Unusual transmission number mystery

[javadog]

I have a 1977 Carrera 3.0 that I have been restoring for the last several years and I have run across something I can't figure out. I was looking at the transmission numbers the other day and they don't jibe with any I have ever seen before. A couple of the numbers are illegible but those that can be seen are:

92_ /02 506_128

The transmission is a mag-cased 915. It has the auxiliary spring, although I don't think it is original to that case, as the dowel in the case that normally is used to hold one end of the small spring attached to the positioning lever is not present, and the end of the spring is just hooked over the rib on the case that has the identifying numbers.

The only type number I am familiar with that starts with 92_ would be a sportomatic, which this is clearly not. Also, the serial number should start with a 7, not a 5, which I have never seen before. The rest of the digits in the serial number don't add up either.

The car was originally delivered to a customer in Germany, went through several owners and then was imported to the US in 1983. It had three or four owners in California, then made its way eventually to my neck of the woods. According to the last of the California owners, it had a shorter ring and pinion installed along the way, which I took to mean a 7:31, since that is what it feels like. No records of the work though. None of the documents I have, from the original owner's manual through the DOT paperwork and subsequent servce records, has any record of the transmission number.

Anybody up for a challenge?

JR

[Bill Verburg]

923/02 w/ #506 0001 and up was a '76 912E 4spd

925/02 w/ # 764 0001 and up was a '74 4spd
925/02 w/ # 765 2001 and up was a '75 4spd
925/02 w/ # 764 0001 and up was a '74 4spd
925/02 w/ # 765 2001 and up was a '75 4spd

[javadog]

Bill,

Good thinking. I had forgotten about the '76 912. The other numbers you posted are all sportomatic numbers, which we can rule out. So, perhaps they used a case from a 912 and rebuilt the thing. It's definitely got 5 gear sets. I haven't opened it up or tried to work out the individual ratios yet.

That was quick.

Thanks,
JR

[Bill Verburg]

yeah, I only included them because they are the only other 92x/02 trans #s

[Grady Clay]

OK, what were the gear ratios in a 923/02? R&P also.

Best,
Grady

[javadog]

Grady,

I don't have all of my reference books here at the house but I have done a little digging. I have some listings for a 923/02 and a 923/05. If I remember correctly, a 1976 912E had a five speed gearbox as standard, so that would rule out the existence of a four speed, since only the US got the 912E that year. PET lists it as a 923/02 in one place and refers to it as a 923/05 in another. Other references list the 923/02, again as a five speed, not a 4 speed. So, my guess at this point is they swapped in this transmission for some reason, in it's entirety, rather than using the case from the 912 and the original internals from the correct gearbox.

I think the gearing is very similar to a 72-73 era 915, with the exception of a slightly shorter 4th gear. Here are the ratios I think it will have:

1 11:35
2 18:33
3 23:29
4 26:25
5 29:21
F 7:31

Thanks,
Jeff

[Bill Verburg]

923/05 was the '76 912E 5 spd

11/35
13/33
23/29
26/25
29/21
7:31cwp

[javadog]

Bill,

I'm thinking Porsche has an error in some of it's literature. I don't think there ever was a '76 912E four speed. I think the five speed was actually 923/02.

JR

[Bill Verburg]

It wouldn't be the first error in their docs

[javadog]

Bill,

Do you have time to create a couple gear charts for me? I don't have the Carquip software and have to do it the old-fashioned way. I'll be using 2 sets of tires. For street, they'll have a diameter of 25.2; for autocross, I'll use some with a diameter of 22.8. I'll use a 6800rpm redline.

Thanks,
Jeff

[Bill Verburg]

Another mistake, 2nd can't be 13/33 so I guessed 18/33 like the other trans from that era
street
3400rpm in 5 = 73mph
2000rpm in 5 = 43mph

[Bill Verburg]

A/X

[javadog]

Bill,

18:33 is what I had too. I think it's correct. The first chart you did, you have a typo; 23.2 as the tire diameter instead of 25.2, the actual size I'll use.

Thanks for taking the time to do the gear charts. That's a handy program to have.

Thanks again,
JR

[Bill Verburg]

here it is w/ 25.2

[javadog]

Bill,

Thanks again. I owe you one.

JR

[Grady Clay]

Thanks guys.
OK, what is the difference between a 923/02 and a 923/05?
Was there a 4-speed?

For the uninitiated, the big difference between a 923 and a 915 is the input shaft clutch splines.
The 915 (20 splines) are of the SAE form. The 923 is the same at 356, 901 and 914 (24 splines) and the same as early VW.

Best,
Grady

[javadog]

Okay,

Now that I assume I have a 912E gearbox in the car, a question remains. There are part numbers for gears and ring/pinion sets used in the 915 and 923 gearboxes that have a mix of 915 and 923 prefixes. Some 915 gearboxes used some 923-prefix gears along with the usual 915- gears and a 915- R&P; the 923/02 used a mix of 915- and 923- gears, with a ring and pinion having a 923- part number. Any idea what the difference might be between 915- and 923- gears of the same ratio? How about the 915- and 923- 7:31 ring and pinion sets?

Would Porsche have used parts with a lower strength in the 923 gearbox, in light of the lesser torque output of the 4 cylinder 912 engine?

JR

[javadog]

Originally Posted by Grady Clay

Thanks guys.
OK, what is the difference between a 923/02 and a 923/05?
Was there a 4-speed?

Best,
Grady

Grady,

I don't think there was a 4 speed. Perhaps one was planned, and not used.

I don't have any spec books for a 912; all my stuff is 911 or 914 related, so the 923 designation went right past me when I first thought about it. Since Bill identified the 76 912E as having this gearbox, I looked through PET and in the parts section, they list the 923/02 as a five speed. Thus, part numbers for all five gears are shown.

In the transmission number section, they list a 923/05 five speed for the '76 912E, but there is no reference to a 923/02. The serial number range also matches what I have. Looking through the press introductions for the 912E and period test reports, some have the type number listed (as 923/02) and all have the five speed as standard. Since there wasn't a ROW version of the car, and it was a one year wonder, I have to conclude that they just have a typo in some of their material. Most of the evidence points to the same conclusion, only a 923/02 five speed.

Thanks,
Jeff

[javadog]

Originally Posted by Grady Clay

For the uninitiated, the big difference between a 923 and a 915 is the input shaft clutch splines.
The 915 (20 splines) are of the SAE form. The 923 is the same at 356, 901 and 914 (24 splines) and the same as early VW.

Best,
Grady

That's interesting. In my notes from the previous owner, listing the modifications made to the car in the past, he lists the clutch as "RS clutch and plate."

That doesn't seem very likely now.....

I really need to track down some of the prior owners from California.

JR

[Grady Clay]

JR,

To fit that 915-compatable "RS clutch and plate" the 923 transmission would need a 915 input shaft. Easy to do with the transmission apart.

I'm assuming that a '77 Carrera 3.0 engine has a 6-bolt crankshaft.
An alternative is to use the small 215 mm clutch and flywheel from the '69 and earlier but the clutch release bearing and linkage won't work as original. Another alternative is the '70-'71 225 mm 24-spline disc in the '77 3.0 pull-type clutch regardless of the number of crankshaft-to-flywheel bolts The second would be a suitable solution.

Best,
Grady