The pulleys are of different sizes, the DRIVEN pulley (on the crank) is smaller so that it moves the belt less with each rotation, and the accesorie pulleys are LARGER so that they take more belt to make a rotation. This all adds up to slower accesories, and less power loss too them. When i put on my 130 amp alt, i could run all the accesories (ac, lights, etc) at idle, and be just below halfway on the volts gauge. Previous to the 130 amp, at idle, the volts gauge would be there with NO accessories on. With the underdrives, it charges a little less, so i would NOT reccomend them if you have a stock, 75amp, alternator.
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