alternator was too large. The alternator wasn't charging the battery any ideas on why the other won't work???

**Troll** · 05-15-2008, 11:01 PM

Originally Posted by 94svtcobra

I recently bought a set of summit racing under drive pullies for my 94 cobra. I installed them but my local garage told me that the pulley I put on the water pump was too large and t he one I installed on the alternator was too small. The pulley that I put on the alternator I believe fits on the tensor pulley, but any ideas on why the other won't work???

that's the way they work. hence --- underdrive. I'm not trying to be a smartazz. The crank pulley is 25% smaller, the water pump is bigger and the alt is smaller to make up for the crank pulley. I don't understand what you mean about the tensioner pulley. That one doesn't change.

did you install them or the shop? did you follow the directions and the car runs fine? maybe your shop doesn't know what they're talking about.

**94svtcobra** · 05-15-2008, 11:05 PM

a family friend installed whos a mechanic installed them.

**Pure Stock** · 05-16-2008, 10:16 PM

Underdrive pulleys decrease the speed of the accessories. The smaller the "drive" pulley (crank) and the larger the "driven" pulleys (water pump and alternator) the more the accessories speeds are slowed. If you are having problems with charging replace the larger "driven" pulley (alternator) with a smaller pulley to gain some speed for that accessory. As far as fitment issues, cross reference the part numbers to be sure that they are designed for your application. *If this question pertains to the 93-95 Cobra there may be less H.P. gain when switching to U.D. pulleys since those models came with a certain % of underdrive from the factory compared to the LX/GT models.*