3g Alt Question

Discussion in '1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-' started by TOOLOW91, Sep 25, 2013.

  1. Alright. That should do it. Any idea whether I should get the overdrive or get a smaller alt. pulley? I was thinking that the smaller alt. pulley might be better just because it'll cool the it better.
  2. If you have a stock crank pulley, you should be good as it is. Going a half inch smaller on the alt pulley may not make a difference with a full size crank pulley.

    The issue really applies to those with under drive crank pulleys.

    Bit it's worth a try.

    Stock 2g alt pulley is 2", but you'll need a 5/8" ID washer as a spacer to avoid hitting the case
  3. 14 volts at idle no jumping with blinkers or anything life is good !

    Sent from my iPhone using my fingers
  4. How do you measure the pulley? My car killed its alt and it has what appears to be a tiny crank pulley ( not much larger than the balancer)


    its grande!
    #24 GroverDill, Sep 29, 2013
    Last edited: Sep 29, 2013
  5. measure them from outside diameter on one side to the other outside diameter. The stock crank pulley is 6.5"? most U/D crank pulleys are around 4.5 inches, H2O 5.5 innches, and alt 2 3/8 inches. The Powermaster or full charge kits have a smaller 1 11/16 inch alt pulley.
  6. That's an Sn95 pulley. You can sort of see the ford OEM part number near the nut.

    ASP makes some good pulleys. If you have a 4" crank pulley, you can run the asp 1 7/8" alt pulley with a 5/8" ID washer as a spacer.


    As you can see, it's much smaller, so it will speed the alternator up to compensate for the crank pulley being an under drive.

    This is what I've run on my car with a 4" crank since the late 90's
    TOOLOW91 likes this.
  7. That's what I just took off mine the smaller fox pulley makes idle a delight

    Sent from my iPhone using my fingers
  8. so using just very rough math.

    crank is 4.5"
    alt is about 2.5"

    So the alternator goes around 1.8 times per crank rotation. my car idles at 950 ish so at idle the alternator is spinning approx 1710 rpms. According to PA page alternator needs 2k rpm to make power at idle. PA also states 16k rpm alternator speed is where it will fail. Sooooo I ordered at 46mm pulley

    Thanks boys!
  9. Yep, that is why I have the 1.68 pulley. The car idles at around 750

    700 x 2.67 (4.5crank /1.68alt)= 2000 alt rpm at idle. Even if I spin my motor to 5500, I'm good. 14685@ rpm.
  10. Like i said, that's been my exact setup for years. I'm using a OEM 3G alt from an SN95. Have had great idle power and no issues with upper RPMs
  11. I was just doing my research as my car seemed to be killing alternators on the dyno while being tuned, I thought it was overreving.

    plus you never drive your car :)
  12. That's true now, but when i first did my 3G setup, the car was a daily driver. I learned to trust it :)
  13. To be fair the alt that just died was on my car for almost 4 years with the stock SN95 size pulley. I never really had any problems but "since I am replacing it anyhow...."
  14. Here is a interesting alternator website I found, when you click on the type of pulley required it has them measured from the top grove of the pulley and not the total diameter. But they sell every type of pulley if people are searching.

  15. I personally would measure where the belt rides...the top of the groove.

    That's the diameter that matters when it comes to calculating belt travel

    I'd call the pulley in Grover's post a 2.5" pulley, not 2.75"