altenator pulley ratio to crank

[west coast]

I am in process of buying a new altenator and I am wonder what kind of ratio to use will have to buy a new crank pulley or make one. Only turning the motor 7000 rpm.

[Dragsinger]

A general guide line for full alternator output is to overdrive the alternator 30% - 50%

Example; a 5" crank pulley combined with a 3" alternator pulley = 40% overdrive. In this example, 1500 engine RPM will equal about 2100 alternator RPM

If the alternator is of the proper amperage output for your load the above example would have full battery voltage to everything with an engine speed of 1600 - 1800 RPM

This type setup will allow you to race all day without battery charging.

[Ed Wright]

I just bought a low-mount alternator mount & pulleys from Jegs, and a pully set, meant for street rods. Has worked very well.

[Tom Goldman]

East Coast Auto Electric recomends a 2 to 1 ratio .
Most pulley kits made specificaly for drag racing are around this ratio.
I have run higher ratios in the past with no problems.

[Robert Simpson]

Does anyone run a cog belt set up for a low mount sbc 69 camaro? Just curious...Thanks

[Tom Goldman]

Originally Posted by Robert Simpson

Does anyone run a cog belt set up for a low mount sbc 69 camaro? Just curious...Thanks

I run a low mount cog on my BBC.
Check out Jones Racing Products in Ottsville ,PA.
They have a huge selection of cog drive parts..
Almost every circle track racer runs their stuff.
Tom

[Tom keedle]

here's something else to think about,
http://www.decouplerpulley.com/
i see potential...

[Tom Goldman]

I tried one on my car when I had a micro v belt system on the car.
Two of them failed ,altho they were OEM units. ... locked up the overunning clutch.
may have been the rpm range , as I was running a small engine at the time and hitting about 8400 in the traps.
The most common application of these pulleys is on diesels ,to help minimize belt screech at shut down. ...They also cut down on belt whip when a automatic tensioner is employed .

[rod]

try to limit the alt. rpm's to less than 20K. and then only for a very short period of time.
let's say U used a 6" crank pulley and a 3" alt. pulley. that would put the alt. turning 14K with the engine at 7K. the engine would only be at 7K at the top of some gears and for a short period of time.
now if U used a 6" and a 2" the alt would go 21K. way to much.
if you have no choice, then 1. make sure you use an alt. than can be easily [meaning not some special alt. which has to be ordered and takes three weeks to show up] and cheaply replaced or 2. have all the windings epoxy coated to keep it from flying apart.

in my case on my 265 i used a nippo denso alt from a 3 cyl chevy something. i used an old smog pump pulley [5" dia] drilled for the chevy three bolt dampner pattern and the original 2 3/4" alt pulley. that runs the alt at just a tad less than 2 to 1. my 265 hit 7K twice. and again only for a very short time.

be careful of some low mount kits on camaro's. the alt. pulley runs into the steering box. i spent 3 ours reconfiguring a friends. ugh.

rod in AZ

[Wade_Owens]

Originally Posted by Tom Goldman

East Coast Auto Electric recomends a 2 to 1 ratio .
Most pulley kits made specificaly for drag racing are around this ratio.
I have run higher ratios in the past with no problems.

x2, I have an east coast alt. also. 3" crank, 5" alt. no issues at all.

Wade O