SharkSkin

Tom, IMHO such performance curves are "ideal" and "as designed". Variations in the manufacturing process will result in slightly different performance curves, and as steep as that curve is around 1400-1500 RPM it only takes a small shift to see actual performance fall off the cliff. I think the loss at the high underhood temps that we see on 928s may be quite a bit higher than your 85% number would indicate.

tveltman

It could very well be so. It seems to me, though, that the alternator output is going to follow the same general shape, so the higher the rated output, the more likely you are going to be to get decent amperage at idle. I think that I will try out the 140-amp CS-130 and see how it goes. Maybe the 170 would be useful for all the blown cars out there, or those of you who replaced the rear "seats" with equally-sized subwoofers.

SharkSkin

Same general shape, but as I understand it, aside from manufacturing variances, the shape of the curve is affected on both axes by temperature.

That is, the curve starts off further to the right(higher RPM to start charging) and has a lower peak(less max output).

BTW I just noticed that set of curves is for a CS144 -- I don't think it will fit if you have oil cooling lines. The case is big enough it may interfere with other stuff as well.

tveltman

Oh yeah, I am well aware of that (but thanks for pointing it out anyway, never hurts to be too careful). I was really just citing it as a general example. I plan on putting a remanufactured CS-130 case that contains higher-amperage innards. That was why I was concerned with the mod on later-model cars. It isn't safe to assume that the curves are the same for the 140Amp 130 and the 140Amp 144, but I was hoping to show a general trend. As the amperage rating goes up from 108 to 140, the amperage at any given speed increases, and there is a HUGE difference in output between the 108 and 140 down around 1500 RPM. This was the basis for suggesting that the high-cap CS-130 would be an appropriate modification for any 928, because higher amperage rating clearly results in higher output at idle, regardless of the specific temperature or performance-level at that temperature. (I suppose it's possible that the high-capacity alternator loses more efficiency with heat than the normal capacity alternator, thus resulting in very little or no net gain at idle, but there is no way to know that without testing it out, and I doubt that it would be the case).

EDIT: Ahh i see what you are saying about starting off further to the right with temperature. Yes, that is a definite concern, and you are right to point it out. However, I think the point still remains that the higher the rated output, the better you're going to do at idle.

SharkSkin

If you have found out that a particular frame style offers options where a higher-output alternator puts out more at idle, that's great, but that's not usually the case. One anecdotal example -- on my Ramcharger, I asked my local rebuilder to give me more output at idle. I ended up with double the capacity at idle, but I gave up 100A max output and ended up with 80A max output. This was about 15 years ago, but physics is physics.

Here are a couple of examples randomly selected via Google where, on the same frame, higher peak output results in lower idle output. Please talk to your rebuilder, they ought to be able to explain the specifics better than I can. Images are linked from this page:




Keep in mind that most alternators are designed to run 3x crank speed or higher -- the CS130D I fooled with, as it turns out, was designed to run about 5x crank speed.

My feeling is, if you are not running a lot of accessories, you would be better off with a 75A alternator that puts out 50A at idle, than with a 200A alternator that may only put out 20A at idle. I tend to plan for worst-case scenarios, e.g. warm humid rain, stuck in traffic needing the defroster -- which means AC is running, which means rad will get hot enough to call for fans -- lights on, brake lights on(surprising how much current the brake lights draw), wipers on, rear defroster on. You may set up the system so everything is all fine and dandy, then you get stuck in traffic one day and you have a problem.