ADias

How can we test for a worn LSD?

RonCT

I remember a tech telling me to lift the car, rotate one rear wheel and the other should firmly rotate in the opposite direction. He also said something to the effect that a couple of DEs will toast OEM LSD, so if you bother to rebuild, have somebody like Guard provide upgraded friction disks.

cello

Another test: Lift car so 1 wheel is off the ground, while the other remains grounded; take a torque wrench and turn lifted wheel. Should take a certain described degree of force to turn. If the lifted wheel spins 'freely', LSD is shot and needs rebuild of the clutch plates (if a clutch/plate design, like Guard). Test can be done in a simpler version by hand.

LSD can help higher powered cars on deceleration as well and that is why it is standard on the GT cars, for example. Also, when set up correctly the car is easier to rotate when trail braking.

Not to complicate matters further but there are also two types of LSDs: Plate or mechanical and torsional (gearing derived; e.g., Quaife). In general terms, the former is can be set for differing accel and decel locking rates described in %ages and is better for track use. The latter is not adjustable and is better for street/AutoX use. The other differences are that the torsional does not require servicing and does not assist on decel, IIRC.

akim47

excellent discussion. sorry to veer off topic a bit, but BMW touts their DTC or dynamic traction control (at least this is what the tech told me) as an "electronic version of LSD"... the DTC in combination with the DSC (dynamic stability control) basically delivers powers to the necessary wheel, especially in a track situaion.

My question is since this is contolled electronically and not via the differential, do I not have to worry about friction plates wearing out? I am definitely not savvy enough to know how DTC or DSC works in this case. Anyways, this is some really good info.

CJT

i think marisa tomei does an excellent job explaining what LSD is in My Cousin Vinny . . .

Mona Lisa: The car that made these two, equal-length tire marks had positraction. You can't make those marks without positraction, which was not available on the '64 Buick Skylark!
Vinny Gambini: And why not? What is positraction?
Mona Lisa Vito: It's a limited slip differential which distributes power equally to both the right and left tires. The '64 Skylark had a regular differential, which, anyone who's been stuck in the mud in Alabama knows, you step on the gas, one tire spins, the other tire does nothing.
[the jury members nod, with murmurs of "yes," "that's right," etc]
Vinny Gambini: Is that it?
Mona Lisa Vito: No, there's more! You see? When the left tire mark goes up on the curb and the right tire mark stays flat and even? Well, the '64 Skylark had a solid rear axle, so when the left tire would go up on the curb, the right tire would tilt out and ride along its edge. But that didn't happen here. The tire mark stayed flat and even. This car had an independent rear suspension. Now, in the '60's, there were only two other cars made in America that had positraction, and independent rear suspension, and enough power to make these marks. One was the Corvette, which could never be confused with the Buick Skylark. The other had the same body length, height, width, weight, wheel base, and wheel track as the '64 Skylark, and that was the 1963 Pontiac Tempest.
Vinny Gambini: And because both cars were made by GM, were both cars available in metallic mint green paint?
Mona Lisa Vito: They were!
Vinny Gambini: Thank you, Ms. Vito. No more questions. Thank you very, very much.
[kissing her hands]
Vinny Gambini: You've been a lovely, lovely witness.

jason952

Both wheels should turn the same way if LSD is intact. Open diff will spin opposite.

cmb13

I remember that scene! That was classic!

aggie57

For cars without a high rear weight bias (hence high rear traction) an LSD makes a greater difference and affects handling as well as grip. Basicly the car will tend to understeer more at moderate speeds or in low speed corners, particularly if the locking ratio is high. It can also let go at the rear far more easily in damp or slippery conditions which can of course be fun but hard to control for novice drivers. A stability control or traction control system behaves quite differently.

Being newish to 911's (14 months now but who's counting...) I'd assumed the lack of standard LSD is because the front/rear weight bias makes it unnecessary. Certainly in the 3 track days I've done so far I've not felt that I've wanted one. Maybe as I explore the cars handling more that will change.

RonCT

Good point - the 997 in particular is extremely grippy in the back-end whether on stock or R-compound tires. With or without LSD I've never lacked exit grip on the track.

mander

Slightly off-topic but my 91 318iS had LSD standard. One very cold day my differential decided to head south and that was one very expensive repair relative to the value of the car.

RonCT

Ironically, I had a great 964 at one point and I asked the very respected race and restoration shop about whether I needed to replace the friction disks in the LSD. They said "nah, don't worry about it, the car hasn't had a functional LSD for many years..." So, if the disks are worn out, their opinion was to just leave it alone.

Coochas

Agreed.
My 99 986 on the other hand definitely would benefit from LSD. At NHMS there is a sharp uphill right hander and my right rear tire just spins all the way up the hill (it's fun but obviously could benefit from LSD).

Erik@GBox

Well for those who know me already, you had to know that sooner or later I was going to see this post and chime in. LOL

I think that a very important aspect of the LSD may have been missed here, that or I simply missed it. That being that a LSD offers lock up on the deceleration side of the differential, as well as the Acceleration side. Depending on how it is set up, this can vary from differential to differential.

This can be a very good think for those who like to drive hard, especially in the mountain regions, although, you can feel the effect pretty much anywhere. This deceleration lockup is what sets a true limited slip differential apart from other diffs on the market. It helps to keep the car in trim, which as many 911 drivers will tell you is pretty important when you start to play harder.

A torque biased differential, ie. quaife, will only offer you lockup on the acceleration side of the differential, and then only when both tires have the same amount of grip applied to them. They offer no deceleration lock up of any kind.

My recommendation is this, if you have any intentions of driving your 997 with any aggression at all then a LSD would be a very good option for you. If your intention is to never drive your car in the rain, or hard in any way, then a LSD is probably not worth it. I would certainly agree though that any true sports car, that is going to be driven as such would benefit greatly from a LSD.

There are several options for you to choose from with respect to which LSD to use, and feel free to contact me if you would like some suggestions. I am always here to help. They do require fairly consistent maintenance to keep them working correctly. This is more so in the Porsche Differential, but most will require re-plating at least once every couple of years.

I could go on and on here, but it would probably be easier on everyone if you just gave me a call sometime.

Warmest Regards,


Erik Johnson
GBox Performance Transaxles
(303) 440-8899 work
(303) 895-4828 cell

RonCT

I can remember reading articles in the various car magazines and in online articles as well. Something about the engineering in the 987 / 997 platform negated the need for LSD. At both Lime Rock (1:02) and Watkins Glen (2:14) in my 2007 C2S I found no need for LSD. The GT3 had it, but there was no change in "attitude" - and same with my 09 C2S that has Sport PASM and LSD.

But, as you say, everyone I know with earlier Porsches that track regularly either have LSD or want it.

cgfen

<snip>

LOL
my wife and i just saw that scene recently.
after seeing it, she felt better about checking the LSD option box


Craig