Thread: Alternator RPM?

Originally Posted by Davo

Here's where I'm planning to put a second alternator. It will become the main alternator and the original will charge the second battery, on the LH side. This makes more sense than each alternator being on the opposite side of the engine bay.



The plan is to have it run on its own belt off the second groove of the aircon pump. All up, this involves a few pulleys on the way, and calculations show that if the crankshaft is doing 2000rpm, then this alternator will do 4125rpm. Is this too much?

If you work out what RPM the original alternator is doing, that will be a guide as to what the new alt. should be running at, Regards Frank.

That's a good point, and I did - it calculates to be 2468rpm. But it's a replacement I put on and it often takes a little tap on the accelerator to get the warning light to go out, so I'm not sure that it's running as fast as it should.

Do you still have the original alt. if so compare the diameters of the old and the replacement, the replacement may be running too slow, if so can you swap the original alt. pulley with the replacement.
The 2nd. alt. looks new, did it have any paperwork with it that may tell you what the max RPM should be, if not ask an Auto electrician what speed your model alt. should run at, good luck, Regards Frank.

Prestolite - Leece Neville

According to this link you are about right. I'd said you'd want a bit higher ratio but the best advice is to see if you can get the data sheet for the alternator.

Hi Davo,
I had a 1983 Rangie for 23 years and ran two batteries. I ran one high output alternator which kept both batteries happy. From memory the output was either 85 or 160 bummies. It had a heat shield under it keeping the manifold heat at bay and never gave trouble. The main reason I got it was that it had sufficient output to weld from.
Two alternators in those hot engine bays is twice the fittings and potential trouble.
Perhaps what worked for me may be of interest to you.

I have had 2 in my RRC 92 for about 8 years now with no problems.

Oh and it was great in Kakadu when the regulator went on my Bosch 85 amp, to just connect the Ingram 85 amp to both batteries and continue to Darwin.
Regards Philip A

Thanks guys, as always. My apologies for getting a number wrong - I was using this: Pulley Belt RPM and Speed Calculator and didn't do something right, so it would seem after all that the present alternator is doing 4387rpm at a crankshaft speed of 2000rpm. And the specs show that the light goes out at 1755rpm, which is 800rpm at the crank, just on idling speed, which must be why it sometimes needs a little help.

The original alternator had a 2&1/2" pulley, and the new one is 3", so the old one must really have been revving. These two new alternators are from Baxters, part NAB900, off eBay, so they're easy to get, which is important up here. They are rated at 65 amps, and come with a printout, which shows this amperage at about 3000rpm. A graph shows that 4000rpm is supposed to give 80 amps, and 87 amps at 6000rpm.

So the question is whether a constant 5700-6000rpm at a cruising engine speed of 2800rpm would be bad. The old one must have done been doing 7371rpm???

I'm going with two alternators after having a scare with the Landie years ago while visiting friends on the Gibb. I never worked it out, but the charge on the voltmeter seemed to drop. Since then, two batteries and alternators have sounded like a damn good idea!

Now I find that I can't get a belt with the right size, 10A850 or so. Any ideas?