Mark Wheeler

What's the functional difference between locking differential and active brake differential in reference to Traction Control on a 996?

Thanks.

wwest

Locking diff'l is a mechanical system that prevents the diff'l spider gears (or equivalent) from free-wheeling, and thereby forces engine torque to the wheel with traction.

"Brake" diff'ls use the Anti-lock braking system to moderately apply the brakes on the non-tractive, free spinning, wheel, thereby forcing engine torque to the wheel with traction.

Assuming ABS already, no additional mechanicals to wear out, slightly less life for the brake pads.

"Functional" difference is minimal, ABS LSD is possibly quicker reacting.

aktundramonkey

I'm confused. Does my '02 C4 have both?

JeffES

No. 996's have ABD (Automatic Brake Differential)
J

Karl S

The locking diff was an option called Traction Control and I believe was only available on 1999 model C2's. I know I had it on mine.

Karl

Viken

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Mark Wheeler:
<strong>What's the functional difference between locking differential and active brake differential in reference to Traction Control on a 996?</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">As Karl stated, LSD (LIMITED-SLIP DIFFERENTIAL) was only available on the '99 C2 models. Here are the differences in Porsche's words:

"ABD (Automatic Brake Differential) A type of traction assistance which applies the brakes to a slipping wheel, transferring torque to the opposite wheel. ABD systems generally operate at lower vehicle speeds.

LSD (Limited-Slip Differential) A type of differential which limits the degree of variation in wheel speed between the two driven wheels, thus reducing wheelspin under conditions of poor traction. The limited-slip differential begins to restrict the torque supplied to a spinning wheel by a predetermined amount, reducing the speed of rotation of the spinning wheel to that of the wheel which still has sufficient grip. The result: traction is not interrupted appreciably, allowing the car to keep moving."

With the advent of PSM, LSD is no longer compatible on the Carrera and Carrera 4 but ABD is now part of PSM.

Viken

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by aktundramonkey:
<strong>I'm confused. Does my '02 C4 have both?</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Your Carrera 4 has PSM. Here's Porsche's description of it:

Porsche Stability Management System (PSM):

The 911 Turbo and Carrera C4 all-wheel drive models are equipped with a significant handling innovation designed to stabilize the car when driver loss of control is impending. It’s called Porsche Stability Management (PSM). The system is optional on both Boxster models and the two-wheel drive Carrera.

PSM incorporates the components of the previously used Boxster and Carrera 2 Traction Control system — Automatic Braking Differential (ABD) and Anti-Slip Regulation (ASR) with additional sensors.

But a new and innovative element of PSM is also present: a high-tech control unit that responds to help the driver maintain control when the vehicle oversteers or understeers. Here’s how this system works:

A number of sensors provide input to the master control "brain." These include the four ABS wheel-speed sensors, plus a yaw-velocity sensor which detects movement around the car’s vertical axis, a transversal-acceleration sensor which responds to sideways movement of the vehicle, and a steering-angle sensor which detects the direction and speed of the steering wheel input from the driver.

The master control unit continuously checks and compares input from the sensors to determine the vehicle’s direction and movement and how it differs from the direction chosen by the driver based on steering-wheel input. If the system senses that the vehicle is deviating from its proper path while accelerating or coasting — by oversteering or understeering — it reacts with the following corrective action:

If oversteer is sensed, the brake at the front wheel on the outside of the bend is activated

If understeer is sensed, the brake at the rear wheel on the inside of the bend is activated

If selective brake applications are unable to stabilize the car, the engine management system reduces or increases torque by varying the ignition timing or by varying the throttle opening

In cars equipped with Tiptronic S, the automatic shift process is controlled in order to ensure that changes in torque do not cause the car to be unstable

Hence, the Porsche Stability Management system has components that help to maintain control during braking, as well as components that help maintain control during acceleration, cornering, and coasting.

While PSM significantly increases the stability at the limits of adhesion, it cannot overcome the laws of physics, and must not be thought of as an antidote to irresponsible driving.

Porsche Stability Management provides numerous benefits to the Porsche driver:

Promotes optimum traction and steering accuracy when accelerating, even on surfaces with varying adhesion levels

Helps compensate for load-reversal reactions when coasting, or during partial braking applications

Helps avoid oversteer, such as might occur during rapid steering movements

Reduces understeer when turning sharply into a bend — especially on compromised road surfaces

Promotes shorter braking distances and improves stability when brakes are applied, especially when cornering on surfaces of varying grip

Maintains high level of sportscar handling and driving safety by maintaining outstanding agility

Allows driver to switch PSM system off in order to adopt a deliberately more-active driving style

carrerajack

Great post, Viken!

A somewhat related question:
I assume that the algorithm in a C2 is different than a C4 - the front wheels now also drive, so it's not just about braking. Is the algorithm for a C4 or C4S different than a Turbo?

Mark Wheeler

Great info gents! Thank you.

Mark

willard

C4...

Since the front wheels rarely have high engine torque applied it's unlikely that PSM would need to apply "trac" (moderately brake a front slipping wheel) and to do so could "upset" ("radical" torque steer) the steering, like having a front LSD, not very common even for most SUVs.

I would guess dethrottling would be the first effort the algorithm would use with front wheel slippage.

SPR

Very great post. Basically if you have traction control you typically have an open or peg leg, non limited slip differentail that applies brake to the wheel slipping typically the right rear- hence peg let. with the psm it brakes the wheel spipping at that corner. MB calls it esp. It uses wheels sensors and in some circumstances yaw sensors- come car to monitor the force, then applies the brake. Personally I hate it. It is typically very invasive, however Porsche is the best at doing it and is managable but I would still turn it off if I had the option. I opted to get a 99 because of the fact that is has nada! I do however have traction control, which means I dont' have a limited slip, which is really lame but at least I can do what I want where I want. A small footnote however, with the average driver these control systems time and time again are a life saver for someone going in over their head. For someone more experienced or otherwise they are invasive and try to do the work that you should be doing with by limiting a nice slip angle or a power oversteer slide etc. Porsche is definitely the best of the bunch for sure though concerning these systems.

willard

What are the functional differences between PSM and wider tires thereby more contact patch and traction?

SPR

psm just uses wheel sensors to modulate the braking and possibly yaw sensors or something like it but don't know, Wider tires and more traction would just make it not kick in as soon. Also with larger rims it would make it looks slower. Basically it seems to me that it looks at all the wheels and if one is going x amount% faster then it brakes it as it is loosing traction. I don't know that much about the inner workings but the theory typically is the same amongst systems. I do seem to remember that the psm uses the same sensors that the litronics use for adjusting height which are mounted front to back on the track arm or something? but I could definitely be wrong. I bet loren would know more. I don't have psm, although I do have traction control and it does get in the way as i have a c2 and like to get maximum slip angle which like to kick the traction control on here and there and upset the balance a bit, so I turn it off rutinely. Around town however I could care less it seldom comes on like a good system- read PORSCHE does.

enthusiast

I searched this set and the 06-23-2002 postings for 'traction control' systems but need to clear up the limited slip differential issue for the 996. (I might buy a 996 for street and track)

On the 1999 USA Spec 996 two wheel drive, manual transmission model...

It is a fact that there was an option of ' traction control' officially called Automatic Brake Differential (ABD).

Was there a traditional 'limited slip differential' for this car? If so was it standard or an option?

I think SPR and some others are saying there wasn't any factory rear LSD for the normal US Spec street 996, ever. If that is true my choices within the 996 C2 fleet are; no special traction system, ABD, and PSM (starting MY 2000)?

SPR

no just that I don't believe that you can have traction control with a limited slip diff. That is how my c43 was as it had to be open to work with the brakes etc. I am pretty sure though that they did offer limited slip in 99 just not with traction control as they should be mutually exclusive-