mark kibort

dont know what an "upgraded" engine might effect, but my 6.5 liter is perfectly cooled, on street driving in 110 temps with two electric fans... by the way, these are stock S4 fans. ..... same effect with the after market fans of the 18" variety, with a pusher in front of the radiator as well. both are 10amps each.
i think the stock S4 fans were 10amps each as well... maybe im wrong there, but its the stock stuff, so its a known quantity. so, if we are debating the current draw, fine, either way, the stock stuff does it. it doesnt push my little 13lb battery at all, and the alternator (stock GT) works fine for the draw of the fans and the current the engine requires, plus a 500wattt stereo blasting away.

nahhh, there is something wrong with that alternator. 800rpm should still keep the voltage at 13+ with the current draw you are talking about. 10.6 is more like a 300-500amp load.... wanna bet?? just put a meter on your starter motor and measure current draw... or easier, look at a volt meter when you start your car. the battery will easily keep voltage above 12.6 Nominal with a normal alternator running and systems draws of over 60amps. i have video of this happening. 13.7volts with a 60amp draw. (at idle)

alan, ive run for many years in the 100 to 110 temps. ac working well, stereo blasting, lights on, etc. no isssues with voltage decaying down past 12.6 volts.
if you do, you have an alternator issue.

Lets talk about voltage drop, im talkiing about the alternator response to "voltage drop".... the alternator puts out more an more voltage,which produces current as it sees the voltage drop of the battery. that's how it works. it tries to keep the voltage at a set level of 13.5-14volts. if it cant sport 13.5volts, that's when the voltage starts to decay and this means you are above its power capabilities and thus its current limits.
all you need to do is find a good running 928, turn on the works and let me know what your fluke meter shows.... the stock alternator will work fine at high temps and will not go below 12.6 volts.

j.kenzie@sbcglobal.net

Hi Mark,
I always enjoy your information. However, I don't think your fans are 10 amps. On my car, cooling is a challenge because I have a large supercharger right behind the radiator, blocking air flow and taking the space of a puller fan, factory or otherwise. Your voltmeter is disabled while the starter is running, so I can't tell what my system voltage is, probably 12.4 for a while. Anyway the alternator plays no role while the starter is running. The alternator does not put out its full rated output at idle; it makes maybe 30 amps, even yours. If your load exceeds that, the battery supplies some. The battery will not do this for long. I don't know if you have expressways with rush hour traffic jams near you, but for me, when I'm stuck in one, there is no way my voltmeter stays above 13 volts. This is the fourth alternator I have had on my car, and none of them delivered good voltage when heatsoaked at idle. I am really glad you have had such good success, but I have not. Until lately, I have finally been doing better.
Thanks,
Dave

Carl Fausett

Be sure your battery is in excellent condition. An older battery will have resistance to charging and that appears as low alternator output when testing. It is for this reason that many alternators come with a specification on their warranty that requires the battery be replaced when the alternator is replaced. It prevents false warranty claims.

An FYI: a lot of 928's don't get driven very much. My 91 is one of those, and I can put a brand new battery in it and that battery will be garbage in 3 seasons because the car sits so much. The lead plates just sulfate until he battery will not take a charge at all.

Better: Get rid of the wet lead-acid battery and get an Optima Red. Because it is a gel-cell and not wet, (and because of a few other technological advancements) the Optima's are unaffected by sitting for long periods and still work great. The first Optima red that was in my race car is now in our fork lift and still going strong - probably 12 years old now. They aren't cheap in the short term (about $150), but less expensive in the long run.

You do not need a big one. The 24R works fine for me.

I don't sell batteries - not selling anything here - just offering advice....

j.kenzie@sbcglobal.net

My after market alternator is not compatible with the stock cooling shroud. It has internal fans rather than the one external fan of the stock unit. The stock unit is really very specially designed, but only 90 amps. I tried two of them with their stock cooling duct and shroud.
Dave

dr bob

Carl is spot-on with the need to look at the batery at the same time the alternator is replaced. Most alternator failures, and certainly many of our 'undercapacity' symptoms are the direct result of tired batteries. A good battery will not allow as much voltage to maintain charge current, or conversely will need more current for a longer time to recover after a casual discharge. Low voltmeter readings at idle are a symptom of a battery that's sucking current. The battery can't tell that the car is at idle, it just asks constantly for more current. At idle when current capability is reduced, all that current sent to a tired battery pulls the whole system voltage low. Then there's the deal with resistance between the alternator and battery that keeps the battery from ever reaching full charge; the battery can't support voltage needs at idle when the alternator is overworked and underfed. Hence the directive to replace the battery ground strap and the engine ground strap as early steps in the fight against the low-idle-voltage syndrome.

The Group 24 batteries have the terminals on top of the casing, rather than recessed slightly as the spec'd Group 48 that my can came with. The Group 48 batteries also have the correct "lips" at the bottom ends to fit in the factory retaining tabe and to use the factory hold-down. So if you decide to fit the Optima as Carl suggests, remember to get a standard 'murikan car hold-down bracket, the J bolts to go with it, and some adequate urethane pads to keep the positive terminal insulated from the battery box lid.

The Optima batteries don't seem to outgas the way wet-cell lead-acid batteries do, so you could forego the external vent hoses that fit the Group 48.

--------------------

My new-to-me this year CTEK battery charger/maintainer has a dedicated charging profile for Optima and other gelled-electrolyte batteries. IIRC it has a different peak-voltage profile.

dr bob

[...]

WallyP

An interesting thread. I do have one question...

Every "standard" automotive alternator fan that I remember seeing is basically a centrifugal type. They blow air outwards from the fan after pulling it from the rear of the alternator. Do the alternators that were discussed as blowing in from the ends and out in the center use axial-flow fans? If not, how does a centrifugal fan blow inward?

j.kenzie@sbcglobal.net

Hi Wally,
My hose directs cool air to the rear center of the alternator, so the centrifugal fan can blow it outward.
Dave

mark kibort

I'll have to check the fans.. I measured them a while ago for proper fusing, and lets say they are 15amps each... still that's 30 amps and the stock it takes to have the engine run. never had the voltage drop below 13.5. alternator has no problem producing that net 30, or 40amps including what again, I don't know what alternators you have been running, but this seems to be the constant for the stock 928 alternator if it is good shape.
certainly you might have more challenge because of the blocking air inlet due to the supercharger, but that would only have the fans run longer on a hot day. certainly they might be a higher rating fan than the stock 928 S4 fans. you can easily put a volt meter on the battery during starting.... it very accurately measures the voltage drop, which pertains to a current flow from the battery. depending on the battery, the voltage drop will vary. generally, 400 to 500 amps will drop a battery in good shape to below 10volts. 60amps, on a health battery wont make much of a voltage hit to the battery. nominal battery voltage of 12.5 might go down to 12.0volts..... with the alternaltor running, it shouldn't go down much more than 14 to 13.5v.

j.kenzie@sbcglobal.net

Thanks, Mark,
I completely agree with you. However, a 60 amp load will eventually lead to reduced voltage if the alternator is only making 50 amps, given enough time. Say 30 minutes in traffic at idle. I could be wrong. That is just the way my car seems to have worked.
Dave

WallyP

"Hi Wally,
My hose directs cool air to the rear center of the alternator, so the centrifugal fan can blow it outward.
Dave"

So does every other one that I have ever seen, thus my question...

j.kenzie@sbcglobal.net

Sorry, Wally,
I guess I did not understand your question. My alternator is an open cage design with fans at each end. They suck in from the end and blow out the edge of the case. I am unable to make a tight fitting shroud, so I direct forced cool air onto the rear face of the alternator where the diodes and voltage regulator reside. It is not as elegant as the factory cooling setup, but it does seem to help.
Dave

Alan

It sucks inward from the vent - to the center then blows it outward inside the case to exit the vents in the middle of the case. There is always an in & an out they are just reversed here. There are lots of dual internal fan styles with central exhausts.

Alan

WallyP

OK - I have never looked at one of that type of alternator. So, if I understand what you are saying, the centrifugal fans are open to outside air in the center of the fan, and blow outward inside the closed end of the case, thru the case and outside thru vents at the middle of the case.

Of course, if you didn't have a source of cool outside air for the fans, you would still be using 200 - 250 degree air (heated by the AC, radiator and exhaust manifolds) to "cool" the alternator...

Alan

Yes exactly.