Outer Limits 4x4 Board

I have been fitting a 2nd alt to the GU today, I got one out of a patrol and the other off a hyundai excel. Both were 75 amps but the hyundai one is less than half the size of the other. how can this be if both are putting out the same?
cheers.

Different operating speeds?

Bullshit ratings?

-Scott- wrote:Different operating speeds?

That'l be the one...

Patrol motor would be a slugger, especially if it is a diesel (general rev range 800-3500 rpm) Hyundai would be suited to a higher speed engine (up to 5500 rpm).

You will probably find that the Hyundai unit will not put out anything like 75 amps at the engine speeds of the patrol.

This would all be assuming comparable sizes of the pulleys on the front of the alternators. One way around the problem would be to get a smaller pully for the Hyundai to speed up its effective rpm (tricking it into believing it has a "faster running" engine).

thanks for replies, but i might be missing something here. I was more interested in the physical size difference.
cheers.

numbnuts wrote:thanks for replies, but i might be missing something here. I was more interested in the physical size difference.
cheers.

Although not an absolute rule, alternators make more current the faster they turn.

The Patrol alternator would be designed to work well at low speeds, as it is hooked to a relatively low-revving engine, and is expected to work well at low engine revs. To get good current from low revs requires a physically larger alternator.

The Hyundai alternator is designed for a buzz-box which must spin higher to produce reasonable performance, and low speed operation isn't much of an issue. So the Hyundai can get away with a design requiring higher revs to generate its output, so it can be a physically smaller alternator.

Well, that's the theory, and Gary and I are sticking to it.

If you want to use the Hyundai alternator you may need to "change its gears" by fitting a smaller pulley to it. This will cause the alternator to spin faster at any given engine speed. If you do this, make sure your replacement pulley is for the same belt width.

thanks for that mate, i understand that now.
cheers.

Echoing what the others have said...

IMHO it's ultimately going to be like an engine, the bigger one is going to be more durable and have more capacity to spare, assuming roughly equal age and tech.

The Hyundai one was in a cheaper, smaller car and was probably selected on price as well as anything else. I would guess that the Patrol one manages something like peak amps over a much wider rev range, while the Hyundai one probably gets to those peak amps when you are reving the engine's tits off. I would fit the bigger one if you have room, no hesitation.

But... if it is really just a second alternator you may not need the full 75A out of it anyway.