Which new alternator should I get?

[Jameel]

'79 911SC

Alternator died yesterday, and I have a local mechanic who is a whiz at rebuilding, but he says the stator is dead, and new stators are not available. This is the SEV Marchal with external regulator. According to my mechanic the SEV Marchal is still available new, but I'm guessing made by someone else who bought the SEV Marchal name.

Is this a case where the consensus is that's its better to find a good used stator and rebuild the original unit? Or are current new units good quality? And if so which brand?

[Dmitry at Pelican Parts]

Hello Jameel - our customers seem to like the Valeo alternator which is an OEM supplier. If you prefer to get a rebuilt Bosch unit which is also a great quality unit; those are available as well. Please let us know if you have any other questions and we'll be happy to help out.

1979 Porsche 911 SC Coupe - Charging System & Battery - Page 1

[pete3799]

I went with the Valeo......no complaints.

[Jameel]

Ok. Thanks. So I did some reading. Looks like the Tech doc had us upgrade to the internal regulator version. Anyone have any pics/method of the air duct modification?

[pmax]

If I were getting a new alternator, I would seriously consider upgrading to the high output version.

Who makes that 90W alternator on the PP page ?

[xbmwguy]

You should definitely look into this one. Obviously it depends on what your electrical demands are but in my case it has been fantastic. Stereo amps,twin plug COP, 3 ac fans etc. 160 amp. 2 years and going strong with a price that is normal even for a P CAR

[Jameel]

Thanks for the recommendations. This is just a stock SC, no A/C, no hi power stereo, nothing fancy. Seems like the standard alt will be fine.

Anybody used their earlier air duct with this update?

[Jonny H]

^ Smells fishy. There is absolutely NO WAY that an alternator based on the standard case can sustain an output of more than around 90A for a prolonged period. I am not disputing that the output can peak at 160A, just that it cannot sustain that output in the real world. How do I know? Well...

We have improved the windings in the standard case to increase power output. Once you get above 90A continuous current, the heat generated by the alternator cannot be cooled by the flow of air through it, especially if the engine fan speed is low. The main reason is that the standard alternator case has no internal fan and secondly, the alternator is encased by the engine fan, blocking air flow.

Most alternator manufacturers test alternators on machine using a pulley. They run the machine and record the max output in ambient temperature. For the 911, this test is fundamentally flawed UNLESS you encase the alternator in the fan and housing. Bizarrely, the engine fan does not really help cool the alternator, it actually blocks the cooling holes.

Anything above 90A continuous, the alternator will get so hot it will melt the insulation on the windings and short them out. Several alternators were harmed in these experiments.

We are working on a completely new 6 phase alternator with internal fans that should be able to offer 120A continuous and 150A peak. We currently offer a 110A peak version which is the highest you can safely go in the standard casing.

[seafeye]

Is there a difference in size of the alternators? I had to use spacers in the back of mine to get the pulley to line up correctly.

[mysocal911]

Originally Posted by Jonny H

^ Smells fishy. There is absolutely NO WAY that an alternator based on the standard case can sustain an output of more than around 90A for a prolonged period. I am not disputing that the output can peak at 160A, just that it cannot sustain that output in the real world. How do I know? Well...

We have improved the windings in the standard case to increase power output. Once you get above 90A continuous current, the heat generated by the alternator cannot be cooled by the flow of air through it, especially if the engine fan speed is low. The main reason is that the standard alternator case has no internal fan and secondly, the alternator is encased by the engine fan, blocking air flow.

Anything above 90A continuous, the alternator will get so hot it will melt the insulation on the windings and short them out. Several alternators were harmed in these experiments.

Yes, it's as simple as that. Totally agree!

1. The number of winding turns and wire size used in the stator determines max output.
That then determines the required physical size of the alt.
2. The diode rating (amps) and the size of the diode mounting plates determine the max current output.
3. The rotor windings (rotor current) also affects the max output.

The above constrains the overall physical size of any alternator design.

[Jonny H]

Originally Posted by seafeye

Is there a difference in size of the alternators? I had to use spacers in the back of mine to get the pulley to line up correctly.

Yes, there are 3 different lengths. Over the years, the fan housing was machined deeper to accommodate the longer alternators.

Broadly speaking, the long hood cars take the short alternator and the Carrera 3.2 has the long alternator. Everything else is the middle size.

[mysocal911]

Originally Posted by Jonny H

Yes, there are 3 different lengths. Over the years, the fan housing was machined deeper to accommodate the longer alternators.

Broadly speaking, the long hood cars take the short alternator and the Carrera 3.2 has the long alternator. Everything else is the middle size.

The 911 alternators used were:

Bosch - 35 amps, SEV Marchal - 55 amps, SEV Marchal - 70 amps (long), Paris-Rhone - 90 amps (longer)

All used 6mm bolts, except the Paris-Rhone - 5mm.

[Flat6pac]

The only one he has to consider is the verlo 81 style for the proper depth for his fan housing.
Bruce