Alternator pulley larger size?

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QuickDart360

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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:
 
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)
 
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.
 
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.
for more on pulley ratios see the many cooling threads lately
 
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.
 
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.
 
Also saw some larger aftermarket 5" i believe moroso pulley for chryslers. But doubt this big pulley will work on a street ride.
 
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
 
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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.
 
They are designed to send you straight to the help section on the site.
 
Also saw some larger aftermarket 5" i believe moroso pulley for chryslers. But doubt this big pulley will work on a street ride.
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
 
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

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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