Chapter 7 Transaxle
1 - Overview, Manual Transmissions
2 - Overview, Automatic Transmissions
3 - Torque Tube Rebuild
4 - Transaxle Calculator (PDF)
4 - Transaxle Calculator (XLS)
5 - Transaxle Applications Table
Porsche 911 Transmission, Transaxle, and Gearbox includes model types 901, 915, 902, 905, 911, 925, 930, G50, G64, G96, G97, A50, A96, and A97. (2) 2 product ratings - Porsche 911 964 C4 (1990-1994) OEM G64 5 Speed Manual Transmission 95,588 miles.
1 - Overview, Manual Transmissions
General description at Darrin's
Maintenance - Porsche recommends replacementof the transmission oil every 30,000miles with the following specification:
924: Approx 2.64 qts hypoid oil SAE 80 or 80W90 GL4
924 Turbo: Approx 2.64 qts hypoid oil SAE 80 GL4
Except 1979 924 and 1980 924 Turbo: Approx 2.64 qts hypoid oil SAE 90 GL5
In other words - buy at least 3 quarts! Preferably 4 to allow for the remainder you can't get out of the bottom. Conventional (dinosaur) oil will do, but synthetics such as Redline, Mobil 1, and Swepco are widely favored and do quite well. The different specifications listed above reflect that the 1979 924 and the 1980 924 Turbo had a different transaxle type, the 'snailshell.' More on this below.
The transmission oil is replaced by removing the lower of the two plugs on the side of the transaxle, draining the oil, and refilling through the top plug hole until the gear lube just starts to come out of or is level with the top hole. The plugs are on the left hand side of the transaxle, and are a 17mm Allen key. This Allen key is readily available either from the online/mail-order parts dealers or VW shops (that deal with air-cooled VW's). Some have had success using bolts with 17mm hex heads. Part of the end of the allen wrench can be chopped off using a bandsaw, hacksaw, or die grinder, so that a 1' section can be used with a standard 17mm wrench. This can make service much easier on some vehicles where space around the exhaust is limited.
In servicing the transmission, it is a good idea to first remove the upper plug when changing fluid for the very first time. Then remove the bottom plug and drain the old fluid. In addition to speeding the flow of old oil out of the trans, it will ensure that you are able to remove the upper plug - which can be hard - before draining your transmission! It's good to know you will be able to put in new fluid before you remove the old stuff! Getting the new fluid into the transaxle can also be very difficult due to the lack of space in which to work. The best thing is to find a hand-pump with attached hose that will fit into the top of your gear lube bottle. Lubri-Matic makes exactly such a small pump, a little plastic one that is like the squirt mechanism in a glass-cleaner bottle, which costs in the neighborhood of $15USD. It greatly eases the process, allowing the entire gear lube bottle to be below the trans but still easily allowing you to pump the fluid in.
Identification
Which transaxle is it, or, are you confused yet? Haynes really confused me. Hint: look at the specifications where the code letters for each type are listed. The code letters are how you can tell exactly which is installed by checking the casting.
There are two basic types of Manual transmission, Audi and Porsche based. The 76-78 cars were delivered with a 4-speed transmission of Audi origin. This transmission is fairly robust and common, but not the fastest for the car.
Then, in '79, a 5-speed trans based on 915 internals (the trans used in late-70's 911's) was introduced. This is referred to the snailshell, because of the pattern of the diff housing (see picture below). However, it is even more easily identified by the shift pattern - 1st is directly below Reverse, with the main H-pattern formed by 2nd-5th. Due perhaps both to the unusual shift pattern and the internal design, these transmissions tend to be in fairly bad shape in most 924's by now, with synchros on 1st and 2nd gear usually gone (identifiable on a test-drive by crunching when shifting into those gears with the car moving). Unfortunately, they were only in the '79
924's, so not all rebuild parts are readily available.
If the synchros are only slightly worn, and the basic gears are OK, the synchro parts can be replaced successfully with 915 parts and the transmission can continue to be used. However, if the box is allowed to continue to be abused, damage to the gear teeth can result. The good news is that these, based on the 915 design, were intended to be replaceable separate of the main gear teeth. Bad news is, they are unique to the 924, and are not available. If they must be replaced, an entire new gear must be fitted, for about $1000 each from Porsche. For this reason, a '79 924 with a bad gearbox is often most economically repaired by swapping in another 924
5-speed or 944 5-speed gearbox. More about that later.
In 1980 an Audi-based 5-speed was used, for the remainder of the life cycle. It looks identical externally to the earlier 4-speed, except that the tail housing is longer. It is fairly reliable and robust, and has a normal shift pattern.
A 3-speed automatic was also available for the 924, and it was of Audi origin as well. More information about caring for this beast below. Fortunately, it tends to be the most robust of the lot, due to the fact that it's very hard to abuse. The only issue here can be the rubber damper that bolts to the back of the flywheel, replacing the flex-plate in a modern automatic. After 20 years, they can crack and break.
The 931 started out in 1980 with a snailshell type 5-speed, but with larger internal shafts to handle the extra power (see picture below). The torque tube also has a larger internal shaft, and the clutch diameter is larger, concurrently. The same reliability and repair issues for the 924 snailshell apply to this gearbox. In 1981, the 931 switched over to a 5-speed Audi-based trans, but with larger shafts again, as with the torque tube shaft. This transmission then was used in the 944, and then the 924S, with slight differences in gearing. Consequently, a 944 trans can be used in a 931 as a direct bolt-in.
Removal and re-installation of the transmission and torque tube are covered well enough in the Haynes manual that they need not be reiterated here.
Transmission Swaps
So, now, how to fix a broken snailshell? Well, there are details of the procedure in the factory manual. If you are lucky enough to have a
salvageable unit, sue the manual. Otherwise, you're looking to swap in a trans, either into a '79 924 or an '80 931. This is somewhat bolt-in, but does require removal of everything behind and below the flywheel, so expect major surgery and plan time.
To swap a 924 80-82 5-speed (Audi-style) into a '79 924, you will need the torque tube and trans and shifter and linkage from the donor car, including the trans mounts, and the crossmember that holds the torsion bars. The latter must be swapped out due to the different mounting scheme of the transmissions (see below). I recommend trying to find a torsion bar housing with rear sway bar mounts, if at all possible, for later use.
If you wish to use the 931- or 944-style trans, you will need all of the above, and you will also need to change your clutch. Actually, you should change your clutch (and pressure plate and throwout bearing) regardless, while you're in there. It is also advisable to check the operation of the throwout bearing on the guide tube, and make sure it is smooth and does not hang up. If it does, the guide tube (which bolts into the bellhousing) should be replaced; it's available through the dealer, relatively inexpensive. If it does not, a fresh swab of grease on the tube is sufficient.
However, to install the 944 type trans with the large shafts, without changing over the flywheel and pressure plate (though that is an option), a different clutch must be used to accommodate the larger shaft of the torque tube. Turns out, the clutch for an 83 Mustang 5-speed works just right; perfect fit on the splines of the torque tube shaft (1'x23), and the right diameter to fit under the stock pressure plate. After installing the new clutch disc, the rest is easily installed.
Porsche Transaxle Strength
To retrofit a 944, 924S, or later 931 trans into an '80 931, all of the above applies, except that the clutch work is not necessary; the shaft size is consistent for all 931's and the 944 and 924S.
Transmission Mountings
The pertinent question is, can the different types be interchanged? In a word, sometimes. The two styles are mounted to the car differently.
This picture is of a 5 speed snailshell and torsion bar carrier from a 1980 931. Note how the transmission sits on top of the arms. 1980 was the last year this transmission was used for US spec cars, and it was only used in the 931 in 1980. This setup is preferred by some who are interested in a track car. The transmission is easier to remove and replace. It is lighter. There are more gearsets available and it is easier to change gearsets than the Audi based unit. Here's a few more pictures of the different torsion bar carriers for the different mountings:
The Audi based transmission hangs on mounts bolted into a beam under the hatch floor. We do know that from 79 on all bodies have provision for these mounts.
The 931s have larger driveshafts and input shafts. The coupler on the left is from an 80 931 with snailshell. The right from an 82 924 with Audi unit.
Shifter Linkage
It is fairly common for all of these transmissions to have problems with sloppy shifters, due to wear in the linkages. This is not surprising, given the length of the linkage. The Audi-based transmissions all share a similar layout, and suffer from wear at both ends. Wear at the shifter is easy to evaluate, just by pulling up the shifter boot. Replacement of bushings or, if wear is extreme, actual linkage parts may be necessary. Wear at the linkage located on the transmission is harder to isolate due to its location. However, there are a number of plastic bushings back there which may wear and need to be replaced. The most popular choice for this is to actually install a short-shift kit, such as sold by Paragon Products, DST Performance, or Tweeks, which have the advantage not only of replacing some or all of the worn components, but also allowing a reduced (and, in some cases, adjustable) shift ratio, resulting in quicker throws and a more enjoyable drive. General consensus on these seems to be that it does not increase the shift effort to an unbearable degree.
The snailshell trans also can suffer from wear in the linkage, but is less likely to require replacement of metal parts. In this case, only bushings must be replaced. However, some of them are less easy to get at than with the Audi design trans.
Easiest to access are the bushings at the shifter; 4 identical bushings accessed simply by pulling up the shift boot. Part number for these is 999 924 002 40. Simply remove the e-clips (2) holding the shift rods to the shift lever, remove the old bushings, and slide in the new bushings.
The bushings back at the transmission, accessible only from underneath the car (reaching around to the top side of the transmission are harder to change. The are part # 911 424 223 03. They are also used on the 914, and can be purchased from more common sources like Performance Products as such (914 shift linkage bushings). In this case, the cross pin holding the joint together (it's designed like a u-joint) must be driven out to one side. Then the joint is separated. A 13mm bolt is removed and the aluminum joint casting can then be removed from the car. The plastic bushings can be replaced by hand at this point.
Porsche 911 Transaxle For Sale
Transmission Input Seal