If you want to build a better limited slip

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03 Jan 2012 21:02 #7613 by 928mac
For Example if you have a used LSD unit and you want to turn it into a Greg Brown Limited Slip Differential, then this is what you do

Greg gave me his ok to post this.
**************************


They, Porsche, "bounced" around quite a bit on what pieces they installed. Lots have two friction discs, but they are touching each other, without an outer lamina between them. The result is the same...there are only two friction surfaces touching outer lamina on each side.

If the friction disc is completely toast. The moly coat is worn down so that the "peaks" are almost all gone and the surface is virtually worn flat. The next step is for that surface to start "smearing" the peaks and valleys together.

If you want to build a "better" limited slip, buy yourself 4 new inner friction discs, instead of just the two....from Porsche...or from someone that bought them from Porsche (do not use aftermarket friction discs....they are not of adequate quality.)

Assemble each side this way, starting from the "inner pressure ring" and working your way towards the outside:

Inner Pressure ring
Inner friction disc
Outer lamina
inner friction disc
Outer lamina
Spring washer

Start with 1.9 or 2.0 outer lamina pieces.

Assemble the entire unit, using a gear oil with the proper friction modifiers that work with this style limited slip. Mobil 1 LS is easy to get and ecconomical (Auto Zone).

Bolt the entire thing together. Install 6 temporary bolts and nuts (with washers on each side of the differential to not damage the surface where the ring gear sits), where the ring gear normally bolts (to make sure the unit is completely squeezed together.

Install a CV stub axle on each side. One stub will have two 10mm bolts (or studs) inserted 180 degrees apart. You will clamp these two bolts into a vise.

For the other stub, you will need to make a little flat plate that bolts across the flange with two 10mm bolts 180 degrees apart. In the center of this plate, weld a 17 or 19 mmm headed nut.

This nut will allow you to turn the limited slip with a torque wrench. measuring the amount of friction the unit has. You can turn it 20-30 times to get the inner friction discs to "settle" in. Now use a torque wrench to turn the unit. I think that the "ideal" friction, for new friction discs is right about 40 ft lbs. You can switch the "outer lamina pieces" to adjust this pressure. They come in 1.9, 2.0, and 2.1mm thicknesses. You should be close, if you assemble the unit like I described. If your pressure is very low, install new "spring" washers on the outside...they get "soft" over time.

If you don't understand, I can take some pictures of the flat plate with the nut all mounted in the vise.
I've got a couple of these things to "set-up", next week.

If you have any trouble finding any of the pieces, I stock all the pieces, multiple times over....including new prssure rings, inner friction discs, outer lamina (in all three sizes), spring washers, and thrust washers.
__________________
gb

Greg Brown

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03 Jan 2012 21:51 #7616 by pcar928fan
Replied by pcar928fan on topic If you want to build a better limited slip
NICE! GB is THE MAN!

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

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23 Feb 2012 15:37 #8309 by turbohugh
Replied by turbohugh on topic If you want to build a better limited slip
Im still trying to visualize this.

Will this work in the 'open' diff also?

Thanks in advance,

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23 Feb 2012 15:46 #8310 by 928mac
Replied by 928mac on topic If you want to build a better limited slip

Im still trying to visualize this.

Will this work in the 'open' diff also?

Thanks in advance,


in a word (no) ;)

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25 Mar 2012 19:38 #8602 by Normy1
Replied by Normy1 on topic If you want to build a better limited slip

For Example if you have a used LSD unit and you want to turn it into a Greg Brown Limited Slip Differential, then this is what you do

Greg gave me his ok to post this.
**************************


They, Porsche, "bounced" around quite a bit on what pieces they installed. Lots have two friction discs, but they are touching each other, without an outer lamina between them. The result is the same...there are only two friction surfaces touching outer lamina on each side.

If the friction disc is completely toast. The moly coat is worn down so that the "peaks" are almost all gone and the surface is virtually worn flat. The next step is for that surface to start "smearing" the peaks and valleys together.

If you want to build a "better" limited slip, buy yourself 4 new inner friction discs, instead of just the two....from Porsche...or from someone that bought them from Porsche (do not use aftermarket friction discs....they are not of adequate quality.)

Assemble each side this way, starting from the "inner pressure ring" and working your way towards the outside:

Inner Pressure ring
Inner friction disc
Outer lamina
inner friction disc
Outer lamina
Spring washer

Start with 1.9 or 2.0 outer lamina pieces.

Assemble the entire unit, using a gear oil with the proper friction modifiers that work with this style limited slip. Mobil 1 LS is easy to get and ecconomical (Auto Zone).

Bolt the entire thing together. Install 6 temporary bolts and nuts (with washers on each side of the differential to not damage the surface where the ring gear sits), where the ring gear normally bolts (to make sure the unit is completely squeezed together.

Install a CV stub axle on each side. One stub will have two 10mm bolts (or studs) inserted 180 degrees apart. You will clamp these two bolts into a vise.

For the other stub, you will need to make a little flat plate that bolts across the flange with two 10mm bolts 180 degrees apart. In the center of this plate, weld a 17 or 19 mmm headed nut.

This nut will allow you to turn the limited slip with a torque wrench. measuring the amount of friction the unit has. You can turn it 20-30 times to get the inner friction discs to "settle" in. Now use a torque wrench to turn the unit. I think that the "ideal" friction, for new friction discs is right about 40 ft lbs. You can switch the "outer lamina pieces" to adjust this pressure. They come in 1.9, 2.0, and 2.1mm thicknesses. You should be close, if you assemble the unit like I described. If your pressure is very low, install new "spring" washers on the outside...they get "soft" over time.

If you don't understand, I can take some pictures of the flat plate with the nut all mounted in the vise.
I've got a couple of these things to "set-up", next week.

If you have any trouble finding any of the pieces, I stock all the pieces, multiple times over....including new prssure rings, inner friction discs, outer lamina (in all three sizes), spring washers, and thrust washers.
__________________
gb

Greg Brown


I have a problem with both tires going up in smoke if I get on it in first gear. 5 mph rolling start in first, nail it, roll about 75 feet...and then it gets into the power band and both tires loose traction. S2 5 speed, rear tires are 274/40-17 Falken RT-615's.

Any suggestions? If I could "hook up" the car would be way easier to drive!

N

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25 Mar 2012 23:01 #8604 by 928mac
Replied by 928mac on topic If you want to build a better limited slip

I have a problem with both tires going up in smoke if I get on it in first gear. 5 mph rolling start in first, nail it, roll about 75 feet...and then it gets into the power band and both tires loose traction. S2 5 speed, rear tires are 274/40-17 Falken RT-615's.

Any suggestions? If I could "hook up" the car would be way easier to drive!

N


Hmmm never had any one complain that they had to much power before.

To answer your question
To get traction you need soft compound tires and as wide as will fit.
Then you need weight transfer.
1 way is to go to a shock that will allow the front to pop up when you nail it, but that will also change the way the car handles and I have no idea what is available for a 928
Ask on Rennlist if you want but I don't think anyone there will help, then again, never know till you ask.
You are looking for a 60/40, or 70/30, or 80/20 shock if you get the drift ;)

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25 Mar 2012 23:41 #8605 by srshaw3
Replied by srshaw3 on topic If you want to build a better limited slip

Any suggestions? If I could "hook up" the car would be way easier to drive!
N


Install a PSD from a newer 928..... :D
Just kidding.

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30 Mar 2012 20:36 #8701 by Normy1
Replied by Normy1 on topic If you want to build a better limited slip
Yeah, that's one way to grow some grey hairs!

I wondered if maybe a Quaife differential would help, but one day...I lost traction coming off a light, and decided to go back and examine the street. Just as I thought...two short black streaks. Both tires are turning, which confirms that my pidgin test of having the woman hold one tire in her hands while I turned the other, both rear wheels off the ground told me: My posi is still working.

Tires: Falken RT-615's, which are WAY soft, and typically used by autocrossers.

Engine: 4.7 liter 2-valve, S2 with rising rate fuel pressure regulator, Louis Ott X-pipe, Porken ignition chip, and Borla twin exhaust. I have an A/F ratio gauge, and I've spent some time fixing all the vacuum leaks and tuning the car. The car was rated by Porsche at 310 hp DIN at the crank, and I cannot believe that this setup is making more than 270 hp at the rear wheels, so power can't be the issue.

The Falken RT-615 is an AWESOME tire for about 12 months or 1000 miles in my opinion; That's how much time I'm getting from a set! I drive aggressively, but rarely that fast. I'm sick of watching a late Volkswagen GTI pace me to 60 mph because my rear tires are spinning like a stern-wheeler boat!

Thoughts:

N!

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30 Mar 2012 22:37 #8704 by 928mac
Replied by 928mac on topic If you want to build a better limited slip
Norm you do realize that 2 tires spinning is better then 1 tire going up in smoke. can you verify that it is starting out in 1st and not 2nd?

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02 Apr 2012 21:29 #8756 by pcar928fan
Replied by pcar928fan on topic If you want to build a better limited slip
Brad, he states his car is a 5spd, so I am pretty sure he knows if it is in 1st or 2nd...

I am guessing old hard tires...

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

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03 Apr 2012 16:00 #8765 by 928mac
Replied by 928mac on topic If you want to build a better limited slip

Brad, he states his car is a 5spd, so I am pretty sure he knows if it is in 1st or 2nd...

I am guessing old hard tires...


Thanks James I missed the 5 speed fact duuu. :o

I agree that tires have a lot to do with it, when my tires are near finished it is real easy to go up in smoke

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14 Oct 2012 18:38 #11843 by Normy1
Replied by Normy1 on topic If you want to build a better limited slip
Brad if you can post a picture of what is being talked about regarding the rear limited slip in your first post I'll slide you a beer or 4-

Thanx again!

N

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14 Oct 2012 22:10 #11845 by 928mac
Replied by 928mac on topic If you want to build a better limited slip

Brad if you can post a picture of what is being talked about regarding the rear limited slip in your first post I'll slide you a beer or 4-

Thanx again!

N


Normy;

This repairupgrade is a greg brown fix.
For more details or pictures you will have to contact Greg Brown

Brad

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