Pulleys depend on the type of pulley available. There are two main types. Underdrive pulleys and lightened pulleys.
For they xD only the underdrive pulleys posted above are what I have found.
Underdrive Pulley - Basically as stated it underdrive the other accessories. It's makes the crank pulley smaller (which also makes it lighter). By underdriving the accessories pulleys it causes less parasitic drag caused by these accessories which means less HP lost due the accessories , i.e. A/C, Alternator.
Lightened Pulley - It's just a lighter pulley. Same Size/Diameter as stock, but much lighter. The first gen xBs have lightened pulleys available. When added act the same as taking about 100 lbs out of the car. Lighter pulleys cause the crank to work less to spin it and there for freeing up HP.
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