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LSD identification


gwb72tii
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Some time ago I picked up a LSD for my tii off a seller on Ebay. The LSD is in an oem 2002 differential housing, but without the normal identifying marks you would expect on a 2002 LSD. That leaves me wondering if the seller, who was racing 2002's at the time, built the LSD from scratch in the oem housing. The diff is in my tii and was a straight out swap.

My question is how I might identify if my diff is a 25% vs 40% or something else. And why I should care?

And secondly exactly what 25% or 40% means? lol

I should add  the diff is a 3.90:1 and my car is a 4spd.

 

Thanks for any help

 

Edited by gwb72tii
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The only way to be 100% sure is to take locking unit apart and see its internals. Case has ZF part number ending with last tree digits like S25 or S40 etc. That tells only what it had been from factory and it might have been rebuild to other specs.

 

25%, 40% or 75% refers units "lock up". 100% is spool = no slippage between rear wheels.

https://drivetribe.com/p/how-limited-slip-diffs-make-you-aKo9i9p4TdiBurut_Y4zSg?iid=ferTwyYQSLaxAe9gHGgj4Q

 

If you happy with units performance you shouldn't care one way or the other.

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The casings are identical, except for the stampings on oem lsd diffs but you could stamp the numbers on. I bought a second  second hand diff for my E30, that was an expensive object lesson. If its important that it works, I would have it checked to ensure that the clutches are good and the contact angles are good, i.e. it will apply the correct rate of lock up. My E30 diff hat shot in it from blasting the case, the ramp angles were wrong and it was generally speaking improperly assembled...so while all the parts were new it was not set up right and would have failed (ignoring the blasting medium getting washed into the roller bearings). A 25% is the standard road going diff, 40% is somewhere in between road and fast road, Toby and others can share views on the higher lockup %. For normal use, I would say proper function is more important than the %, both are ok. The clutch pack (clutch and washers)varies by the lockup you want to achieve, the friction plates have obvious signs of incorrect wear and if worn out its just an open diff. I would take it apart before installing. If it is newly and properly buikt, you should check the oil level after install and run some slow figure 8s in a parking lot for 15-20 minutes, then swap out the oil again. The abrasives from the clutch are in the oil after the initial embedding and damage the diff....you can strain filter and reuse though.

Andrew

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I just weld mine.  Bkuz race car.

 

I too always wondered how precise the 'percent' number could be-

I've had 'junkyard' LSDs in track cars, and they did pretty well under

lapping day conditions. 

 

I mean, at the factory, you can set preload and ramp angle and

such, but in 1st gear, there's 3x as much torque available

as there is in 4th, and those liddle widdle clutches just aren't

going to transfer 25% or 40% of the 1st gear (or more realistically,

2nd gear, really) torque for very long.

 

Personally, getting more clutch area is the best argument I can

see for going to a 188mm diff, myself.

 

t

cheap- ass.

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My E30 has the 188 case, you can double the clutches in the 188 case, so you have 4 in total and milling the plates to adjust the ramp angle determines to my understanding how progressive the lockup is, not the %. A guy that builds racing drivetrains built mine, after I had bought it "rebuilt" from a well meaning but inexperienced E30 fan. My builder had a lot of time on his hands, since noone is racing at the moment here, so he explained a lot about which surfaces are key, how they are milled etc. There are even petrol and diesel LSD units for the E30, which are different. He says that the oem clutch packs are really good, they are interchangeable with the ones Porsche sells as oem, so he usually uses those. I don't know if you can double up in the small 168 case, components are all different. I would agree with Toby on the size of the clutch pack in the 168 diff, but my 168 had a broken pinion, clutch pack was ok.

Andrew

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On 12/23/2020 at 1:49 PM, gwb72tii said:

 

... My question is how I might identify if my diff is a 25% vs 40% or something else. And why I should care?...

 


You probably shouldn’t care. If you occasionally but regularly track or autocross the car, you might prefer the 40% lockup. But if the car is strictly for street use, you’ll be fine with a 25% unit.

 

The factory ‘02 LSD’s were 25%, while the factory e21 units were 40%. Oh, wait, that’s probably backwards. 😯😏😉

 

Best regards,

 

Steve

 

 

 

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2 hours ago, Conserv said:

The factory ‘02 LSD’s were 25%, while the factory e21 units were 40%. Oh, wait, maybe that’s backwards. 😯😏😉

It is backwards indeed.

The Ramp angles are different but you can omit the solid shims in a factory 40% 02 LSD and add some bellwashers ans spacers to get some preload or more friction discs to make it something like 60% with preload or 75%

Adding more than 40% is on fast tracks with lots of grip inducing understeer.

 

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1 hour ago, uai said:

It is backwards indeed.

The Ramp angles are different but you can omit the solid shims in a factory 40% 02 LSD and add some bellwashers ans spacers to get some preload or more friction discs to make it something like 60% with preload or 75%

Adding more than 40% is on fast tracks with lots of grip inducing understeer.

 


Thank you, Uli!

 

Merry Christmas,

 

Steve

 

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