Lim slip for dummy's help. stock G50 diff. Skool me?

[vintagecarlover]

im trying to understand ratios on lim slip differentials. What is factory diff ratios on G50 diffs and what do the numbers really mean? Do u have a lim slip and what do u have type and ratios? Can u explain ratios. Thx for the help

[Bill Verburg]

the lsd used in g50 depends on the year
so what year do you want to know about?

[vintagecarlover]

Hey bill. Thx for offer to assist. G50 is from 96 993 car. What can you tell me about going into an 87 Trans. Lmk. Thx.

[Bill Verburg]

Originally Posted by vintagecarlover

Hey bill. Thx for offer to assist. G50 is from 96 993 car. What can you tell me about going into an 87 Trans. Lmk. Thx.

The lsd from a '96 993 is a GKN 25/65 asymmetric lsd. nominal 25% lockup on acceleration and nominal 65% lockup on decel. The actual brake away torque depends on how it's setup and how worn the internals are.

The stock friction discs wear quickly and the unit can be greatly improved by rebuilding it w/ 4 GT friction disks

It will bolt into an '87 G50

the rebuild and install should be handled by a competent shop, GT has a list of shops that they deal w/.

[vintagecarlover]

Bill, is that Matt?

[Bill Verburg]

Originally Posted by vintagecarlover

Bill, is that Matt?

yes

[vintagecarlover]

Can you explain what u mean by break away torque and how can that change by the way it's set up?

[Driven97]

Originally Posted by Bill Verburg

the rebuild and install should be handled by a competent shop, GT has a list of shops that they deal w/.

This is important and should not be overlooked. Shims down to .05" (half the thickness of a sheet of paper) are used to adjust the total width and lateral position of the diff. Total width is for the preload on the cone shaped carrier bearings - too tight and they'll wear prematurely, too loose and the ring gear can shift under load. The lateral position is for the gear mesh between the ring and pinion. Too close or too far and they'll wear out very fast. Even very small production differences can make a big impact, so you can't just bolt a differential in without setting this up properly.

Pinion depth is even more complicated to setup, but if you already have a working R&P in the car that shouldn't need to be touched.

Originally Posted by vintagecarlover

Can you explain what u mean by break away torque and how can that change by the way it's set up?

Break away torque is similar to the "initial" setting in the GranTurismo games if you play that. It's how much lockup the diff has when there is no torque applied between the input and output - aka when you have the clutch in. It's usually adjusted by a preload spring, that squeezes the clutches together. In this photo it's the dome shaped (Belleville) washer on top of the stack of clutch discs:

[Bill Verburg]

Originally Posted by Driven97

This is important and should not be overlooked. Shims down to .05" (half the thickness of a sheet of paper) are used to adjust the total width and lateral position of the diff. Total width is for the preload on the cone shaped carrier bearings - too tight and they'll wear prematurely, too loose and the ring gear can shift under load. The lateral position is for the gear mesh between the ring and pinion. Too close or too far and they'll wear out very fast. Even very small production differences can make a big impact, so you can't just bolt a differential in without setting this up properly.

Pinion depth is even more complicated to setup, but if you already have a working R&P in the car that shouldn't need to be touched.



Break away torque is similar to the "initial" setting in the GranTurismo games if you play that. It's how much lockup the diff has when there is no torque applied between the input and output - aka when you have the clutch in. It's usually adjusted by a preload spring, that squeezes the clutches together. In this photo it's the dome shaped (Belleville) washer on top of the stack of clutch discs:

That's right
stock the late lsd's have very low preload, if you spin them on a lift they behave more like an open diff. When the drive line is putting torque through them the ramps incorporated in the end plates wedge the pinion and raise the break away torque value.

The older ZF lsd's had symmetric 30° ramps which were provided a nominal 40/40 or 80/80 action depending on how the disks were set up. I don't have the angles of the street 993 lsd but on my PMS lsd the angles are 50° on the acceleration side and 37.5° on the deceleration side. The smaller the ramp angle the more 'wedge' effect and the more torque through the drive line the more 'wedge' effect. The more 'wedge' the more torque it takes to spin L/R wheels at different speeds.

zero or low preload is desirable for mostly street use as it is less intrusive in slow speed turns or backing while turning, the actual clamping force is less under light loads than under heavy acceleration.

[Matt Monson]

The only additions I will make are that the 993 LSD is not going to have an allowance to mount the speedo drive tone ring.

Also, it will currently be 2 clutches if it hasn't ever been rebuilt. For street use that is fine but if for a track car you want to remove a big spacer in there and convert it to 4 plates. The ramps are adequate for a racecar as long as it is stacked tightly enough. While Porsche standardizes preload numbers it is not the end all. I don't even bench test diffs. I build to a stack height instead.