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Alternator RPM

Old 11-06-2018, 10:32 PM
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Default Alternator RPM

I bought a Powermaster alternator (Summit PWM-37461) for my LS3. The alt specs say not to exceed 18,000 RPM.
They suggest a larger alt pulley or a smaller crank pulley. The crank pulley is 7.5" dia and the alt pulley is 2.5" which is a 3:1 ratio. So with the engine at 6K (not even at redline) the alt will be at 18K. I'll be going close to 7K so I'd like to change the alt pulley but Powermaster doesn't sell one that would work, need at least a 2.8" OD.
Have any of you toasted an alt by over reving it?

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Old 11-06-2018, 11:00 PM
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It's actually a fairly common thing that the stock alternators fail if they're over-revved. But they're pretty much bullet-proof if you don't over-rev them. As far as an aftermarket one - no idea.
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Old 11-06-2018, 11:11 PM
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They make a large pulley for the alternator. Its like $40
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Old 11-06-2018, 11:23 PM
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Originally Posted by JoeNova View Post
They make a large pulley for the alternator. Its like $40
I know they do but I haven't been able to find a 6 groove one with a 17mm bore and a 2.8" OD.
Powermaster doesn't have one.
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Old 11-08-2018, 01:53 PM
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Found one from https://www.aspwholesale.com
It has a belt dia area of 2.62" as opposed to the 2.0" that came on the alt.
So at 6600 RPM the alt will be spinning at 18,893 RPM a little higher than recommended but much lower than the 24,750 if I was using the stock pulley.
The back spacing is less so I'll need to add a 2.5mm spacer between the alt and the pulley.
I bought their p/n 24-1271.
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Old 11-08-2018, 03:34 PM
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Hopefully it produces enough at idle to keep your electronics working. I’d keep an eye on the volt gauge.
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Old 11-08-2018, 08:52 PM
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It should be fine, there isn't much in the car that draws a lot of amps. It's got an Odessey AGM battery.
The alt pumps out 140amps. I don't expect it to be spending much time idling.
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Old 11-09-2018, 12:13 AM
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I had the same problem with my car and did not think about alternator speed when I was racing my combo. 7500 rpm with a 2.75" alt pulley and 7" crank pulley. The voltage on my data logs was all over the place at high rpms. Anyway, I changed it to a 3.5" and had no problems with charging at a 1000rpm idle. I actually tried to match the rpm the factory would have on the stock alternator. The pulley I bought was from https://www.krcpower.com/ . It solved all my voltage variations. At least you found this out before it was too late.
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Old 11-09-2018, 09:23 AM
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Thanks for that link. My pulley should be here tomorrow or Monday but we're leaving for a weeks vacation.
The Ls3 has a 2 year warranty which will be 3/4 up before the car is finished. When the warranty is up the plan is to replace the cam, maybe add nitrous so an even larger pulley will be required.

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Old 11-21-2018, 08:26 PM
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I put the new 2.6" pulley on the other day. At idle, 625 RPM, the alt is putting out 14.5 volts.
I should be fine.
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Old 11-21-2018, 08:32 PM
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Originally Posted by JimLev View Post
I put the new 2.6" pulley on the other day. At idle, 625 RPM, the alt is putting out 14.5 volts.
I should be fine.
Good to hear. Itís nice to see people actually report back to a thread and give results.
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Old 11-21-2018, 08:36 PM
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I'm a mod on a BMW forum and always try to get peeps to end their thread with a conclusion that will help others.


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Old 11-21-2018, 08:47 PM
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Originally Posted by JimLev View Post
I'm a mod on a BMW forum and always try to get peeps to end their thread with a conclusion that will help others.
I agree! Just common sense and good manners!
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