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Do you know what it is that will not allow it to bolt up? Is it that the bolt holes are in different locations? Or is the differential longer or shorter?
Last edited by S4-on-your-back-door; 10-17-2023 at 04:41 PM.
Do you know what it is that will not allow it to bolt up? Is it that the bolt holes are in different locations? Or is the differential longer or shorter?
Find a late model limited slip, for your S4.
Rebuild it, install the ring gear from your existing differential, and have it installed. (Bearing preload and backlash must be very accurately set, when changing any differential.)
Sell the early limited slip to a person with a '78 tp '82 928 who thinks they need a limited slip.
The difference between the two units, in terms of performance, is so incredibly tiny that the number of people, on the planet, that could positively tell you which one is installed (by driving the car) is incredibly small.
Greg,
is there another option for a lsd from you beside the Porsche options?
I have here a new G28/10 manual gearbox without LSD, if I recall your meaning here the before PSD units are a solution with mediocre performance.
So if going this way of changing the diff is the only way?
Greg,
is there another option for a lsd from you beside the Porsche options?
I have here a new G28/10 manual gearbox without LSD, if I recall your meaning here the before PSD units are a solution with mediocre performance.
So if going this way of changing the diff is the only way?
The 928 limited slips are all "friction style" limited slips.
Internally, there are moly coated friction discs, under pressure (which needs to be set-up when rebuilding any of the friction style limited slips) that provide the resistance to keep both rear tires spinning at the same rate.
There are also internal "clutch" plates, which the friction discs rotate on.
The friction discs have internal splines and the internal "clutch" plates have external splines. They can not rotate together as a unit....these plates (as a unit) make the internal friction.
All of the limited slips ('78-'89), as they left the factory, are suitable for rain/snow conditions. (The factory continued to call them all 40% limited slips. At best, they were all 20% limited slips. Some were barely 10% limited slips.)
They will provide power to both rear tires, up to a very limited point.
They are not anywhere near a performance limited slip, as they left the factory.
Here's the differences between the different years of limited slips:
The "early limited slips" ('78-'84) have four total friction discs (two on each side.) As the left the factory these friction discs were installed touching each other.
The result is that although there are four total friction discs, there is no outer clutch pieces between them. This makes the available friction surface four total surfaces (two friction discs touching each other equal two friction surfaces.) Two per side. Four total friction surfaces.
Internally, there is enough room to make changes.
The later limited slips ('85-89) have two total friction discs (one on each side.) These left the factory with a clutch plate on each side of the friction discs. Four total friction surfaces. (Same as above.) Internally, there is no way to add any more friction discs or make an changes.
Anyone who has swapped out an "stock late limited slip" for a "stock early limited slip", using some "custom offset compensation pieces", has wasted their time and money. They have been "conned", into thinking that there was an advantage.
Building a "performance limited slip":
On the early limited slips ('78-'84), it is possible to separate the two friction discs with an outer clutch plate. (You will need new friction discs, as one side will be worn and the sides that touch each other will be brand new, making then unsuitable for performance use.)
This will allow there to be eight total friction surfaces touching outer outer plates, essentially doubling the available friction ability, from stock.
This arrangement of friction plates with outer plates converts these limited slips into TRUE 40% limited slips.
There are downsides to doing this.
There are also important limits to the amount of friction pressure during "set-up".
A transmission oil cooler (friction results in heat) may be required.
I have now updated my option 220 description for the VIN decode thread:
Limited Slip Differential, 40% (but actually somewhere between 20% and near-zero..)
Porsche has been calling limited slips 40%, virtually forever, regardless of the amount of friction required to make one actually "slip"
Very early 911 four friction disc 911 limited slips 40% = 7.2 to 10.8 ft.lbs. of release force
Later early 911 four friction disc 911 limited slips 40%-50% = 30 to 57 ft.lbs. of release force. Above 40 ft.lbs., see note below.
Later 911 competition limited slips 75% to 80% = 43 to 85 ft.lbs. of release force. (Almost undrivable, by anyone but drivers of extremely high caliper. As the temperature of the transmission changes and corner entrance/apex points change, the release point (on the track) of the limited slip will vary considerable. The skill required to control oversteer and understeer, from these varing points of release (on the track) are considerable. Oil cooler absolutely required. Frequent rebuilds required.)
G50 and later limited 40% limited slips= 7 to 25 ft.lbs. of release force.
"Early" 928 40% limited slip (4 friction discs. two sets touching)=4 to 14 ft.lbs. of release force
"Late" 928 40% limited slip (2 friction discs) = 7 to 25 ft.lbs. of release force.
Needless to say, 40% locking effect has a relatively broad range of actual locking ability....
Choose wisely, not emotionally.
.....Or by reading the Internet.
Here's a video of me misquoting the spec on an early rebuilt LSD from my '79, shoulda said 4-14, not 7-25... This is what the breakaway torque on a rebuilt (ie- flipped the discs) early LSD looks like:
So to recap on options. If one already has a early LSD with the P/R gears and wants to install it in a late model 928 Auto.
1. Install both early model pinion and ring gear in the late model AUTO TRANS.
2. Send your ring gear to a machine shop and have them Mill off 5 mm off the
back side of it. Bolt that to the early model LSD.
3. Sell it and buy one from Mark Anderson 😃
Porsche has been calling limited slips 40%, virtually forever, regardless of the amount of friction required to make one actually "slip"
Very early 911 four friction disc 911 limited slips 40% = 7.2 to 10.8 ft.lbs. of release force
Later early 911 four friction disc 911 limited slips 40%-50% = 30 to 57 ft.lbs. of release force. Above 40 ft.lbs., see note below.
Later 911 competition limited slips 75% to 80% = 43 to 85 ft.lbs. of release force. (Almost undrivable, by anyone but drivers of extremely high caliper. As the temperature of the transmission changes and corner entrance/apex points change, the release point (on the track) of the limited slip will vary considerable. The skill required to control oversteer and understeer, from these varing points of release (on the track) are considerable. Oil cooler absolutely required. Frequent rebuilds required.)
G50 and later limited 40% limited slips= 7 to 25 ft.lbs. of release force.
"Early" 928 40% limited slip (4 friction discs. two sets touching)=4 to 14 ft.lbs. of release force
"Late" 928 40% limited slip (2 friction discs) = 7 to 25 ft.lbs. of release force.
Needless to say, 40% locking effect has a relatively broad range of actual locking ability....
Choose wisely, not emotionally.
.....Or by reading the Internet.
Geez, compared to an OE E30 90's BMW in our spec racing, 60FtLbs.
In my early 1st gen RX7 racing, there was no rule, we would shim the pack with beer can aluminum to get 80-90lbs.
Greg, you‘re a living 928 and racing encyclopedia!
At the beginning of my 928 ownership in 2007 my car was an auto without the lsd and the tires weren‘t on the grip level from the actual Performance tires like a Michelin Pilot Sport 4.
I know several occasions my back of the 928 did a 180 degree spin after driving around a wet corner.
Maybe the auto box had a harsh kickdown behavior. Driving a five speed is without this surprises.
At the moment I sell my to big homestead and will build a loft above my Porsche wrenching place in my barn on my second farm so my time is limited.
It‘s a lot about family tradition so it‘s for me at the moment not so easy to work on complicated new technical projects on the 928.
Mother and siblings are emotionally involved and my wife and I had a lot too discuss with them to beware the good relationships in our family.
Than is where the engine damage on my tractor, I think the shop will need time till Christmas for te repair.
After bad starting behavior they changed all injectors. We used the machine after the repair 2-3 hours and had suddenly very rough engine behavior.
Called the shop and they found additional 10 gallons diesel in the oil pan.
Now the drama begins, all new injectors deinstalled and on the test bench, no failure so we had no failure for the five year insurance of the machine.
Injectors back in the machine but on the test drive the engine don‘t run well.
Newest theory is the oil return line in the head is broken because the head has a small crack. So engine again apart on this 14 ton beast.
Not to easy to beware Contenance in this case. The master and his team in the shop are doing their best . I think we have here a minimum 50.000 € damage.
My 928 hobby is good release for all the stress, nice to have this great community all around the world.
I know a bit about Porsche stuff.
55 years of doing not much else than work on Porsches and a photographic memory has a tendency to do that.
Your tractor engine intimidates me.
Plus, climbing up and down every single time you need a different tool sounds like way too much exercise for this old man!
10-17-2023, 04:09 PM
