Can a ZF 915 LSD locking rate be modified?
 

I've read through countless threads on this topic but I can't seem to find agreement on this. I just sourced one from an 84 that is set at 40%. Can I make any adjustments to get it to 80% (for a track only car)?

Yes, it is very simple. It's explained in the factory manual. You just change the order of plates. Get the parts from Paul Guard. He may be able to supply the manual pages too.

Have fun!

There were single plate (friction) per side LSD's that can't be made into 80%. IIRC the later Carreras had these. And all the 964/993 too.

Someone correct me if I'm backwards on the years, but I recall the SC's had the 4 friction plates (total) and the later Carreras came with 2 total.

If you've got the 4 frition LSD, then all you need to do is rearrange the discs so that the 2 frictions on each side are not touching, but rather have a static plate in between them, which doubles the friction surfaces.

You may want to reduce the pre-load when making the switch to 80%, depending on how you want the car to feel.

Right on, Tyson,...:)

Tyson,
What does reducing the pre-load do? How does it affect the lock-up characteristics?

I bought mine from Hayden and he set it up for me, but I've always been curious what it means. Mine is an 80 / 80 ZF w/ load pre-load, per his suggestion.
Thank you,

Doug

Hmmm, I need to check which one I bought. I thought they were all the same for the 915. Darn. If I have the type with only 2 plates, can I simply add two additional plates? I'm sure it can't be that simple.

Here is a cross section of a 915 LSD
http://forums.pelicanparts.com/uploa...1177976521.jpg
Preload is set by 2 mechanisms
1)the outer disc(#2)which has a dished profile
and
2)the thickness of the stack of the other inner splined(2 different thicknesses) & outer outer splined(3 different thicknesses) disks(#12 & 13)
the % lockup is determined by the way the outer and inner splined disks are arranged
the sequence reading from outside to in, on a side of; dished outer, 2x inner, 2x outer, 1x inner gives a 50% lockup

the sequence 1x dished outer, 1x inner, 1x outer, 1x inner, 1x outer, 1x inner gives an 80% lockup factor

but that still doesn't tell the whole story because the preloads can change the breakaway torque from 30 -57 ft-lp on a 40-50% to 43-85 ft-lb on a 75 or 80% diff these high breakaway torque diffs are considered to be too inconsistant for many apps and are not particularly desirable, neither are the later diffs described in the next section.

later street diffs(~84 Carrera -964 and 993) only have 1x dished outer, 1x inner, 1x outer and have considerable lower breakaway torque than earlier LSD w/ the same % lockup.

Additionally PMS diffs starting w/ 964T and 964RS and street 993 have asymmetric lockup on acceleration and deceleration. Asymmetric lockup is achieved form varying the ramp angles on the thrust rings. the PMS diffs have the 4 disks per side the street only 2.

how would you define breakaway torque?
how would you define pre-load?

I thought Hayden set mine up with very low break away ~ on the order of 20 ft. lbs w/ 80 / 80. I believe mine is an early ZF unit due to the idea that there is a "stack" of lock up discs.

Doug

You need to put the LSD into a bench vice, meausur ethe torque needed to turn an output flange.

It's confusing because the 4 disk early 40% will have much higher breakaway torque than a later 2 disk 40%. The difference comes from the internal friction of the extra plates(and possibly higher preload) they are both called 40% though.

Sounds like I might have the wrong one for my track application. I could probably get by with the 40% - although not ideal, but the low breakaway torque sounds like a problem. Can you identify the 2 versus 4 from part number?

Are there any shops that specialize in these things?

Send it to Paul Guard at GT or Hayden at Wevo

Reducing the pre-load will make the car more neutral during maintenance throttle. It will allow you more control of the amount of lock-up by modulating the throttle.

If you run a high pre-load, it will not tuck in as well when you go from throttle-on, to maintenace throttle. But the plus side is, it will be more stable during breaking when the clutch is disengaged and there is ne decel or accel going on.

So you'd probably want high pre-load at Willow Springs, and low pre-load at Streets of Willow for example.

Arguably, there might be more situations where a lighter pre-load is desirable. But that depends on a lot of factors, plays heavily into the entire set-up of the car.

HI sfoster13 do's your diff look like this,

http://forums.pelicanparts.com/uploa...1178032208.jpg

http://forums.pelicanparts.com/uploa...1178032239.jpg

if it is then you can adjust it , if not it is the cast diff and you are stuck with the 40%, the one on the photo is for sale on DDK- online for "euro 1500".
what is the part No' of the diff??

regards mike

it doesn't look like that

May I ask:

What type of LSD would this be:

915/63/12 - 74C/2841

It came with the 1980 911 SC from the start.

Any thoughts on what % this might be?

That would be a 40% w/ the higher break away torque than the later 40% version