trackjunky

Can anyone clarify the PCA stock class rules around Limited Slip Differentials?

If a car was available with a LSD, stock rules say that you can add a factory version or one equivalent. Is there an advantage either way? What are the cost differnces? Let's use a 964 for reference.

Sean F

i just went through this with my SC this season. You can add a like LSD. For me that meant a clutch type LSD with the same lockup available from the factory...40 or 80. I wanted to buy one from Guard because I understand they are stronger and I wouldn't have to worry about the condition of an original part (oems often need to be rebuilt if you can find them). Unfortunately, Guard wasn't going to have any available this year. I found an original 915 ZF LSD that I had tested and put in. The Guard is more expensive, but again, I would have gone that route if available. The other advantage is that he will set the lockup you want. Most of the oems for the 915 were 40 and couldn't be switched. Oems for the 915 are hard to find. I paid about $1000 for mine and the Guard is more expensive.

Veloce Raptor

Dang, Bill, when I saw you (of all people) post a thread about LSD, I naturally thought....

analogmike

You can change the lockup in the stock LSDs, the factory shop manual shows you how. Just rearrange the plates differently.

MarkM

Dosen't the ZF style also provide stability under braking, where the guard does not. I think there is a machining mod required for this?

And yes the rate can be adjusted by aranging the plates differently.

analogmike

The Guard is the same as the ZF, just in a stronger housing. They both help quite a bit under braking depending on how they are setup. If you have a stock ZF and it needs a rebuild you can get the parts from Guard.

trackjunky

Hmm. This is what I thought. Good info here.

I guess that you can pretty much "dial in" the lock up you want with the stock LSD, so there may not be an aftermarket advantage.

What is the big hangup with aftermarket LSD's and the rules?

trackjunky

Bill Verburg

Any clutch type LSD either Guard or ZF oem will provide additional braking stability over an open diff.

The ZF diffs used on all 915 and G50 up though all G50/05 were either 40% or 80% and operated symmetrically. That is lockup was the same on acceleration and deceleration. There was a reduction of the # of friction plates and thus the effectiveness(despite the #s) ~'84

Starting w/ the 964RS G50/10 and 964t G50/52 an asymmetric 20%/100% clutch type LSD was offered. These trans had an acceleration lockup of 20% and 100% lock on deceleration. Even better for braking and fast cornering.

the 993 6spds continued w/ 40/65 from GKN on RS and 25/65 or 22/40 used on n/a and tt models.

Guard sells a beefed up version of these asymmetric diffs w/ several different lockup options. The cases are much stronger than the ZF or GKN cases and there are more friction plates which are much more durable.

a version of these asymmetric diffs is available for 915 transmissions

PCA rules specifically state that oem lockup factors may not be altered w/o progressing 1 class.

Sean F

Prior to 84 the plates can be changed to change the lockup...not post 84 (and there seems to be a lot of 84+ on the market). You can tell the difference between those that can be changed and those that can't by the shape of the hole in the side. If it has a circle, it has the extra clutch plates that can be changed. If it is an oval it cannot be changed. The breakaway torque on the post 84 years is not good either.

Sean F

One other point. If you go the OEM route, you need to have a reputable shop that knows how to test the torque of the LSD and inspect the clutch plates (e.g., Gary Fairbanks). There's a good chance you'll be buying a beat to sh*t LSD.

trackjunky

to all.

Thanks for the education. This is exactly what I need to know.

Can you tell, without opening the trans and diff, what the lock up rates are set at?

Veloce Raptor

Oooo....touchy.

trackjunky

It's touche.......dumb ***!

Veloce Raptor

there you go again....drinking the