On Feb 9, 7:42 am, "Bill Noble" wrote:

FWIW, my car idles at 800 rpm, readlines at 7500. without measuring, I'd
agree with the aprox 2:1 ratio of diameters, so that suggests that on my car
at least, the alternator must be OK at 15K RPM.

True, but there is no need to speed up the alternator in this case.
With equal sized pulleys and the engine running at 3000 rpm, the
output will be the same as if it were in a car with the engine running
at a very high idle. Alternators will charge batteries when the car
is at a low idle.

Dan