Quote:
Originally Posted by Ryland
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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Purely from a power/acceleration perspective: When it comes to low end boost, I have yet to find anything "bolt on" that can beat an under drive pulley, especially for the price. I've done it to several cars, but notably, all my Neons and the Scion xB (which, with it's auto trans, was excruciating to accelerate prior to the pulley). The gains are noticed from the bottom end all the way up, unlike any intake and exhaust work (minus boost! lol). I think a lot of it comes from the weight difference in rotation, though. Shedding several pounds off the crank, as well as a slightly tighter diameter brings what little weight there is closer to center seems to really where it shines. MPG wise, I've never done a good A-B-A comparison, because they aren't exactly easy to swap out.
Chiming in back on direct topic, I'd be curious how much distance people are getting on no-alternator setups. With alternator and small battery, I would find myself having issues keeping decent voltage when pulse and gliding engine off over 40+ miles. Note that it's with a very small battery, not some standard or larger deep cycle batteries. Since, I've added my "solar sunroof" that helps trickle charge and voltage hasn't been an issue since (haven't driven the distance at night, though, where the lights make the engine off portion nearly impossible).