my 84 did that, but never knew about the redlight at the voltage gauge secret. i would have to rev my car to 2k to get the dash lights to turn out. if a shop didnt know about this, they would kill the battery doing engine running tests.

Mk

 

MK
Did you ever get any video of the race......I wanna see the "bump"

 

Not realy relevant to this discussion but Tony Lapine mentioned at the 25th Anniversary Euro 928 meet that the S4/GT rev limiter was dictated by the rpm that the alternator could sustain.

They had to make the 928 to sit at idle in a very cold climate with all the lights on and the heated rear window / mirrors and the AC and everything else you can think of, yet not disharge the battery.

 

Jon I can attest that summer in Phoenix - both cooling fans on full virtually continuously, AC on, blower on full, headlight + audio is much worse than those winter conditions for loading - and I'm not sure they actually fully allowed for it... !!

Interesting note on the alternator rev limitation... I'm only surprised porsche didn't come up with some variable speed gizmo to "fix" that.

Alan

 

I presume that you don't have the heated rear window on in AZ ?

And the internal fan takes no more current hot or cold ?

 

the rev limit shouldnt be effected by the alternator. the car draws near 10amps just to run it. yes you have a chance of a weaker spark as the voltage goes down, and subsequently a miss at high rpms, but that only happens at under 11 -11.5 volts. i proved that you dont even need an alternator if your battery is good! there is not much more current needed for running the engine at 6500 rpm vs idle. its probably only 1 amp diff or so. (ie 10amps vs 11 amps at redline)

as long as the alternator is charging the battery (ie producing more than 12.5 volts)and usually this is easy to do for a 928 altnerator rated at over 100 amps, the battery will not be discharged. the voltage drop is what you see when you start turning on things in the car, like lights, fans, etc. if the total current is not over 50amps, the voltage presssure will stay high. in fact, in my BMW with the eRAM turned on which is 60 amps, the lights will dim, but the voltage still stays at 13.5 dropping to 13 and coming back to 13.5 when the 60amps is drawn.

if you dont see 13.5 to 14 volts on your battery when running, you have an issue with your charging system.

mk

 

fans are between 13 and 15 amps a piece hot or cold. actually, the alternator and other motorized components when cold have less winding resistance, which can make them work a little better and draw more current, with slightly less of the power going up in heat in those devices. ever heard of superconductors? copper kept at liquid Nitrogen temps to reduce resistance to 0. more efficient.

mk

 

Mark I think Jon's point was that part of the reason the gearing to the alt is low @ about 2x (=> problems at idle) is that the alt can't take much over 13K rpm - presumably its a physical thing - it probably would just self destruct as the rotor windings or poles seperate or deform and crash into things... maybe something like that?

Jon the cooling fans are the big hit item at about 50A + AC clutch - heating also doesn't typically need as much airflow for extended periods for effectivenees as AC where its not just transmission of heat from the heat exchanger but airflow to your skin that cools you. Like with a ceiling fan.

I've driven in both extremes - my car electrics are far happier in the cold.

Mostly I'm happier in the hot (hence where I live!)

Alan

 

I hope this doesn't mean you're seperating. Think of the kids.