altenator pulley ratio to crank
 

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.

Re: altenator pulley ratio to crank
 

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.

Re: altenator pulley ratio to crank
 

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

Re: altenator pulley ratio to crank
 

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.

Re: altenator pulley ratio to crank
 

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

Re: altenator pulley ratio to crank
 

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

Re: altenator pulley ratio to crank
 

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

Re: altenator pulley ratio to crank
 

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 .

Re: altenator pulley ratio to crank
 

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

Re: altenator pulley ratio to crank
 

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

Wade O

Re: altenator pulley ratio to crank
 

Wade are your pulley numbers backwards or is it just me. I have found pulleys for my crank with ratio of 2.26 and 2.10 for my altenator pulley size. Thanks guy's for the helpjust need to make brackets now.

Re: altenator pulley ratio to crank
 

Is there any benefit to underdriving the alternator?

Re: altenator pulley ratio to crank
 

Wades numbers are right for a Stocker that needs no real alternator output.
carburated Stockers run faster with a fully charged battery and an alternator that is really just along for the ride.

Re: altenator pulley ratio to crank
 

I agree with Tom and Wade.....That how we run all our stockers (Although not the truck we use to tow them Home).

Re: altenator pulley ratio to crank
 

i was kinda thinking that might help on gear changes...just a thought.

Re: altenator pulley ratio to crank
 

Me too.......................

Re: altenator pulley ratio to crank
 

I should have mention this is a stocker. Wade and Bob are you guys just running off the battery and not running any electricity to the altenator?

Re: altenator pulley ratio to crank
 

We use Denso Alt. and still have 13.8 to 14.2 volts above 2,500 with above ratio.....I like all the voltage I can get with MSD. We are seldom below that RPM even in Pits.

John Irving

Re: altenator pulley ratio to crank
 

With my ICE Ignition, he likes to see over 14v. By about 3000-3500 rpm's, I have 14.4

I did have a racepak on for awhile, after peaking at 14.4, it stayed there for the duration of the run.

We do have to have a working alternator, hooked up and charging as per rules.

Wade O

Re: altenator pulley ratio to crank
 

My alt. turns on at approximately 3000 to 3200 rpms and puts out 14.2 volts.