The main idea as I understand it of an under-drive pulley is when you are pretending that you own a race car and you have put so many after market parts on that it no longer idles at a reasonable speed and you drive with high revs you install an under drive pulley because the engine is always spinning faster, but it is true that the larger the pulley the less energy is wasted bending the belt and the looser you can have the belt as well because it's making more contact with the pulley.
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Originally Posted by 07aveoSVM
I'm confused about a few things. I'm getting the impression that underdrive pulley's either hurt your mileage (by bending the belt) or do not make any noticeable improvement. I thought underdrive pulleys were promoted to improve both horsepower and efficiency? I thought the smaller, lighter pulley was supposed to have less angular mass and therefore use less energy to rotate. At the same time, it is supposed to give the crankshaft more leverage over the other belt components which should reduce the resistance seen by the engine. Is this not true?
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