915 LSD rebuild & upgrade

[gears]

Early 915 LSDs have multiple discs, which allow setting the LSD to either 40% or 80% “anti-slip effectiveness”. In the 40% setting, one friction disc per side is active (2 total), while the 80% setting requires 2 friction discs per side (4 total) to be utilized. The later Carrera 915 LSD was handicapped, in that it only has room for one friction disc and one plain plate per side .. so these LSDs only allow a 40% setting. This thread shows how to easily convert one of these 40%-only LSDs to 80% without any special machine work or expertise. (Keep in mind that 80% is ONLY for track use.)

First, the ring gear is removed. This leaves two short 6mm screws as the only means by which the cover is held onto the main body. Because of internal Belleville washer preload, the cover begins lifting off as these screws are removed. So, in order to avoid damaging the threads of both screws and cover plate, the cover must be held in place .. either with 4 nuts & bolts (as used to draw the cover back on during reassembly) or with a shop press. I opted to use my press, as it makes the job a few minutes quicker.

The short 6mm set screws are then replaced with longer 6mm bolts, making it easier to tap the tight-fitting cover off the main body without inserting pry tools between the machined surfaces. (The small gap between cover and body in this photo is evidence of relieved Belleville preload.)

With the guts openly displayed, it can be seen that the Carrera 40% LSD has only one friction disc and one plain plate per side.

Our simple conversion to 80% is facilitated by using special custom discs and plain plates that are half the thickness of original factory parts. The single 2.0mm plain plate on each side is replaced with two 1.0mm plain plates, while the single 2.0mm friction disc is replaced by two 1.0mm friction discs.

This method of squeezing twice the number of friction discs & plates into an LSD was first done by Porsche, when (in 2005) they began replacing the thick plates & discs in their Motorsports LSDs with 1.0mm versions. Twice the number of friction discs dramatically increased the amount of lock-up in these LSDs .. something Porsche deemed desirable in their GT3 class ALMS and ELMS race cars.

Belleville washers provide static preload to the disc / plate stack. This preloading assures smoother transition (thus enhanced driver control in turns) as the LSDs pressure rings increase / decrease their clamping pressure on the LSD's plate stack. Although the Carrera LSD's Bellevilles are made from beefy 2.5mm thick steel, they don't have much spring pressure to them, as the Belleville shape is minimal. So, to be considered when converting the Carrera LSD for track, is the use of Bellevilles having increased static preload (greater overall height).

When setting up an LSD from scratch, everything is measured meticulously to assure proper clearances within the LSD body for the stack of parts squeezed into it. Because this conversion kit is specific to the Carrera LSD, none of these measurements are required. However, I'll quickly touch on this procedure anyhow ..

First, the body depth is measured:

Then, the depth that the cover sits into the housing is measured:

The pressure ring / cross shaft assembly is measured:

.. and the individual plates, discs, and Belleville washer thicknesses are all measured. When measuring these thinner pieces, what we're determining is their nominal thicknesses, which we will later total up on paper. Contrary to what one might believe, this individual measurement is more accurate than trying to measure these thin components stacked up together. Keep in mind that when measuring the Bellevilles, we are interested in material thickness, not the Belleville's height.

Making sense of our measurements:

Housing depth minus cover depth = room within housing
95.49 - 13.72 = 81.77

Pressure ring assembly + discs + plain plates + Bellevilles = total stack height of parts squeezed into housing
68.18 + 2 x 2.0 + 2 x 2.0 + 2 x 2.5 = 81.18

From the above figures, we can see that approximately 0.6mm “free space” exists within this LSD. All of this free space is of course more than taken up by the “spring rate” of the Belleville washers (difference between Belleville's material thickness and Belleville's measured height).

Reassembly:

First, install the two small thrust washers with grease on their backside, so that they won't fall out of place during reassembly. (Although not shown in this demonstration assembly, all other parts are liberally coated with gear oil as they are installed. This is extremely important.)

Unlike side gears in an open diff, the side gears in an LSD are snugly held in place by preloaded discs. For this reason, we use the LSD's output flanges to facilitate assembly, thereby keeping the internal splines of the side gears perfectly aligned with the flange bores. If you don't do this, it could be an absolute bitch attempting to pound flanges into slightly misaligned side gears after the LSD has already been installed into your transaxle.

The first Belleville washer is installed with convex inward, concave outward (outer edge contacting diff body, inner edge contacting plain plate).

Next, the right side discs & plates:

The inner pressure ring:

When installing the inner side gear, it likely will require a bit of wiggling and turning, so that its outer splines plug into both friction discs while its inner splines plug onto the flange.

The cross shaft / spider assembly is dropped into place. Proper side gear installation is confirmed by the cross shafts seating fully in the pressure ring's notches. If the cross shafts don't seem to properly seat, the side gear wasn't properly aligned in the previous step.

This is a good time to squirt some extra gear oil onto the spiders and inner side gear teeth.

.. then install the left side gear and left side pressure ring:

.. and the 2nd set of plates:

Outer Belleville:

Properly installing the diff cover is the most critical step. We're going to use a pair of long 6mm bolts in the two set screw holes for initial alignment, and four 10mm nuts & bolts with HD washers to pull the cover on perfectly square. Slow going with tiny twists of each consecutive nut minimizes the scuffing between cover and body. (I smear a bit of moly anti-seize mixed with oil on the contact area.) I very much prefer this 4-bolt method to using a press, as one can immediately feel if something is resisting normal assembly. Keep in mind that if you're now using high performance Belleville washers, this upgrade will significantly increase the effort required to bolt the cover down.

Just prior to the final tightening of the 4 bolts and installing the two 6mm screws, alignment of the 12 ring gear bolt holes in cover and housing must be assured. If you fail to do this, there may be problems when reinstalling the ring gear. Lacking proper alignment pins, a 31/64” drill bit can be used. As this isn't the perfect size, just make certain that the drill bit cocks equally off-perpendicular in both directions. (Ignore the fact that this photo shows checking alignment before the cover has been 99% bolted down.)

A few light taps around the rim of each output flange before removal assures that the internal side gears are perfectly aligned. The flanges should slide out effortlessly.

Normally, the LSD's breakaway torque would now be measured in order to establish a baseline for later LSD servicing. As this is a DIY bolt-on kit thread, we'll skip measurement of breakaway torque (just as we could have skipped discussion of internal measurements).
_________________
aka Pablo, Geary
9.36 @ 146 in '86 Hot & Sticky
'90 Syncro Westy SVX
'87 Syncro GL 2.5
https://guardtransaxle.com

[raygreenwood]

Very nice write up and pictures! Ray

[gears]

Some interesting discussions on the merits of using LSDs over other types of locking differentials in 911 race cars:

Greg Brown's article in the POC Magazine, Velocity, February 2001
http://members.rennlist.com/emcon5/velocity_lsd.pdf

http://rennlist.com/forums/performance-modifications-forum/673572-wavetrac-quaife-or-gt-for-lsd.html

http://rennlist.com/forums/racing-and-drivers-educ...-slip.html
_________________
aka Pablo, Geary
9.36 @ 146 in '86 Hot & Sticky
'90 Syncro Westy SVX
'87 Syncro GL 2.5
https://guardtransaxle.com

[gears]

.
Kits now offered to rebuild your own Porsche, ZF, or GT LSD:
https://www.thesamba.com/vw/classifieds/detail.php?id=2091565
.
_________________
aka Pablo, Geary
9.36 @ 146 in '86 Hot & Sticky
'90 Syncro Westy SVX
'87 Syncro GL 2.5
https://guardtransaxle.com

[raygreenwood]

[gears]

Thanks, Ray. It took years to perfect the parts, which now last years in serious track cars, and indefinitely in street cars ..
_________________
aka Pablo, Geary
9.36 @ 146 in '86 Hot & Sticky
'90 Syncro Westy SVX
'87 Syncro GL 2.5
https://guardtransaxle.com

[raygreenwood]