Alternator pulley larger size?

Mopar General Discussions

  1. QuickDart360

    QuickDart360 Well-Known Member

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    Ok I have come across an alternator pulley that is 3.25" in diameter. Have found some vague info that it came on some hemi cars and 440 also 340 cars. Most stock alternator are 2.50" diameter roughly. So my question is why did some have the larger pulley?:eek:
     
  2. diymirage

    diymirage HP@idle > hondaHP@redline

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    Underdrive, it takes less power to run the larger pulley, because it is basically a lower gear
    (One might also want to have a larger pulley on a higher revving engine to keep it from frying the alternator)
     
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    • AJ/FormS

      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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      Do the math; 8 inch pulley on crank, 2.5 on alternator ;7000 crank rpm is 22400 on the alt. But only 17200 on the 3.5.
       
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      • 72bluNblu

        72bluNblu FABO Gold Member FABO Gold Member

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        Yup, the larger pulley will make fewer revolutions compared to the smaller one, spinning the alternator slower. Although that would make charging worse at low RPM’s, which can already be an issue with the stock alternators

        Keep in mind not all the crank pulleys were the same diameter either. The larger alternator pulley, paired with a larger diameter crank pulley, wouldn’t necessarily be under-driven. You have to look at the ratio of the diameter of the alternator pulley to the crank pulley.
         
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        • kursplat

          kursplat FABO Gold Member FABO Gold Member

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          for more on pulley ratios see the many cooling threads lately
           
        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          IIRC @Rainy Day Auto has been collecting info on OEM pulley diameter combinations and applications
           
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          • QuickDart360

            QuickDart360 Well-Known Member

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            I am willing to give it a try. My motor will have the 292 high rev cam. 3.91s out back and a holley strip dominator intake. So yeah high rpm's will be more than likely! All in a dart sport 360 a body. Also a 9.5" 4200 stall dynamic converter in the near future.
             
          • AJ/FormS

            AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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            I have the bigger one on my 360..... cuz I like to shift it at 7000 whenever I have opportunity, but with 3.55s, and a 3.09 low; 7000=52mph. Then I hit the splitter and go again to 7000=66mph, Heck-Ya.
            She's never had a problem keeping the battery pumped up,lol.
             
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            • QuickDart360

              QuickDart360 Well-Known Member

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              Also saw some larger aftermarket 5" i believe moroso pulley for chryslers. But doubt this big pulley will work on a street ride.
               
            • QuickDart360

              QuickDart360 Well-Known Member

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              Hey AJ, so your car does ok @ idle in traffic? Charge stays normal?
               
            • AJ/FormS

              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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              yes, never a problem....... because the only thing it has to run is the ignition,lol, and she's a manual trans with a 10.97 starter gear , so 800rpm is already 5.9mph. What I mean is even just crawling, the rpm is up
               
              Last edited: Sep 7, 2018
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              • Hellrats

                Hellrats Just another dumbass FABO Gold Member

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                My 72 Scamp came from the factory with the larger alternator pulley. It's started life as a 318 car with A/C.
                 
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                • Johnny Dart

                  Johnny Dart Well-Known Member

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                  They are designed to send you straight to the help section on the site.
                   
                • QuickDart360

                  QuickDart360 Well-Known Member

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                  Help section?:eek:
                   
                • Johnny Mac

                  Johnny Mac www.blueprintengines.com FABO Vendor

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                  It actually works backwards with a smaller crank pulley. The Moroso is smaller than the factory pulleys, thus slowing the alternator speed. I think my factory crank pulley was almost 7 inches. (makes a good wheel chalk now) I kept tossing the alternator belt at 7000 RPM. The moroso pulley helped my case a bunch. My alternator is a 140 amp aftermarket. haven't had idle charging issues since swapping to the moroso.
                  (FYI the moroso has a single offset bolt hole that won't lineup with some aftermarket balancers...so you may have to run one less bolt if you don't have the early type balancer with the one offset hole)
                  kinda off topic, but may help someone else in the future
                   
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                  • Johnny Mac

                    Johnny Mac www.blueprintengines.com FABO Vendor

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                    So to pop this back to the top...my words above came prior to the install of a bolt on electric water pump. I had a little moroso add on pump fail, and went with a bolt on direct bolt on water pump.
                    Since that pump install I had been chasing low voltage and charging issues if the race day ran long. Threw a new monster powermaster alternator at it, and voltage regulator, no change. Installed a voltmeter, and realized that at anything under approx 1500 rpm, the little 5 inch crank pulley is not spinning my alternator quick enough to charge. I am only putting out 10 volts at "idle" so in my quest to not throw belts, I shot myself in the foot with idle charging.

                    Yesterday I had some labored starting, and hooked up my trucks jumper cables. I could actually HEAR my electric fan pick up NOTICABLE rpm.

                    My conclusion... the small moroso pulley is not the fix all I thought it was, and I am putting major strain on my electric accessories, and slowly discharging my battery even with the engine running.

                    Now the search begins for a larger crank pulley... or maybe a serpentine crank/alternator Pulley. And hoping my geometry is correct enough to not roll/toss belts

                    Wanted to post, although an old thread, to save someone the problems involved with this underdrive pulley BS
                     
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