Quote:
Originally Posted by MetroMPG
... If that's the case, except at very low alternator RPM, I don't believe there's anything to be saved by underdriving the alternator.
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I think that's right, as far as the electrical aspect goes. Mechanically, I think that with the engine spinning the alt at lower rpms, the alt's purely mechanical load on the engine would be reduced by whatever percentage you reduce the alt's drive ratio. I'm talking about bearings, belt friction, that stuff. Those are the engine loads you can't remove by only disconnecting the alt electrically, you can get rid of that load by removing the belt or likely reduce it by underdriving the alt.
Likely the alt's max amps output would be reduced at the lower spin speed. If you have a 70A alternator and you ask it to deliver only 30-40A, it will deliver them just like before. You might not get the full 70A if you ever "ask" for it (cabin fan, headlights, and kicka$$ stereo all running at max). With an underdrive or with stock drive pulley, output is reduced near idle speed rpms; you can see that on a voltmeter at idle.
If you're hypermiling you're likely already working to reduce electrical system load; you probably won't need the max limit of what the stock alternator was able to do. Until your uncle rides with you and expects the full luxury treatment.