993 Limited Slip??

[Switzer]

Does anyone know how to determine whether a 993 has a limited slip differential (e.g., build slip, VIN, etc.)? It's a 96, by the way.

[Don Plumley]

The easiest way is by lifting the rear end and turning one wheel. If the other wheel turns in the same direction, you have LSD. The other way is through the option codes printed under the trunk lid. Then use an on-line option decoder, such as:

http://www.kindel.com/Porsche/options.asp

or

http://p-car.com/technical/option.html

Don

[kang]

Checking your option codes is the only way to be sure. I also seem to recall that you can look at the transmission (from underneath, you need to remove the cover) and check the transmission code. There is a code on it, like on the engine, and if the code has a -1 or -2 or something at the end that means you have a limited slip.

It is a common misconception that you can test for this using the “turn one wheel, if the other turns in the same direction, you have a limited slip” method. That is not a valid test on these LSD’s. From what I’ve heard, this only works in brand new limited slips. I don’t know all the physics behind it, but it has to do with preload. For this test to work, e.g. for both wheels to spin in the same direction, the wheels will need to spin the ring and pinion, which will in turn spin the main shaft of the transmission. There is a fair amount of resistance in all of this. It is typically too much resistance for the preload to overcome. What ends up happening is that the spider gears turn inside the diff, leaving the ring (and therefore pinion and main shaft) still. The other wheel turns in the opposition direction, but you still have a properly functioning LSD.

In a brand new LSD, there is enough friction on the clutch plates to overcome the resistance in the ring & pinion & main shaft, so the other wheel turns in the same direction. This resistance wears down quickly, and then this test fails.

There is a proper procedure to test a LSD, but it is not the “spin one wheel” test. It is a complicated shop procedure, done with complete access to the LSD (e.g. tranny out of the car). A certain amount of load is placed on all the components of the diff (e.g. each half shaft and the pinion). You can’t do that without complete access to it.

[RarlyL8]

Dump the clutch at 5000RPM.
Count the black marks.
2 = Posi

[Bill Verburg]

The 84 up ZF and GKN 2 disk lsds wear very quickly. Best to assume that if you have one it is worn out.

[kang]

I believe that due to the fact that they fail the "spin one wheel" test, people think they are worn out, when in fact they are not.

[Bill Verburg]

Believe me, I know what a working lsd diff feels like. I have never found one working properly in a 993 w/ > ~40k mi.

You are correct that the break away torque even w/ a new 40% 2 disk lsd is much much less than in the older 4 disk 40% units.

[Switzer]

You guys are awesome. My options code confirmed the locking differential, yet you confirmed it's probably too worn to be useful.

[dvkk]

Originally posted by kang

It is a common misconception that you can test for this using the “turn one wheel, if the other turns in the same direction, you have a limited slip” method.

It is not a misconception. If the other wheel does turn the same way, you do have a LSD. An open diff will never do this. This test does work on some high mileage ZFs, not just new ones.

However, if the other wheel goes the opposite direction, you may have an open diff, or a worn out LSD.

The condition of the gear oil also has an effect on whether the other wheel turns the same way or not.

[kang]

It is not a misconception. If the other wheel does turn the same way, you do have a LSD. An open diff will never do this. This test does work on some high mileage ZFs, not just new ones.

Those points are true: if the other wheel turns the same way, you do indeed have an LSD, and with an open diff, the wheels will never turn the same way. And yes, some high mileage LSD’s do pass the test, just not all.

However, if the other wheel goes the opposite direction, you may have an open diff, or a worn out LSD.

Also true, but I would add “you might have an LSD that is working, but can’t overcome the internal resistance of the transmission.”


Imagine doing the “spin the wheels” test with the transmission in gear and the clutch engaged. In order for both wheels to turn in the same direction, you’d have to turn the engine as well. I suppose that if you had two big lever arms, one on each wheel, and turned them simultaneously, you could turn the engine. But if you just have one person gently turning one wheel, the engine won’t turn. The motion will be transmitted to the other wheel, through the spider gears, which will turn in the opposite direction.


Basically the same thing is happening even if you have the tranny in neutral. There is too much resistance inside the tranny for it to turn, so the motion is transmitted to the other wheel.

The condition of the gear oil also has an effect on whether the other wheel turns the same way or not.

True, in that the condition of the gear oil effects how much resistance there is in the tranny.

I had my engine and tranny out a few years ago. It had quite a few miles on it at the time, way into the range where people say these LSD’s wear out. I asked the mechanic to check my LSD. He did the out of the engine test I mentioned above. He said it was fine, but my car fails the spin the wheels test. The other wheel spins in the opposite direction. He gave me the explanation I posted above (well, he actually gave me a better explanation. My post above is my understanding of what he told me plus several years of elapsed time. )

I trust the mechanics opinion, especially since he could have made a tidy sum by replacing or repairing my LSD. Had he said it was worn out, I would have believed him and taken the opportunity (tranny already out) to have it fixed.

Further evidence that my LSD is fine is that I never spin one wheel when coming on power out of a turn, say at an autocross. When that starts to happen, it’s time to do something.

[dvkk]

Originally posted by kang


Imagine doing the “spin the wheels” test with the transmission in gear and the clutch engaged. In order for both wheels to turn in the same direction, you’d have to turn the engine as well. I suppose that if you had two big lever arms, one on each wheel, and turned them simultaneously, you could turn the engine. But if you just have one person gently turning one wheel, the engine won’t turn. The motion will be transmitted to the other wheel, through the spider gears, which will turn in the opposite direction.


I had my engine and tranny out a few years ago. It had quite a few miles on it at the time, way into the range where people say these LSD’s wear out. I asked the mechanic to check my LSD. He did the out of the engine test I mentioned above. He said it was fine, but my car fails the spin the wheels test. The other wheel spins in the opposite direction. He gave me the explanation I posted above (well, he actually gave me a better explanation. My post above is my understanding of what he told me plus several years of elapsed time.

I've never bothered to try to turn one wheel while it is in gear. You'll never be able to turn the engine, so you are just testing the friction of the discs.

One thing I have observed is that sometimes a dragging rear rotor will stop the other wheel from turning, forcing the input shaft to turn.

The first ZF I installed in my car was well used when I got it but still in good condition internally. After about 20k miles, the opposite wheel wouldn't turn with the turned wheel. I figured it was dead but left it in and continued to drive it. By 30k miles it would again start to pass the "turn one wheel" test. I ran it up to 70k miles where it would still pass the test. The only reasons I could think of that caused the 20k test failure were cooked gear oil, and a dragging rotor.

[Geary]

If one wheel turns in the opposite direction of the other, the friction discs have gone "slick" and/or little or no preload exists. A breakaway torque test would register quite low. The factory-installed 993 discs go slick in one or two track weekends, which equates to perhaps 5-10,000 street miles. While a low performance LSD such as this may be perfect for slower speed autocross or conservative street use, it likely wouldn't be acceptable for a guy who's stiffened up his suspension for DE track use.

The major issue is the "carbon" type friction discs found in the 993 street LSDs. These are not of durable quality and should NOT be replaced with new carbon discs, but rather the durable plasma-sprayed type.

[edward993]

OK, here's another reason why I don't believe the "spin one wheel" test. Can someone explain why my stock, never opened 993 tranny/diff that has NO LSD (confirmed with option codes), have both wheels spin in the same direction when on jacks and in neutral? I've heard before that there may be friction in the spider gears, but was told it was no real concern. Car behaves great, w/o any noises, so I'm really just wondering why it is so. TIA.

Edward

[Bill Verburg]

Any useful lsd can be felt tugging at the car in a 90&deg turn as when turning from a stop on a side street and acelerating onto a main. If you don't feel the tug your lsd is useless.

[Geary]

Edward, my guess is that you have an LSD despite what your option codes say. It wouldn't be the first time a mistake was made at the factory.

[edward993]

Bill,

Thanks for that tip. I haven't driven my SC (with LSD) back to back with another sans LSD, but I definitely feel that tug when doing any 3-pt turns ...I thought something was weird with the ackerman, or something rubbing. Now I know it's my healthy LSD. Cool!

Geary,
Well that would be very good news, but somehow I doubt I'm that lucky. No "tugging" as Bill mentioned, ever, even from when it was a "low mile" car at 50K. One day I'll get it confirmed.

Thanks, guys!

Edward

[Geary]

Edward, something would have to be causing the wheels to turn in the same direction. LSD (or some sort of locking diff) or a damaged open diff are the two things I can think of. Are you CERTAIN that the second wheel is turning forward when you manually turn the first wheel forward?

The "tugging" that Bill mentions is typical with a more aggressive type of LSD, but NOT typical with a "worn in" 993 street LSD (for reasons already posted). I would guess that if you in fact have a 993 street LSD (and not a damaged open diff), your friction discs still have enough bite/preload to cause the opposite wheel to spin, but not enough bite/preload to provide obvious resistance in a slow speed turn. Just an educated guess.

[edward993]

Hi Geary,

Sorry, I guess I wasn't clear. On my trackmeister, an '82SC WITH LSD, I get the tugging. It was purportedly a freshly rebuilt tranny from the PO (and shifts buttery smooth so I believe it), so I guess the LSD is working as designed.

It is my 993 that according to the option codes has NO LSD, yet turn one wheel forward on stands, and the other turns in the same direction. It has been like this for who knows how long, at the very least 40K+ miles. It has always driven well, on the track and daily driven. Any thoughts? No problems, really, just been curious about this.

Edward

[Geary]

Well, needless to say, I'm curious too. A stock open diff is "as loose as a goose", with the other wheel freewheeling in the opposite direction from the one manually turned. If it's truly an open diff that's doing this, then something is binding. Not likely for 40K miles.

[edward993]

Oh well...thanks for thinking about it, Geary. I guess I'll just have to keep driving it until she drops ...bummer.

Edward