ahmed

i want to get a 25% underdrive crank pulley. dont know if i should keep stock alternator pulley or go underdrive or over drive with it.


what really determines what i can do. the voltage i need to run the car. i dont have subs or anything. cheap cd player and 2 speakers. power windows. thats about it. its a 1990 mustang.

Golf&GM

iTrader: (22)

I had a 25% underdrive ASP pulley with stock alt pulley on my 6-speed for over a year and never had any charging issues. I hear that if you have an auto you might need the overdrive alt pulley, but I don't have any experience with that.

ahmed

im 6 speed. im sure just the crank pulley alone i wont have any issues. just wondering on what i should do with the other pulleys.

NVR_SPDS

iTrader: (39)

I had the 25% UD pulley on my auto, with the rest of the pulleys being stock and didnt have a single issue.

ls1 1990 VN

Never had any charging issues for 4yrs now

ULTIMATEORANGESS

iTrader: (4)

ASP for 10 yrs no problems.

bene

iTrader: (16)

I have powerbond 25% one, 6 spd also, no issues at all with charging.

ahmed

i know the crank pulley wont give me issues im wondering about if i went underdrive on the alternator pulley

Golf&GM

iTrader: (22)

You mean underdrive alt pulley with underdrive crank pulley? Engine speed determines how many RPM the crank pulley will spin and the circumference of the pulley determines how fast the accessories will spin. With a 25% underdrive crank pulley, because the circumference of the pulley is smaller, it will spin the rest of the accessories slower. This along with less rotating mass increases horsepower. If you add an underdrive alt pulley the alternator will spin even slower and will charge that much less. With just an underdrive crank pulley, at idle I would notice that my headlights would dim slightly, if you reduce the speed the alt is spinning even further with an underdrive alt pulley you will start to have electrical and charging issues. It might net you a coiuple ponies but you'll probably have a dead battery before too long as well. Not worth it in my opinion.

EDIT: This is kind of misleading, read post below.

ahmed

thank you golf&gm for the info thats all i needed...

imma_stocker

iTrader: (19)

Correct me if I'm wrong but I think Golf got it backwards. If you UNDERdrive both the crank and alt pulley they will spin at a closer ratio. If you underdrive the crank and get a larger, overdriven alt pulley then the alt will spin even slower (counter-productive).

You only care about the center of the alt pulley. The outer edge only determines belt length to wrap around the pulleys.

Your best bet is to get a higher output alternator.

Golf&GM

iTrader: (22)

Thanks for pointing that out, I did get it a little wrong.

The terms underdrive and overdrive do not refer to the size of the pulley, it refers to whether the accessories spin slower (underdrive) or faster (overdrive).

If the pulley drives the belt (crank pulley) then a smaller pulley would be underdrive and larger would be overdrive.

If the pulley is driven by the belt (alt pulley) then a smaller pulley is overdrive and a larger pulley is underdrive.

So we've got:
Smaller Underdrive crank + stock alt pulley = OK for most
Smaller Underdrive crank + Larger underdrive alt pulley = FAIL
Smaller Underdrive crank + Smaller overdrive alt pulley = Charges alt better

Sorry for the confusion.

imma_stocker

iTrader: (19)

Thanks Golf, that cleared up the terms for me too! I thought under/over driven meant smaller/larger pulley, respectively.

ahmed

ordered an slp 25% underdrive pulley.

RedmaroSS

I did the powerbond kit and its great installed 3,000 miles ago on my 00ssm6 and no problems it also came with a new crank bolt and both belts