Adjusting 930 LSD power/overrun split.

[Russ Noble]

Well, I've just pulled my LSD to bits. It's the first LSD I've ever looked at. Everything looks good and measures up nicely and it appears easy enough to tighten it up a little. But what's got me beat is how to change the power/overrun split.

Standard Porsche has much more locking on overrun than it does with power on, in order to help combat oversteer during turn in, which is induced by the motor hanging way out the back. I believe the standard lock up is 80% on overrun and as low as 20% under power. Although this can be changed to as much as 80/80.

What I want to know is how to change the split. Paul Bearman and Pete T have been good enough to send me copies of various factory manuals but the manuals don't seem to address this issue. I believe the LSD units are the same from 915 through to G50. I have tried searching various Porsche forums but haven't come up with anything.

I would like to start off with the total opposite of the Porsche split i.e. power 80% overrun 20%. (80/80 might be OK but my geriatric pitcrew are likely to revolt at having to push it around in the pits! )

If I could figure what the mechanism is that achieves the split, I'm sure I could work out how to alter it. But I've looked at the bloody thing endlessly, tried assembling it different ways to see what happens but nothing has yet clicked in the old grey matter.

I'm sure it's dead simple, I'm just too thick to see it!

Any info much appreciated. Thanks.

 

[Jac Mac]

Perhaps a few pics of the mechanism for those of us who have never had to lower ourselves to this level of technology before could see what you are trying to modify/acheive.

BTW, by the time you get this lot done most of your geriatric crew will probably be enjoying the generous pension/retirement plan that I am sure you have put in place for them as a caring responsible employer.

 

[Russ Noble]

Well Jac Mac, it must be rocket science! But it works this way:- The ring gear turns the diff/LSD housing which drives the tabs on the two halves holding the 4 spider diff. This works as a normal diff. The LS effect torque transfer is achieved by a clutch pack which drives from the housing onto the diff output gear.

1st pic is diff and slip assembled up without housing.

2nd pic is housing. You can see the four slots which drive the tabs on the drive plates and diff assy.

3rd pic is is one side laid out in order. Diaphram, 3 drive plates, driven plate, drive plate, diff. The other side is the same mirror image. The drive plates and driven plate are available in three different thicknesses 1.9, 2.0, and 2.1 to adjust the transmitted torque to spec. Probably easier just to shim it up if nothings worn. The three drive plates together seem like a prime target to replace the middle one with a driven plate which would then act like a twin plate clutch and transfer more torque to the loaded wheel. 351 cubes and 14" tyres could benefit from a lot more LS drive.

Last two pics are just photos of the diff itself

That's all easy enough, I just need someone to figure out how the power/overrun LS split is achieved.

re. the pension plan. I am working on some "black gold" for the crew. Containers of used racing oil. Each team member will get 5 litres as a golden handshake on retirement. Even guys that would put P****** clutches together backwards!

 

[Jac Mac]

Russ, you missed one point in the power flow, the 4-lugged plate housings also drive the cross with the spider pinions which in turn drive the side gears axles- now note the ramps that the 'cross shafts' climb up on to apply clamping force to the clutch plates under differential action- now if those 'ramps' had different angles to apply the 20/80 forces under deccel/accell or whatever then fine, but to me they look to be the same angle & therefore would apply the same amount of clamping force in both directions. Since the clamping force applied by the ramps/side gears is virtually fixed the best way to acheive a more effective LSD is to increase the number of plates per side. ( that increases the friction surface area ).

P.S. There should be more driven plates-The ones with the friction material & splines at ID.

 

[Russ Noble]

Yes Jac Mac, I figured the thing was set up 'square' with the cross shaft ramps. I was looking for something else to change the split. Maybe you have to change the 4 lug plate housings completely to alter the split. This is a trans that I got from local P***** racer Austin Cole and it may have an 80/80 setup. From the wear marks on the output gear there has only ever been one driven disc per side.

I think I need to pull the LSD from the trans I got from Bill Musarra and compare. If it has different, maybe assymetrical,ramps that could provide the answer. That being the case it's just a matter of swapping them from side to side to get a GT40 setup.

Jeez, everything on this car is a mission and everything is in bits. I'm sure the light at the end of the tunnel is the proverbial train coming the other way!

Cheers

 

[EGLITOM]

Russ

Take a look at this article, it explains the function very well. AThere is also a how to change the push pull ratio.

According to this i would be carefull changing the pull ( 20%) to push(80%) ratio.
From what i read, there are different LSD from Porsche available. The newer ones have a dynamic push ratio (from 0% up to 100%, depending on the load) so with one of these you would be fine in your pits and on the track as well.
Don´t know if yours is of the later type. But may be the articlel helps you to find out.
I found it very helpfull. Initialy i wanted to go for an ATB type Diff but now decided for a LSD.

Hope it helps.

Guard Transmission - Velocity

thanks
TOM

 

[Russ Noble]

Thanks Tom,

That's exactly what I was looking for.

That confirms mine's an early symmetrical one. Guess I'll just double up on the drive plates and tweek up the tension a bit in the mean time. Can't see me outlaying the dollars on different thrust plates! Could experiment by machining reduced ramp angles, although I think modifying the existing is a bit trickier than it looks. It's going to be one of those things that's easy enough to change and play around with in the week or so that we generally have between meetings. Constant fettling will get the right compromise for my setup, eventually.

Thanks.

 

[Dave Deerson]

I thought the ramp angle of the side pieces that ride on the cross pinion and apply the side force to the clutch packs determine the lock-up split?Therfore the ratio is fixed unless parts re changed.I may be wrong.
Dave D.

 

[Pete T]

Russ, one of the things that determines the locking ratio is the plate stack-up. Most street cars are set like to one in you picture.

Here is the stack up on the stock G31:

If you notice there are 2 discs that are side by and and two plates also. By placing ont of the discs between the two plates that are together you change the locking ratio. In this case you will double the holding force of the clutch pack by doubling the surface.

The way it is im the picture is 40% locking. Swapping the plates turns it into an 80% locking LSD.

 

[Russ Noble]

Thanks Pete and Dave, confirms what I'd figured.

 

[Bill Musarra]

Russ,
Check with our "Porsche Guru" in Calif. He should know. I seem to remember in a conversation on just this matter several years ago with him that it is the shims or clutch packs that make the difference. Can't remember how it went. I will ask my shop forman here tomorrow as I was planning to call him anyway about another matter.

Bill

 

[Gregg]

Bill, how about a personal favor, take off your rose colored glasses and please stop calling Mr. Craig a "Porsche Guru". He is nothing of the sort. On top of having honesty issues, I asked him the following question, which HE COULD NOT ANSWER. One would think that a "Porsche Guru" as you have so cutely named him would know the answer to.

I have a 3.3 twin plug engine going for rebuild. The original cylinders have fins cast approx half way round. The later turbos have fins cast all around the cylinder. I asked him if I could use the later cylinders on the engine. Guess what, he doesn't know.

For anyone new to the forum and would like to read about MY experiences with Bill's friend, please go to the feedback, buyers, sellers & vendors section and read the sticky, Beware of David Craig.

Ron, I see the sticky was moved from the transaxle section. I was unaware of this and think it may be better suited if it was also copied back to the transaxle section.

Russ, if you get stuck, send me an email and I will check with my Porsche contacts.

 

[BruceA]

Hey Russ, could you do me a favour please. Can you measure the outside and inside diameter of the clutch disks and the outside diameter of the 4-spider setup.
Cheers
Bruce

 

[Jim S.]

Tom,

Thanks for posting the Guard Transmission link very good info rockonsmile

 

[EGLITOM]

Jim, you´re welcome.

Unfortunately i just know the theory. I´m still searching for a LSD for my G50/05
Either 20/100 or 45/65 would be very welcome. Any idea ?

TOM

 

[Russ Noble]

Hey Russ, could you do me a favour please. Can you measure the outside and inside diameter of the clutch disks and the outside diameter of the 4-spider setup.
Cheers
Bruce

Sure Bruce,

The clutch driven discs are o/d 88.1; spline major diam. 48.0; minor diam 43.8;
Clutch drive discs o/d 87.8; i/d 49.0; o/d of the four drive lugs 102.0;
4 spider setup o/d of thrust plates (spider carrier) 90.0;length of spider cross shafts 88.7; o/d of the four thrust plate drive lugs 102.1.

Hope that gives you everything you want.

Cheers,

 

[Russ Noble]

Damn, you notice that my slip only has one driven disc per side. I was going to whip my second 930 LSD to bits and get some more discs out of that. Great idea, pulled it out but it's an open diff! Bummer. Never mind that's the luck of the draw. Now I'll have to hunt around for a couple of discs. Wonder whether I should just weld the spiders up and fit that?

 

[Jac Mac]

PLEASE DO, I need a laugh.

 

[Russ Noble]

Last time we talked about that you didn't think you would be LAUGHING pushing it round the pits! IIRC that thought caused a union walkout! :lol:

 

[Cliff Beer]

When I was in high school I bought a bmw which was fitted with an LSD. Unfortunately, the car had been sitting for about three years and the friction plates in the LSD had all rusted together. Sure made the car drive funny although I did note that burn outs created perfectly symmetrical left and right black lines on the pavement - 100% lock is very effective for that....