Dick:
Too many unknowns to really answer you question so I'll describe my thought
process and you can judge how it will work for you.
From the looks of a normal stock set-up, the alternator is over-driven
about 2.5:1 engine speed. Looking at the Hot Rod pulleys, they reduce this
overdrive to about 2:1 engine speed. Since you daily driver usually
operates in the RPM range of 1,500-3,500 rpm (rarely higher) I would assume
that Detroit expects most alternator output within that range of RPM. In
other words, a max alternator rpm of about (2.5 ratio x 3,500 rpm= 8,500
rpm). Sure, it will go higher but that is why they sell Hot Rod pulleys. .
. to reduce the rpm and also the parasitic power loss.
Since your set-up is pinion driven, the pinion rpm will never be greater
than engine rpm divided by your gear ratio. (6,000 engine rpm /3.00 rear
gear = 2000 pinion rpm). (assuming you transmission top gear is direct or
1:1). Using these assumptions a pulley ratio of 4.25:1 would yield a 8,500
alternator rpm at the lights.
Will that set-up work for you...probably not. And how do you figure
"average" engine rpm over the course (long or short?). My guess, would be
to forget about the time it will take to get to 100 mph (12-15 seconds) and
figure you are going to be pulling for about 2 minutes, trying to get to
terminal speed. So figure your engine rpm in high gear from 100 mph to
terminal speed mph and set you alternator pulley ratios to give you about a
max of 9,000 - 10,000 rpm at your terminal speed,,, what ever that is.
(Raise or lower those numbers in proportion to your "pucker factor"). You
will be underspeed at 100 mph and overspeed at terminal speed but for those
entire 2 minutes you will be at least getting some amps out of the
thing. Are you long or short course? How long will it take to accelerate
after 100 mph? What is your engine rpm at terminal speed? Only you have
those answers and they all effect your pulley ratio. Good luck. . . Your
mileage may vary. -Elon
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