BaysideC4

I have a 1990 C4. Does anyone know conclusively whether or not I have a limited slip differential?

Thanks in advance

Colin 90 C2

you need to locate the options sticker on the bonnet of the car. It should give you your options, usually a 3 digit alpha-numerical.

There are a lot of decoders out in the cyber world and if you post them here, I'm sure the community will help out.

Bill Gregory

I don't believe option M220, limited slip differential, was available on C4's.

garrett376

You mean for the front differential? The C4 has an "active differential" in the rear which is similar to a limited slip but its slip is variable based on the computer's input, not based on the clutch packs like a mechanical LSD.

I'd say you DO have a limited slip in the rear - since it does split torque variably, limiting slip of rear wheels actively. It just does not have deceleration lock capability.

The front is an open type differential - wonder if anyone has ever put a limited slip up there??

garrett376

Come to think of it, even the C4RS, and the 953, don't have "standard" limited slip differentials - they had adjustable ones, like what the C4 has, but the C4's is not driver adjustable, just computer adjustable.

You technically have TWO limited slip differentials in the C4 - the rear (lateral) and center (longitudinal) differentials.

Christer

but in the normal use of the word, no.

BaysideC4

That’s where the confusion is. Is a computerized LSD the same as mechanical LSD? I have to class my car properly for Autocross; do I deduct points for a LSD if the car technically didn’t have that option code?

NotTwinTurboYet

Yes!

Christer

i think to put it in a different way: no. traction control maybe. LSD no.

NotTwinTurboYet

This is the copy of an answer I made to someone else about LSD. We were looking at the numbers on the gearbox:


And to add some comments about that, let me give the following info about numbers on the gearbox. This is the rule for gearbox from 1989 to 1991.
Let's take that example:

G6400 3 K 00501

G6400 is the gearbox type (G64 for C4, G64/01 for Switzerland and G64/00 for rest of the world, G50 for C2, A50 for tiptronic)

3 indicates the type in a group, 0 for "whithout differential" (note that none of the cars were provided with "no differential"), 1 for "Normal Differential", 2 for "ZF LSD (M220)" and 3 for "electonic LSD"

K is fot the Model Year: K for 1989, L for 1990, M for 1991

00501 is the specific number of the gearbox


Now let's start by geabox available for the MY1989 964 C4:

G6400 3 K 00001 Manual gearbox with LSD
G6401 3 K 00001 Manual gearbox with LSD (for Switzerland only)
Z6400 1 K 00001 Front differential (or axle, I'm not sure about how to translate that) and note that "1" means the front differential is "normal".

Therefore according to Porsche books, they are speaking of an "electronic LSD".

NotTwinTurboYet

Ok, I'll try to explain that, apologize for me english in advance, I'm not sure to be clear...

Electronic differentials (center and rear) on a C4 have the same purpose as a LSD. I know that some guys consider it as a traction control, because that system can use the 4 wheels to be more efficient, and also because it is electronically controlled, but to me we can consider it as 2 LSD.
A LSD, such as a Torsen, is Torque Sensitive. If a wheel overspin, torque and power will be transfered to the slower wheel. It's the same on the C4. The difference is that it's not torque sensitive, it uses ABS sensors (and some other accelerator detectors) to identify which wheel is too fast. Then, it uses a high pressure pump to lock the differencial and transfer torque and power to the slower wheel.
This is for the rear wheels.

Let's now talk about front wheels. On the C4, there is also a central differential with the same capabilities than on rear. Again, electonically controlled via the ABS sensors and other sensors, and locked with another high pressure pump activated by the computer. If one of the axle loose traction, torque and power will be transfered to the other axle.

The front differential has no lock capabilities.

What does that mean? Oversteering almost never appears on a C4 (in standard conditions) because there's the equivalent of a rear LSD, and if it's not sufficient, power will go to front wheels. Understeering is something usual on a C4, because there is no front LSD, and therefore one front wheel can be faster than the other because it does not make any difference for the central differential, and therefore there's no power transmition to the wheel which has more grip. To kill the understeering, you have to floor the accelerator, you need space, and a good insurance company because the car will start to oversteer!

When you know how all this work, you can really play with a C4, exactly as you would play with a car fitted with a central and rear LSD.

Therefore, to me, this has nothing to see with a traction control, and I experience it each winter, for around 4 month

I also had a subaru STI, you may know that car. It has front and rear suretrack LSD (made by AP racing) and a central Torsen. I also have an audi quattro, and I would never go for a non 4WD car. And beleive me, the C4 system really reacts as a central and rear LSD.

Note that this system is disconnected when braking, this is the main difference. Because it uses ABS sensors, if you brake, the sensors are used for the ABS system. If you go on a track or search the limit of the car, this can be an interesting information...

NTTY

PS: the same system was used on the late 928, only on rear wheels, and the purpose was to offer the equivalent of a LSD.

garrett376

BaysideC4 - that's a good question about the points! The C4 system does produce the same result as an LSD as mentioned here several times... but I don't think you need to deduct points because it really is not an "option". I'm betting that you don't need to deduct...

I wonder.... someone email their Tech committee club chairs...