Under drive pullies, they are smaller so it takes less effort to turn them so they free up some HP. The only expereance I`ve had with them was not good. They caused some cooling issues with the car I used them on.
25% Underdrive pulleys will add 5-8 hp, they cost around $300 and require changing to shorter belts. Like Rukee said they don't add hp but free up hp needed for the ps pump, alternator and AC.
The best results from a pulley are with superchargers.
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