Help with G-50 LSD noise

[spooldup]

My car makes a grinding sound only when im off the throttle turning-more obvious at slower speed.In a straight line it does not do it,The noise is in the rear end and I put brand new CVs complete with axles-did not help at all.I also just put new Mobil 1 75/90 in and it made a slight difference.I only put about 5 miles on it after the Mobil 1 went in.Do I need to drive it more for it to be absorbed through the gearbox?The car has only 28,000 miles on it.Thanks

[john walker's workshop]

kendall has a limited slip additive that may help. failing that, maybe go back to regular 80/90 GL-5 gear oil and try the additive again if it still makes noise. M1 may be too slippery.

[Grady Clay]

Porsche addressed this with the last of the 915 updates so I
would think all the G50 LSDs included this ~’87 or later update.
However PET doesn’t show the updated parts, it shows the 915
multi-disc configuration all the way up to the G96 (996).

In the P30 Service Training manual (WKS 303 021 dated March
1988) Porsche shows the following:

“A) Differential Case

Multi-disc limited slip differentials have always had the situation
that driving in tight curves would cause a grinding noise.
Research has proven that this noise (also known as stick-slip-effect)
depends strongly on the oil film on the friction discs.

In previous limited slip differential cases it was very difficult to
supply oil against the centrifugal force.



For this reason the new spheroidal graphite cast iron differential
case is open on the outside diameter as well as sides (arrows).
Consequently, the supply of transmission oil to the friction discs
is almost unrestricted.


“B) Friction Discs

The coat on the friction discs for the approx. 40% locking ratio
was simplified (from outside to inside).



1 – Diaphragm Spring
- Material: Spring steel
- Position: large diameter presses against case;
small diameter presses against outer disc (2)

2 – Outer disc
- Material: steel

3 – Inner disc
- Material: steel, provided with lubricating groves, molybdenum coat

4 – Thrust ring
- Thrust ring thickness is greater to correspond with reduced
number of discs

Note:
A locking ratio other than approx. 40% is no longer possible with
these components. If a limited slip differential with a different
locking ratios is required in a car, the limited slip differential
930.332.053.03 with permanent uncoated discs has to be
installed.
However there is then no advantage of better lubrication for the
discs after this installation.


“C) Shafts of Small Differential Gears

The shafts (1) for small differential gears in multi-disc limited slip
differentials have been chamfered on their ends up to now, in
order to be able to slide up on the bearing surfaces of the thrust
rings (2).



The new shafts no longer have a chamfer; they are round up to
the end of the shaft.”

© 1988 Dr. Ing. h.c. F. Porsche A.G.



I looked through PET6 and didn’t find any of these above parts.
I’ll study the .pdf CD some more.
All I see is the normal multi-disc LSD.

’87-’88 950.332.053.00 40% for the G50.00/01/02
’89-’91 950.332.053.02 10-100% for the G50.10/52
’92-’95 950.332.053.05 25-65% for the G50.20/21
’96-’97 950.332.053.31 22-40% for the G50.31/32/33

I can’t tell for the G96 (996) because they don’t show the
internals of the LSD.

I may be wrong but I will speculate that these differentials will
backdate to the ’70 911 transmission.
Does someone have experience?

My overall assessment is that this “upgrade” prevents the LSD
clutches from chattering because there are a total of only four
friction surfaces in play. Of course the big tires and rear weight
overcomes the stick of the friction discs. Added oil must help
also. This was probably a cost-cutting measure and reduced the
annoyance (to some) of the noise. All at the expense of
performance IMHO.

I prefer to use all the friction surfaces of the discs and put up
with some LSD noise in addition to the differential slip of the
tires on some sand in a parking lot.

I have always been cautious about putting “too slippery”
lubricant in a transmission equipped with an LSD. You want
the friction of the discs – that is what supplies the limited slip.

Check this thread:
Limited Slip Diagram/Info ?


Best,
Grady

[cbeers]

Good to see that you are doing well Grady and helping us all have a better understanding of our cars

Hope to see you soon!


-Chris

[spooldup]

Wow what an education,so with that said Grady what I have is a C2 turbo with oversize tires.It made the same noise even when the stock wheels and tires were on so my question here is am I damaging anything by driving it and do I drain the Mobil 1 out and put what oil back in.Thanks so much you guys as I really did not expect a reply from my post!

[Geary]

Paxton, your model has the brass friction discs that a lot of people complain about (although "grinding" isn't a term normally used). Change them out for the superior plasma sprayed (moly type) friction discs for best performance (and probably a lot less noise), or the 993 fibrous type for zero noise (but poor performance).

Most of the hard-to-find individual components for the 915 & later LSDs are carried by Guard Transmission.

[JasonAndreas]

I thought Valeo/Porsche changed the inner disc coating from spray molybdenum to carbon and coated the pinion shafts with nickel on later LSDs (post 91) to fix the groaning and not the other way around?

Paxton,
Before you dump your Mobil 1 go to your nearest GM dealership and buy a $6USD pint of GM LSD lube and dump it in your transmission, that is supposed to get rid of the noise on the early turbos.

[Geary]

All Porsche Motorsports LSDs come with the moly type discs. The brass discs were only used for a short time on select models, such as the street Turbo. The fibrous ("carbon") material discs come in street LSDs, providing marginal performance after a few thousand miles --- but they're real quiet.

[ZCAT3]

While were talking G50 noise, my newly installed and rebuilt G50 tends to grumble a bit when at idle with my foot off the clutch. If I push the clutch in the sound goes away. I am running a centerforce spring centered clutch disk. Is there any thing I can do about this noise?

[Grady Clay]

Paxton,

The LSD listed as original for your 930 is (I think) 950.332.053.01
for a G50.10/52. Go look and post all the numbers under the
transmission and your VIN – that will help.

Lubrication in a Porsche transaxle has always been a compromise
between various, sometimes conflicting, needs. First and
foremost are protecting the gears and bearings. There is a
difference in the requirements for the Hypoid ring and pinion
compared to the other gears. The bearings have slightly
different needs also. Then there is the ability to transfer heat to
the outside world and the protection from rust and preservation
during storage.

With early syncros (Porsche patent molly) in 901, 911, 915, and
930 there is the issue of the lubricant being too slippery for the
syncro to function properly. There is the same issue with the
moly coated LSD clutch discs.

With the advent of the G50 syncro set-up, I don’t know what the
best combination of features a lubricant should have. I’ll
speculate it is different from the prior needs for the syncros but
the same for the moly LSD discs.



Here is some more info about the operation of the LSD. This is
excerpted from “Service Information Technik” ’91 911 Turbo
(WKD 497 021) for your critical review.

“Operation






The limited-slip differential housing (1) is driven by the ring gear
of the drive set.

Thrust pieces (3) and (7) are located positively in the housing (1)
but may be moved in the axial direction. When a load is applied
by the differential bolts (2), the thrust pieces are moved towards
the left-hand and right-hand disc packages. The resultant
spreading force (Pax) is dependent on the torque applied, the
overrun angles (B) or (C) and thrust pieces (3) and (7).

The higher the spreading force Pax acting in both directions onto
the friction disc packages, the higher is the locking value of the
plate-type limited slip differential.

The preload built up by the cup-type spring (6) in both disc
packages causes the limited-slip differential to be operational
even if no torque is introduced yet. When the vehicle is in
motion, tha static friction torque caused by the disc package
preload is increased considerably by the spreading force that
also acts on the disc packages.

On the 911 Turbo, the overrun ramp under load (arrow B) are
designed to provide a locking value of up to 100% (spreading
force Pax + preload built up by cup springs). Under acceleration,
ramp angles of 0 degrees have been selected on purpose (arrow
C), i.e. no spreading forces are built up when accelerating or
when driving at constant load. The locking value of 20% is built
up exclusively by the cup-spring preload; this locking value
remains unchanged irrespective of the traction load (spreading
force Pax = 0 + preload caused by cup springs).


1 – Limited slip differential housing

2 – Differential bolt

3 – Right-hand thrust piece

4 – Outer disc (steel)

5 – Inner disc (Steel with molybdenium [sic] friction coating

6 – Cup-type spring

7 – Left-hand thrust piece


A – Direction of rotation in forward motion

B – Overrun angle – during deceleration

C – Overrun angle – under load

Pax – Axial force built up during deceleration

© October 1990 Dr. Ing. h.c. F. Porsche A.G.


This arrangement is counter-intuitive and is probably designed to
address some Turbo handling quirk under acceleration. Who has
something to add?

Geary & Jason, can you add some more about the change from
the inner discs having the coating to the outer ones having it.

What is the “Valeo friction lining?”
Where in the LSD does Porsche use brass?
What are the other materials?

Best,
Grady

[Geary]

The outer discs never did have the coating --- only the inners. The splines and diameter of the discs and plates changed (gradually among the different models) during the mid-'90s.

The "Valeo carbon" has to be the fibrous (reconstituted horse puckey) discs that normally come in the street 993 NA, which are worthless after a few thousand miles (or a single track weekend). The "brass" friction discs came in the 993 Turbo, and probably a few other models. Chatter is a common complaint with these brass fricton discs.

Either brass or fibrous discs can be changed out for higher quality plasma-sprayed (moly) discs.

The percentage of lock-up is obviously affected by a number of factors, and is not quite as simple to pin down as many believe. There are differing opinions of the "best" way to set LSDs up, which affect percentage of lock-up. Thick (or double) Belleville washers and high static preload vs thin Bellevilles and low preload with more aggressive ramp angles, etc. etc.

Interesting to note that some early Porsche LSDs didn't utilize a friction material, and relied on ramping effect to press the plain steel inner & outer plates together sufficiently to provide lock-up.

[KobaltBlau]

Originally posted by Grady Clay

...
This arrangement is counter-intuitive and is probably designed to
address some Turbo handling quirk under acceleration. Who has
something to add?
...

Maybe I'm not adding anything new, but ... I remember seeing some information about this in Frere's book. The 100% locking factor on decel is intended to take a bite out of lift-off oversteer. Remember this was introduced at the same time as the 964's unique trailing arms with the axially deflecting bushing outer and spring plate style inner, which produced toe-in in lift and braking conditions among other things. I guess Porsche was on an offensive against the tail in the bushes handling reputation during this time (and I'm glad my model doesn't have these tricks).

[spooldup]

While I am trying to eliminate the noise with different additives and oils this weekend am I going to wear on the LSD parts or hurt anything?Thanks again for all this info!

[KobaltBlau]

nah. The additives you are trying to reduce the shudder with should make it easier on the LSD, not harder. any reasonable gear oil (as a base) should prevent the LSD from undue wear.

[350HP930]

Wow Grady, you are certainly the porsche limited slip guru.

I am glad to know that those molly sintered disks that I have are the preferred friction material.

[Grady Clay]

350HP930, hopefully you kept the adjustment shims with their
respective bearings and location on the differential. What were
your measured values for all the discs? Which ones had moly
coating? What is the type number of your transmission? Was it
originally a M220 LSD option?


Speaking of the original question, I ran across this from Porsche
dealing with noisy 40% LSDs in a 944 Turbo:

“Under normal conditions, install the diaphragm springs (A) with
the raised section pointing outward towards the knuckle flange
[CV joint].

"

“Note
If there is noise, the manufacturer turns the diaphragm spring
(left A) on the housing side. These limited-slip Differentials are
identified by a yellow dot (B) on the differential housing.

" "


I wonder if this solution is applicable to your 930 LSD?

Best,
Grady

[350HP930]

Yup, I do have my shims organized to keep the ring gear positioning the same, though it may be a moot point depending on what the lash measurements turn out to be for the new R&P from andial.

I measured all the friction disks and they did measure the same (except for a few scratches on a couple smooth disks everything appears almost perfect) but unfortunately I can not remember the thicknesses.

As far as the type number of the tranny goes, where would I find it?

[KTL]

There's some major knowledge going on in this topic. You guys rock.

I believe the trans. type number is on the bottom of the trans. near the drain plug? That's where it's located on G50 and 915 trans. that i've looked at.

Yours should be a 930.36 trans with a 77H##### serial number where the ##### is 00001 thru 10000

The 77H##### means:

1st digit, 7 = trans. for 6 cyl.
2nd digit, 7 = trans. type = 4 speed 911/930
3rd digit, H = model year 1987
4-8 digits = serial number