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Alt rpm??
http://motorsportsvillage.com/forum/viewtopic.php?f=4&t=9569
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Author:  Beretta [ Sun Sep 16, 2012 1:35 pm ]
Post subject:  Alt rpm??

Just how fast is the alternator turning at a given rpm?? Another words how do you calculate it? So if your using a 5.25 crank pulley and a 2.5 alternator pulley just what is the rpm of the alternator if the engine is turning at 7600 rpm??

Author:  Slowpoke [ Sun Sep 16, 2012 2:19 pm ]
Post subject:  Re: Alt rpm??

Approximately 16,000 rpm.

Author:  Ken0069 [ Sun Sep 16, 2012 2:25 pm ]
Post subject:  Re: Alt rpm??

If my math is correct, that alternator is spinning @ 15,960 RPM. ;-) The big pully is 2.1 times the size of the small one so I multiplied 7600 by 2.1 and got 15,960. :-k

EDIT: I see Donee beat me to it while I was double checking my math. ;-)

Author:  Beretta [ Sun Sep 16, 2012 2:50 pm ]
Post subject:  Re: Alt rpm??

Thanks Guys..My pea brain could not figure that out #-o ...Did not know it's actually spinning that fast and sure it could be slowed down some..Smaller pulley on the crank will speed it up so a larger on the alt should slow it down some but how concerned should I be about that??

Author:  Ken0069 [ Sun Sep 16, 2012 3:26 pm ]
Post subject:  Re: Alt rpm??

Beretta wrote:
Smaller pulley on the crank will speed it up so a larger on the alt should slow it down some but how concerned should I be about that??


NOPE! Smaller pulley on the crank will SLOW IT DOWN.

LARGER pulley on the alternator will also SLOW IT DOWN. ;-)

Author:  shrinker [ Sun Sep 16, 2012 6:24 pm ]
Post subject:  Re: Alt rpm??

A standard Bosch alternator produces its maximum charge at 2500 alternator RPM.

Author:  gearhead1011 [ Sun Sep 16, 2012 8:04 pm ]
Post subject:  Re: Alt rpm??

PSweeney wrote:
a friend of mine whose an electircal engineer for Ford in the UK was telling me the other day that most alternators are good for 18000 RPM. Max ratio in a road car is 3-1 but for a race car reving past 7K 2.5-1 is a good guide for longevity of the brushes and bearings etc.


From here viewtopic.php?f=7&t=5437&hilit=alternator+rpm&start=15

When I first put the alternator on mine I had a 5.25" CS pulley and a 2.5" alternator pulley. When I added the vacuum pump a year or so later I had to use a smaller CS pulley (I think a 4") and it slowed the alternator down quite a bit. I didn't notice a gain in performance but I did notice it doesn't charge as good as it did with the other pulley. When we're racing 1/4 mile it's fine but not so much with the 1/8 mile racing. If I were you I would leave it like it is.

Author:  nomad [ Mon Sep 17, 2012 7:48 am ]
Post subject:  Re: Alt rpm??

My pulley ratio is 2 for the alternator to 1 for the crank. It always worked real well. What really made things click for me was when I went to a three wire alternator. And I spin it backwards. Voltage is sensed downstream from the alternator.

I still charge between rounds as I still have to cool down.

I use a Powermaster 8002 alternator. It's sized like a GM 10SI but, has 12SI guts in it. I found this out when I had to buy a rectifier for it.

Author:  Ken0069 [ Tue Sep 18, 2012 6:48 pm ]
Post subject:  Re: Alt rpm??

nomad wrote:
I still charge between rounds as I still have to cool down.


So do I, cool down that is but I never charge?

If you're logging system voltage I'd be willing to bet that the alternator will have battery voltage topped off by the time you hit the tree! I've checked mine and it's never less than 14 volts at the hit. ;-)

Author:  nomad [ Tue Sep 18, 2012 9:33 pm ]
Post subject:  Re: Alt rpm??

Ken0069 wrote:
nomad wrote:
I still charge between rounds as I still have to cool down.


So do I, cool down that is but I never charge?

If you're logging system voltage I'd be willing to bet that the alternator will have battery voltage topped off by the time you hit the tree! I've checked mine and it's never less than 14 volts at the hit. ;-)


I'm usually at 14 or so when I go around the corner. I use a Toyota minivan fan and shroud and it pulls a good bit I think. Sometimes I wander off when I see a bright shiny object and forget to turn it off.

Author:  ADAMSVEGA [ Mon Sep 24, 2012 5:24 pm ]
Post subject:  Re: Alt rpm??

2 x 16v agm batteries and 12v constant regulator (only starter is full volt) no alt
going lithium with next win !

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