vettegirl: I have done this in the past on a Mustang.
It does not "make more power." Rather, the benefit is that you divert less of the engine's power towards spinning accessories (alternator, AC compressor, etc), so that extra undiverted power is available to accelerate the vehicle.
Downsides:
- Much easier to run the battery down, since charging at a slower rate, and also there is more time (idling for example, or stop and go), where the electrical consumption exceeds the electricity being produced
- Performance of accessories, including AC, may be impaired somewhat, especially at idle and low city speeds
- Cost of the pulleys and the labor.
How much extra power is undiverted? UP TO a few horsepower. Not huge.
Do I plan to do it? No, not enough benefit to offset the negatives, for ME.
Jim G