Alternator and pulley question

[brewil]

Hi all,

What is the smallest size serpentine alternator pulley that I can use (have made)?

I am looking at KRC's pulley system ( for my street car) but their water pump and crank pulleys are only 4 inch.

This will be a street machine with EFI, electric fuel pump, stereo, locking, alarm etc and I am also not sure what amp alternator would be best to use.

HELP HELP HELP

 

[1968FormulaS340]

Need to be careful of the alternator RPM. I know for a fact the the stock Mopar alternator will start spitting its guts out at 8,000rpm with the stock pulley system.

 

[ram250098]

I had a buddy send me this email as I think my alternator pulley is also to big which is why I am not charging at lower idle speeds. Good info, but still need to know how fast the stock alternator spins.

I will try to explain the formula. it is an inverse proportion. i have not measured pulleys, as i keep with stock set-ups. say for example u have a 4" crankshaft pulley, and the alternator pulley is 2". u write it like this:

4" x 4 times 750
------ = ------- this equals x = -------------- x = is the rpm of the
2" 750 divided by 2 2" pulley

750 is the factory specified curb idle speed

4 times 750 = 3000, 3000 divided by 2 = 1500 rpms the 2' pulley is turning.

with a 2 1/2" pulley:
4 times 750 = 3000, 3000 divided by 2.5 = 1200 rpms the 2 1/2" pulley is turning.

with a 3" pulley:
4 times 750 = 3000, 3000 divided by 3 = 1000 rpms the 3" pulley is turning.

u will have to measure ur pulleys obviously. run off ur crankshaft pulley that is what is turning the carb set idle speed. then u can gauge how fast the fan, and the alternator r turning. as u can see the larger the pulley, the slower the alternator turns. sure the fancy billet pulleys look nice at the show, but in practicality if u r using this car as a daily driver the pulleys should be changed. if it is just a show car, well, then an investment in a battery maintainer will be necessary.
Good luck

 

[brewil]

Thanks Ram!

I've check an I will need a 1.3 inch pulley if I want to use this kit which I am sure is way too small!

The alt should be running at idle at around 2400rmp apparently.

Anyone have any suggestions what amp alternator I would need in reference to the first post?