Ron

In pretty simple terms what does LSD and ABD do ?

jscott82

First you have to understand how a plain diff works. There is a great YouTube on the subject.. Search "1937 how does the differential work"...

Then you will understand the limitation of a plain diff. Even though it provides power to both wheels, it only works when both tires have equal traction. LSD and ABD both work to overcome that.

LSD (Limited Slip Differential) works with mechanical clutches to "lock" both wheels together.

ABD (Automatic Brake Differential) is electronic, the ABS computer will apply the brake to the slipping wheel to force some of the power over to the one with grip.

Ron

Thank you, I can do some research with this.

Ron

So when the vehicle is going straight both half shafts coming out of the gearcase are providing equal a Amounts of power to the ground would that be correct? And be turning at the same speed?

So does a car with LSD have more power and be more stable than a non-LSD equipped car?

TJ993

can not answer first question, perhaps JScott 82 or others jump in
I have tested LSD in the rain, on an angle and car pulled hard and straight - as pointed.

Idea is slows and stabilizers the car under braking
Gives grip upon acceleration
Both wheels Grab
Good option to have for safety otherwise power would be undistributed and may pull you to one side.

saw this with a drag car - it shot into the guard rail

Necessary Option (220) for Track Use / racing = Grip

jscott82

Until one of the tires starts to slip, neither the LSD or ABD are active.. So in general use, no, there is no advantage.

Its only when you get to the edges of performance where they start to matter... A standard diff splits the torque between the wheels. If one wheel starts to slip, the system cannot apply any more torque, even though the other wheel may have additional traction available (this would be the case exiting a turn, where the outer tire is loaded and the inner tire is light).. What you really want is to continue to apply additional torque to the gripping wheel until it starts to slip too.

With a mechanical LSD, as the clutches engage they bind the inner and outer wheels together. This tends to straighten the car up and makes it feel better as both tires are operating at 100% of their traction limits and slip evenly across the pavement.

Brain empty now, that is all I know.

Ron

Did Porsche ship 993 to the USA with LSD if it was not ordered by the customer as an option?

nine9six

Since the engine produces the power, the specs of hp remain the same. How that power is delivered and transfered to the road, is a different matter; but in a word, no, there is no increase in hp.

As an example, if you were used to rolling on, say 30 lb. cast RUF boat anchors, and switched to a set of forged RAYS, you woult think you had more hp, and steering would be crisper, but the hp would remain the same.

Typically dealers order and floor certain configurations of the models they sell, so it is certainly possible as well as probable.
HTH...

pirahna

It would have option code 220 and 224 if it has ABD and LSD.

Bill Verburg

On US cars option M220 was a 25/65% lsd and ABD, thru '95 it was a Zf from '96 a GKN, RS used a 40/65 GKN as standard. For these asymmetric lsd's the first # is acceleration the second deceleration

the #s are notional not metrics, the actual breakaway torque is determined byt the actual condition and setup of the lsd.

on acceleration an open diff delivers all of the torque to the wheel w/ least traction, this is why if one wheel is on ice it spins and the other does nothing, there is some friction so eventually the car usually moves unless there is something else preventing motion.
w/ an lsd the torque to the less traction wheel is limited so a % does get to the wheel w/ more friction

on track this is useful because in a corner the inside wheel has less traction than the outside wheel, lsd thuds aids acceleration out of a corner.

On deceleration the lsd couples the drive wheels and stabilizes the rear of the car, under hard deceleration the rear dances around less.

lsd can be dangerous if both wheels have no traction, both will spin generating a chassis rotation torque that cause the rear of the car to go to the right.

abd is couple w/ lsd on 993 it applies the brakes to the rear wheel w/ least traction and only works up to ~40mph, It uses a 4 channel abs where the non abd cars only have a 3 channel abs.

You can tell what the car has by looking at the rear of the abs pump, 4 channel bas w/ abd and lsd has 2 proportioning valves that go to the rear brakes, non has 1
This is a 4 channel ABS the 2 p/v can be seen below the brake lines which go to the bulkhead fitting which has 4 lines

Ron

Thanks again to everyone for providing this interesting information to me. When I had my car appraise recently the appraiser said the car had some uncommon options one of which is the LSD-ABD the sticker on the hood does show code's 220 and 224 the window sticker shows the cost of these options as $ 1257.

It seems lately that when I post things that have the word appraisal or I'm trying to find out more information about my car I get PMs or inquiries about whether my car is for sale so let me just say no my car is not for sale so please don't make inquiry of same.

initial

Unless it's a Carrera 4. In that case, the Carrera 4 wouldn't have it listed in the option codes but still have ABD.

Can anyone confirm that a Carrera 4 always has ABD coupled with LSD?

pirahna

Correct but the OP has a C2S so his would have them listed.

Yes, the C4 and C4S came with both standard.
Here is a photo from the original 95 hard cover sales book.

C4inLA

JS,

On a AWD with LSD if car on all four jack points, in neutral, ignition off, what would one expect rear wheel rotation to be ? Turn driver side wheel forward, does other rear wheel stay still?

Whitesheep

An old thread I know, but while looking for information on the ABD Porsche system I came across this thread. the answer is if your diff has a true LSD both wheels will turn in the same direction. If not, then the opposite will go the other way even if the transmission is engaged. I this this is even true if you have a full locker (opposite direction).

I am looking for comments on Torque Vectoring vs. the stock set up and am finding nothing. The big problem with a clutch pack LSD is the clutches wear out. Most of my knowledge of traction aids comes from off road vehicles, but the theory is the same. Personally I like the ABD as brake pads are cheaper to replace than an LSD is to rebuild.