Anyone have a need for someone to tell us what we do and don't need?

 

It's been covered dozens of times.

You need enough generation capacity to cover what you use and keep the battery properly charged.

The alternators job is to provide ENOUGH output in it's general use mode..to keep your battery at the proper voltage..for good long term chemical stability.

A 200A alternator in a car using 45a, is only supplying...yes, not 200A. Its keeping the system at the proper voltage based on what's being consumed.

And the general use case, is not idle.


if you have a big whompin stereo, etc..and you CANT keep the battery charged with normal use (which is maybe 5% idle, 95% at charging speed), then you may need a higher output alternator.

But if you want to spend a large stack of change that provides a solution that might charge some more, at a very low RPM, feel free.

But..you're likely using exactly 0% of the additional amperage output that the new solution MIGHT put out.


Put your # into refurbishing the 30yr old cabling and grounds. Cuz you wont find anyone with any issues with the stock alternator as long as it and the voltage regulator are working properly.


Many have forgotten the batteries role in the system, as storage capacity.

 

Unlike the generic BS theory that you are relying on here, this is NOT in my head. The theory itself is not BS, pretending that it is applicable in all cases IS. After getting stuck in traffic for two hours and finally arriving at my destination and completing my business, the car was cranking slowly as I started it up to leave. That's just some of the empirical evidence that I relied on to identify and resolve a problem. Other evidence included the fact that the battery was nearly new and tested at ~800CCA when charged. Voltage drop at all critical connections and wires was nil. My gauge is as accurate as can be, within 0.1 volts (I recall it was 14.1 actual volts at the battery when the gauge was precisely in the 14v line).

Perhaps Hans and Franz were in some utopia where there is no traffic to worry about when they designed the system. Perhaps they collaborated with the Bonanza engineers and decided that since a Bonanza never spends more than 10 minutes on a taxiway that the only sensible thing was to engineer the 928 the same way.

None of that is relevant in any case. Decades later, my particular 928 could not keep up with electrical demands in all use cases that I encountered so I set out to fix it. When sitting at idle in traffic the scenario can be more like 95% idle and 5% >1K RPM. No, I don't like to sit in traffic but it happens. And when it happens I want my car to be able to handle it.

Nope.

The general use case may be fine for the general public. That doesn't mean that I am going to accept that if I want reliability under worst-case conditions. Fortunately I didn't need your permission to achieve that reliability. See the bolded section in the quote above. If you're stuck in traffic, idle IS the use case. And the alternator doesn't keep up in that use case. At least, it doesn't keep up on my '78, and that is because they used a relatively small crank pulley to reduce parasitic HP loss and I can't get a small enough v-pulley to compensate. As I mentioned above you probably have a small enough flat pulley on your alternator that you don't suffer from this issue. Or maybe you don't ever get stuck in traffic. Either way, telling people that they don't have a problem because you don't, or because of some general guidance used at the design phase is complete nonsense. Facts always trump theory.

 

I actually agree with much of what you said. But you'd have to be a bit crazy to believe this last part...

Try driving your 928 in hot weather... really hot weather.. The high has been >110F here in Phoenix for the last week. The temperature deration is quite OTT. My alternator works fine except when hot, yours would be the same...

Now you also do carry this much too far... the car should be able to handle extended periods idling without significantly depleting the battery. A little bit net negative would be OK - but it's much worse than that... Porsche just did a poor job optimizing this system IMO, it seems to have had similar issues for all years - always a bit too little margin and way too much derating for temperature.

Alan

 

Agreed, but a 200A alternator wont resolve the derating built into the voltage regulation.

Perhaps the -best- solution is a simple cheap alternator with the straight-to-the-battery sense wire instead of overly optimistic derating on top of the line losses.

 

Let me fix the problem...being..stop & go for hours, and a slow start?

I'll do the same thing in the plane, when in an _extended_ IFR release hold on a night flight...all lit up and nowhere to go pulling down -30a and the ammeter on a tiny *** battery.

Your experience is not typical, and me, myself, would not whip the bishop for $600-800 for one of these solutions to something I can manage on my own with the equipment at hand, for free.

I will also tend to stay in 1st gear so that when the traffic is moving, I'm burning some extended rev's to charge up what I can, when I can.

You're welcome to try to solve the issue with a measure of benjis, I will manage it for free. But in the end, it's still a <insert amp draw number> amp alternator, just like the stock one.

I'd have more fun going down to TechShop and CnC'ing a smaller diameter pulley for the cost of the aluminum blank.

 

I don't really care if my experience is typical, or more specifically I don't care if you think it's typical(it is, on early cars). I would just prefer a more robust system.

I have a different high-output alternator than the one Carl is selling now, and it was a LOT less $$$(about 1/3 IIRC). For a couple benjis I would consider having the rotor rewound to increase low-RPM charging. I'm not sure it's an $800 problem for me though.

You say that you will manage with the equipment at hand -- great for you, but how many times now, in this thread alone, have you ignored the fact that the equipment is different on early cars, less capable?

 

It might - but only because derating factors do vary across regulators - it's nothing intrinsic to a high output version, but I suspect many do adjust the derating factor - see below. Derating is designed to protect the battery (and alternator) under hot charging conditions. When the battery and alternator are very far apart, as on a 928 - the temperature of one tells you very little about the temperature of the other. Porsche made a conscious choice to do this but didn't fully consider all the implications here.

A battery remote temperature sensor would make more sense first (as above) then a remote voltage sense. Temperature is actually more important here.

The primary point in this whole discussion to me is that a high max amp version of any alternator will usually generate less well at low RPM than a low max amp version. Yes it's counter intuitive but here is why: in a fixed alternator case size there is only so much physical room for the windings, if you need thicker windings for higher max current capacity, then you have to also have fewer turns of them. Fewer turns means less magnetic flux sensitivity which is really no issue at high RPM, but becomes a big deal at low idle RPMs. If you specify a higher max output than you actually need (and therefore higher than you will ever actually use) you are trading off exactly no realizable benefit - for worse idle performance.

A terrible and expensive trade off for most folks who need exactly zero additional max current amps - because their cars generate perfectly fine for all uses at 2K RPM cruise and above. All they need - and this is what Dave is looking for too - is a low RPM solution.

Now some other techniques can get you more power if you are prepared to pay more or take risks, higher temperature windings cost more but can survive better at higher currents in the same sized wire, regulators can be made with lower temp derating that will simply stress the same alternator more (a direct trade off of long term reliability against output), smaller alt. pulleys can get you higher alternator RPM's (but might exceed the alternator max RPM @ red line).

So there are games to be played but the trade offs in a given package size are pretty simple: Max current for low RPM current AND reliability against output power.

The MTBF of special high output alternators does indeed seem to be anecdotally quite a bit lower than standard ones, which to me says many reliability vs output tradeoffs are made. That makes a lot of economic sense - because those trade-offs are usually cheaper to implement than heavier/higher temp windings. Manufacturers can offer generous warranties because their prices are high enough to cover the cost of some rebuilds, the consumer still covers the removal, install & shipping, which is likely to be the biggest expense/inconvenience. The ratio of returns is probably also relatively small since many of these vehicles aren't driven that much, and many owners will look for an alternative solution upon a failure after a few years... None of this really works in favor of the consumer of course.

Alan

 

Pricing: The 150 AMP alternator would retail at $542.
For comparison, the current 200 AMP alternator retails at $695

What I'm told by my supplier is that the 150 AMP alternator will provide twice the voltage at idle over the 200 amp unit. It just doesn't have the other end... but frankly I bet very few guys actually NEED 200 amps.

Anyway - just giving you options.

 

Carl - I assume you mean 'current' at idle here.

Certainly true that the 150A version is probably more appropriate than the 200A version for almost anyone here for all these reasons. I'm sure there are more winding turns on that 150A version compared to the 200A version...

Alan

 

Yes Alan. OMG. Good catch!

I will post when the webpage is up. The new alternator will be here:

http://www.928motorsports.com/parts/alternators.php

 

The webpage is ready now - and we can take orders for either model of alternator.

http://www.928motorsports.com/parts/alternators.php

 

I used to have the issue with the voltage dropping considerable when in Dubai at 100F temps at night time, in traffic with everything running, lights, a/c etc etc.

I fixed the cooling shroud, had the alternator rebuilt by Bosch and renewed the front harness.

After this no issues what ever the weather at idle, but my car was mainly stock. I can see the reason for an upgrade if you have added a load of kit or have an old boy however.

 

Anything special about the rebuild that Bosch did for you? I have a relatively new Bosch rebuild, and am in the conceptual stages of a front primary wiring upgrade of some sort. Probably a "next winter" project now that I actually have a winter projects option that doesn't reduce drive time.

 

Doesn't sound like much of a bonus to me

Alan