924Board.org

[keith]

Hey, does anyone know offhand the gearing differences between the 1980 931 snailshell (g31/02 I think?) and other available boxes? By available, I mean any that could be used for this application (in my 924S that will be naturally aspirated for a while, then turbocharged)

I've heard that 915 gears can be used in the 931, which seems to be desireable due to the availability of geariong options for the 915...

And I would think the 915 stuff is tough enough to deal with the projected horsepower...

BTW, what box did the factory run in the 924 Carrera GTs?
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keith
86/9 951
the RUNT.

[924 turbo]

http://www.924.org/techsection/technical.htm

Click on transaxle, then on either the PDF or the XLS transaxle calculator.

Compared to the 931/944 audi box, the gear ratios are almost identical.

The Carrera GT used a strengthened version of the G31 (G31.03).

Just my opinion, but you'd be better off finding a 951 transaxle.
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Jon Furst

[keith]

better off, why?
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keith
86/9 951
the RUNT.

[keith]

better off, why?
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keith
86/9 951
the RUNT.

[924 turbo]

Just my opinion, but the Audi-type transaxle is easier to find, cheaper to fix, and probably strong enough for your application. The G31 is quite rare (about 4000 cars delivered to the US) versus the Audi-type boxes which number over 50,000. The G31 has notoriously weak synchros, a problem which the early 928 suffers from as well. The G31 is a good transaxle, but I just think the benefits (supposedly able to withstand more torque) outweigh the costs (harder to find, more expensive to fix, weaker synchros). Of course, to use the G31, you'll also need a torque tube from a G31 car, plus a torsion tube carrier from a G31 car, plus the shift linkage from a G31 car, etc. YMMV!
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Jon Furst

[keith]

Jon - thanks so much for your reply. I've been advised not to use the snail on many occasions but have never been given any real reasons not to do so, aside from the old-design synchros.

I work in a Porsche shop, so I can get guidance with the synchro replacement until I am comfortable with rebuilding this tranny.

Aside from that, I own a 1980 931 WITH LSD, which is the donor for this conversion. I've already pulled the torque tube, snailshell, rear torsion housing and suspension, and the rearmost section of the exhaust as an assembly to install in my 924S.

I've decided that I want to run late model aluminum trailing arms in this application, so I've got some work to do on the donor assembly before it goes into the car. I'm probably going to replace some tranny seals also.

So anyhow, I have all the stuff ready, and an avenue to maintain this thing, so if there aren't any known issues aside from the synchros, then all systems are go!

This will eventually be used with a 951 motor, so I guess we'll see if it holds up, huh?

BTW, anyone know what modifications were made to the G31/03 box?
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keith
86/9 951
the RUNT.

[CBass]

IMO, teh LSD G31 is the most desirable transmission to have in a 924. It's a stronger transmission, being based almost entirely on the Porsche 915 transmission from the 911.

It's lighter, and mounts further forward than the Audi box, making for a slightly better handling car. Well, initially this will make for a more frontheavy car, but with battery relocation, a fiberglass hood etc, it will make for a lighter car with a better polar moment of inertia.
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'81 931 in various states of assembly

[keith]

hmm - interesting!
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keith
86/9 951
the RUNT.

[CBass]

Of course, premature synchro wear is a factor, and it does have a funky shift pattern... The Audi box has advantages as a daily driver for that, but for a performance oriented car, the G31 has clearcut advantages.

While both trannies have about the same final drive in 5th, the G31 has shorter gears all teh way up, making for a little better acceleration.

http://www.924.org/techsection/PorscheTrans1.PDF
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'81 931 in various states of assembly

[924 turbo]

[924 turbo]

[CBass]

I was comparing to the Audi box, they have a nearly identical 5th gear ratio, but from 1st to 4th, the G31 has slightly shorter ratios.

Compared to the 951 box, there is a huge difference.

I'm a G31 fan, in case you couldn't tell
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'81 931 in various states of assembly

[-nick]

Give the folks at PowerhausII a call. They had a G31/03 box in a while ago and said it was strikingly different than the G31 incarnations.

I'm not sure what the Euro ratios are for the G31 (the G31/01 box I think).

If you've got the tranny out then _at least_ open up the front case and look at the 1st gear syncro. I would recommend pulling it apart and replacing 1-2-3 syncros while it's out. Especially if you don't know the history. There's a great writeup on the procedure in the 931 section here.

Good luck. The LSD in these box's is supposedly from a 930- however the actual part number differs by a digit at the end. They are all 40% lock I believe.

-nick

[keith]

[CBass]

Interesting, I'd like to get my hands on one and see what the differences are.

Keith, here is the article on rebuilding the transmission, or at least 1 part of it.

http://www.924board.org/viewtopic.php?t=7969
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'81 931 in various states of assembly