2.0L Done and Alternator Question

[126coupe]

My 75 amp Alternator does not sit deep enough in my early housing. Do I need to get a pre 1974 alt.??

[Jess]

I had just the opposite problem on my recent rebuild. My alternator sat too deep in the fan housing.

I ended up getting another fan housing with a shallower cutout that the alt. sits in. I've still got the deeper one if you need it.

Also, just to make you aware, the different housings all seem to have the same number cast into the top.

Jess

[304065]

Don't change housings, change alternators.

The later housing has a 930 part number which will cause the scruitineers to give you a hard time. And the bigger alternator weighs more.

Get a 55 amp. What do you need all that power for anyway?

And make sure it's externally regulated. Mount the regulator up on the rear deck, inside the car away from heat, and you will have no problems.

[126coupe]

Thanks John Both my housings (the one in the car on my 2.5) and the one thats going on my 2.0L are 901 part #'s but one digit difference. I ordered an early alt. from SSF and will bring my housing w me to make sure it works in the housing. The size of the earlier alt must be different than the 75 amp style.

[304065]

Hmm, now you got me thinking. . .

Carrera Alternator = 90 amps x 14V = 1260 watts = 1.68 HP
Bosch 55A = 55A x 14V = 770 watts = 1.03 HP
Bosch 35A = 35A x 14v = 490 watts = .65 HP

What does your electrical load look like?

MSD ignition = 5A @ 5000 RPM, 10A@ 10,000 RPM, therefore, 8A
2 Brake lights @21 watts each= 42 watts/14V = 3A when on
Electric Fuel Pump, 10 Amps max
Monster oil pressure light, 3A when on.

So you have a MAX load of 24 Amps. Give that a 50% margin and you've got 36 amps.

The horsepower drain from the earliest alternator is so low I wouldn't even bother trying to cheat by cutting it off at full throttle. Plus, the diodes will last longer.

Warren what do you think, is the induction load sufficient to justify a voltage regulator with a "cliff" at high rpms?

[304065]

So let's suppose you have a 35 amp alternator. How small could we make the wire going to the starter, bearing in mind that it has to carry the load while being as lightweight as possible?

35 amp load
distance from back of alt to starter about four feet (with a little slack)

According to this table: http://www.aeroelectric.com/articles/wiresize.pdf

16 gauge wire has a drop of 4.01 milliohms/ foot
that means .00401 ohms/foot, and you have four feet of it, so that's .01604 ohms of resistance in the wire. You are putting a max of 35 amps through it, so that's a voltage drop of .56140 volts, which means you get 13.43 volts at the starter terminal where the big battery cable starts. That's a voltage drop of 4%, not bad.

Now, Tbitz told us that he recorded an internal engine temp of 46C when driving. From the table in the article, we can expect a 35C rise in temperature with a 12.5A load, bringing the temperature to 81C. Or higher, if you started with a high ambient temperature, like where you race (at least 40C, right?) plus the engine heat. But in any case you are below the temperature limitation of good aviation grade tefzel wire (100c)

That's for a 12.5 amp load- if you are pulling the full 35 amps through there it's going to get hotter. I would think that unless you drain the battery down all the way, most of the time you are going to be using 10A for the ignition system, a couple amps for the senders, and occaionally 3A for the brake lights.

Now, what does it weigh?

Copper wire, 16 gauge, .00995 pounds/ foot x 4 feet =.039 pounds
versus 2 gauge, .239 pounds/foot x 4 feet = .956 pounds

Weight savings= .917 pounds for that piece.

Now, suppose you wanted to be conservative and use 10 gauge wire, it's a 1% voltage drop and still save .81 pounds!

Man, I love this stuff!

[126coupe]

Wow that dialogue is way over my head, well picked up the early alt. and my problem is solved! early alt the housing is 70mm wide, the later style is 77mm wide

[304065]

All's well that ends well. Good luck!

[911pcars]

For a little more power savings, substitute LED light bulbs in place of the incandescents. Maybe there's an LED array you can use in the monster light too.

You can probably go a little lighter on the wire gauge as well.

Sherwood

[Henry Schmidt]

In all our early 911 / 914-6 engine applications we use the Valeo 50 amp alternator. The benefits are four fold.
1) They fit the early fan housing
2) they are 50 amps not 35 as original
3) they have an internal regulator ( much simpler wiring, less chance for failure)
4) the cost of a new one is less than rebuilt and new has to be more reliable.

We can modify early fan housings to fit late alternators.

[304065]

Henry,

What has your experience been with the internal regulator in race cars?

I have an internally reg'd Carrera alternator with an internal regualtor. On hot days (last year's PCA Club Race at Gingerman) the internal regulator was overheating and shutting down, causing the warning lamp to come on.

Unfortunately I wasn't able to distinguish between the VR overheating and the belt breaking, which meant I had to immediately pit and check the belt. Pretty annoying.

Have you ever run into that?

[126coupe]

I have had an internal regulator in my racecar for over 5 years.....No failures

[Henry Schmidt]

What you describe sounds like faulty or worn brushes on the regulator. I've seen them float at high RPM ( causing the light to come on) when they get some time on them.
One part that belongs in your spare parts box is a new regulator whether you have internal or external regulator.
As for being reliable, we have no real issues with either style regulator.

It has been my experience that when you break a belt your oil temp will rise very quickly so when the light comes on watch the temp. If it does go up, finish your race and check the charging system after.
PS: When your belt breaks. if you are in tune with your engines' performance, you should also notice a significant increase in horse power.

[304065]

Thanks for the advice guys. It may be easier to keep my current setup.

[Lorenfb]

"As for being reliable, we have no real issues with either style regulator."

Obviously, you haven't sampled many alternator failures over many years
(20 years) and thus have concluded incorrectly that the failure rate is the
same.

Even if you haven't done any sampling, you can conclude by simple logic that the
failure rate is going to be higher since the internal regulator is running at a much
higher temperature. Since electronic failures are directly correlated with temperature,
the internal regulated alternator, especially in a 911, will have a higher failure rate.

Avoid internal regulated alternators! If I had the extra time, I'd remove the
internal regulator on my 3.2 and use an external.

[Henry Schmidt]

Originally posted by Lorenfb

Obviously, you haven't sampled many alternator failures over many years
(20 years) and thus have concluded incorrectly that the failure rate is the
same.

Wow, I guess that my 26 years of Porsche experience is not adequate to voice an opinion.
But I'll voice it anyway.
I just got off the phone with my alternator rebuilder and he will second my opinion. There is no greater failure rate in regulators from internal to external.
He did state that early externals had moving parts ( lever and points) that are susceptible to vibration and internal regulators are transistorized. There are however external regulator replacements that are transistorized. Moving parts wear out. Your theory about heat is a good one but does not seem to translate into quantitative evidence as to a greater rate of failure.
BTW: all modern Porsche race cars, Nascar Cup cars, IRL, Cart, FI and every modern day Rally car run single wire internal regulator alternators as do every modem day car street car I've seen.

[Lorenfb]

"Your theory about heat is a good one but does not seem to translate into quantitative evidence as to a greater rate of failure.
BTW: all modern Porsche race cars, Nascar Cup cars, IRL, Cart, FI and every modern day Rally car run single wire internal regulator alternators as do every modem day car street car I've seen."

Those applications DON'T have the regulator setting right next to a 180 to 200
degree F air cooled cylinder head. I doubt that you've done any real failure
analysis on Porsche '82 - '83 SCs or '84 thru '89 3.2s alternators nor has your
rebuilder by his answer given.

[Henry Schmidt]

Well Loren I'll bite, what is your expertise? I'm certain your not going to say "well I own one".

[304065]

LOL, the ol' hornet's nest just got broken open.

Henry, do a quick search of the archives and you'll find Loren's strongly held opinions about matters electrical, particularly with regard to DME chips.

He has a lot to contribute, and unfortunately, often fails to temper that enthusiasm with tact. Loren, is that a fair characterization?

Now let's change the subject: have either of you electrical gurus ever experimented with cutting out the alternator field at WOT?

[B D]

Here we go again!