Porsche transaxles are available from several different Porsche cars, including (mostly) the iconic 911 series. As one might expect, the later, faster cars are the prime candidates, with GT2 and GT3 transaxles the closest to being the ideal transaxle from this storied company.
There are several series of interest, starting with the 930 transaxle, found in early turbocharged 911s. These are quite strong, but are only 4-speeds, which makes them less popular than the next series, the G50s, which began their run in 1987,
Of the G50 series, the strongest is the G50/52 from the 1989 911 turbo. These are rare, and priced accordingly.
Recently, the Boxster and Cayman transaxles have found some favor for the lower-powered V8 cars, as they are inexpensive (for a Porsche transaxle) and readily available. More about these options can be found at [http://www.californiamotorsports.net/987.htm 987] also on the CMS site.
The latest production car source is from the 997 turbo, which is the g97/50, a transaxle that has seen yeoman duty in the SL-C 01 race car.
Very rare, the GT2 Porsches sported a very strong transaxle with pretty good gearing for an SL-C with a V8. These are probably the strongest late Porsche transaxles.
These transaxles are widely available from local boneyards, specialty Porsche-specific wrecking yards, several different rebuilder shops including [http://www.gboxweb.com/ Gbox], [http://www.californiamotorsports.net/ California Motorsports], and others.
They can also be purchased new, or remanufactured, in some cases, directly from Porsche or one their parts resellers. For example, new/reman boxes from the Boxster/Cayman series are available from Porsche as well as others.
The transaxles generally have a wide aftermarket that can provide solutions to known problems.
The G50 series have 5 forward speeds, and some come with a factory-installed limited slip. A good source of data about these is at [http://www.californiamotorsports.net/G50%205%20speed.htm CMS] in their G50 section. The same site has a lot of detailed info about all of the Porsche transaxles- check it out if you want to know details about gearing, syncros, etc.
Later turbo cars tend to be AWD, so there is a conversion issue, but they can be used in our cars with small modifications. These can be found in the 996 series turbo cars.
Aftermarket limited slips are available from Guard, OS Giken and others.
The G50-52 is a variant that was available domestically only in the 1989 911 turbo, and is the strongest of the G50 series, but has been somewhat eclipsed by the newer G97 series.
The strongest street-car based transaxles are thought to be the later G97/50 transaxles as provided in the 997 series turbos. These need to be converted from AWD to 2WD. More detail on these can be found at [http://www.californiamotorsports.net/996T%20GT2%20GT3.htm CMS]. The GT2 had a very strong transaxle, but these are quite rare and are hard to find. There is at least one SL-C with one of these special units, though.
All of the transaxles from the 911 series need to be flipped and run upside down, as the original usage was a rear engined car. There are kits for this, and generally these units seem to tolerate this use. The transaxles from the Cayman and Boxster series do not need to be flipped as they are already in the mid-engine configuration, however, the axle angularity and relatively low power ratings of these transaxles make them an unpopuloar choice, and there are no SL-Cs running one of these units as far as we know (let us know if you have used one these units in your SL-C).
The early transaxles have a tendency to break when abused with high torque and sticky tires, or when used to make drag-style starts. Some have reported that even with high power, these units can be made to last if they are not speed shifted, or shock-loaded with hard starts.
Most of the problems have solutions, though they can be expensive. Many people have spent between $10,000 and $15,000 making these last.
The gearing from the factory is designed for a relatively low-torque 6-cylinder engine, not a V8. Consequently, many builders change the ratios, especially the higher gears as the stock gearing normally has the engine revving way too high at highway speeds.
Many different starters are available, both Porsche and aftermarket. Clutches, and flywheels are also easy to source, and can be targeted to many different levels of power.
Long Tail Transaxles
Some G50 transaxles were built with a carrier mount on the tail case that looks like a rubber donut. These "Long Tail" Transaxles will not fit in many mid-engine kit cars. Here are instructions for how to shorten 5 speed and 6 speed transaxles. Porsche G50 Tail case Shortening Instructions.pdf
Installing the G50 in an SL-C is pretty easy. Follow the directions below.
Assuming you have the right adapter plate from Superlite for the G50 and your selected engine, you'll probably want to begin by fabricating studs to attach the transaxle to the adapter plate as shown below:
lTap adapter plate with 10mm x 1.50 thread
lCut high tensile (HT) bolts and thread to create long studs:
3 ea. M10 x 1.50 x 120mm Stud HT
1 ea. M10 x 1.50 x 158mm Stud HT
Note: Stock Porsche studs have a 1.25mm fine thread pitch and will not work.
You will also need 4 ea. HT nuts and 4 ea. flat washers [ASW1] .
The transaxle can be fitted with the engine in or out of the car. It is usually easier to fit the transaxle with the engine installed and secured by the engine mounts. The sequence below describes installing the transaxle to the engine already in the car.
If you haven't done so, see engine installation section for details of installing the engine into chassis.
Proceed as follows:
- Install the pilot bearing into the rear of crankshaft. Lube needle roller bearing with Moly grease.
- Install the above studs into the adapter plate using Loctite Studlock.[ASW2]
- Install the flywheel to crankshaft using the factory [ASW3] bolts. Torque to 100Nm.
- Install the clutch release bearing to the pressure plate.
- Fit clutch pate, pressure plate and ring gear. The clutch assembly will need to be aligned with the pilot bearing. Use an old input shaft or a clutch alignment tool. Clutch alignment tools are available at all Porsche parts specialists.
- Lightly grease input shaft splines.
- Test fit clutch fork, shaft and bearings to G50 bell housing and ensure smooth operation.
- Install clutch fork onto transaxle.
- Carefully install the transaxle onto the adapter plate trying not to disturb the clutch fork. The splines of the input shaft must pass through the clutch plate with the nose of the input shaft pushed snugly into the crankshaft pilot bearing.
10. Once the transaxle bell housing is seated firmly against the adapter plate, install washers and nuts onto the studs.
Many clutches are available for these transaxles and you should select the correct clutch and flywheel solution based on your engine output. There are many specialists who can advise you, including California Motorsports, GBox, and others. For engines less than 350 HP, a standard Porsche clutch assembly from a 930 Turbo is a good choice.
The clutch slave cylinder can be fitted before or after the gearbox is fitted. The Porsche slave cylinder is not well-suited for use when the box is inverted because the bleed nipple ends up at the bottom, and the slave cylinder needs to be removed from box, rotated and bled using a jig before being permanently assembled onto the gearbox. Superlite Cars can supply a suitable slave cylinder that has the bleed nipple in the correct orientation for bleeding the clutch slave cylinder on the vehicle.
It may also be a good idea [ASW4] to use a rear gearbox mount. Superlite Cars can supply a gearbox mount under the rear of the gearbox to attach it to the rear lower chassis cross-brace.
Sometimes there can be issues with ring gear clearance inside the Porsche bell housing. To be sure you don't have this problem test fit the ring gear inside the Porsche G50 bell housing. The clearance problem is slight. You need to ensure you have clearance [ASW5] for the ring gear 85mm back from the front bolt face of the bell housing.
The above photos show a G50 with earlier short tail housing. If your G50 has a long case with large donut mount on the rear, you will need to obtain a short housing and shorten the gear change shaft. Or you can machine the long case and remove the donut mount and shorten the gear change shaft housing and the shaft itself.
For installation of gear shifter, see the Gear Shifter section in the SL-C Build Manual
Since we're describing metric hardware shouldn't we specify, eg, 12.9 rather than "HT"?
Which product (number) and shouldn't they also be torqued?
Does this mean supplied by SL-C? Or by GM? IOW which factory?
Say why. To reduce stress on the engine mounts?
How much clearance?