obvious reason why no LSD on 996s?

i've searched the archives (performance and 996). was wondering about the lack of a limited slip differential on the 996 chassis. PSM obviously is supposed to do this, but it will fry the brakes and my understanding is PSM would die a slow death if left on during track days. so, in fact, there is no LSD. is the partyline thinking that since the most weight is over the driving wheels, the need for an LSD is precluded? strange, indeed.

Only the 1999 Carrera's were offered with Limited Slip it was then replaced with PSM. (except of course the GT3 which had it all years)

I have no idea at all what the "partyline" thinking might be but I'll take PSM's traction control mode over any mechanical LSD any day of the week. Try it, you'll like it!!

In my previous car, a 2002 M3, the DSC system (practically identical in function to PSM as far as I can see, with the capability to brake any individual wheel) is coupled with the new fangled differential that can send up to 100% of the torque to either one of the rear wheels. So, there is both mechanical and electrical "help". Not that this answers any questions, but it's a data point at least. Cheers, Ken

blackdawg "my understanding is PSM would die a slow death if left on during track days." The Porsche Driving Experience cars have the PSM on constantly. In fact, it can't be turned off on those cars. It doesn't fry the system there. I understand with some drivers it gets a lot of use

No PSM on my car now, no PSM on my waiter. I wanted LSD but PAG thinks they know so well what we want. Why couldn't they leave it as an option. If PSM is all that for tracking, how come the Cup Cars and GT2/3 don't come with it??? I want my LSD! wait, that didn't sound good. I want choices! better. <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" />

Ed, The LSD transmission still shows up on the parts lists... perhaps Gert at Carnewal.com or Porsche Motorsports can get you one

Yeah like one of the psters here pointed out woulda been better if PAG offered more options ie PSM/LSD or none. Now it's all(PSM) or none(no PSM/LSD). Is there a reason why you wouldn't want PSM and have LSD instead- I think it's(PSM) a good safety net since we're not as skilled as those race car drivers are? Is it true that all GT3's including street versions in Europe have no PSM? Regards. Mike

You can install an aftermarket Limited-Slip Differential in a 996, the 4340 Chromoly, multi-disc unit costs $2200 and is made by Guard Transmissions. I think The Racers Group uses this unit and probably sells it too.

Since its not a factory item, it drastically changes classing for racing :-( Even though it is a nice safety factor, in my opinion, it slows the car and makes it harder to perfect you driving as it interrups every time you get out of shape. Especially bad for autocrossing. <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

I'm getting the impression some of us here don't exactly know the purpose of a Limited-Slip-Differential. An LSD isn't a safety device like PSM or DSC is. It's a go-fast item. For example, you're at the apex of the turn, and you wanna power out. Without an LSD, and with the amount of power Porsches typically have, you will have the inside rear wheel spinning, sending you into horrible understeer and generally leaving you going nowhere. This is because without LSD, the differential will simply send power to the wheel with least resistance, in this case the inside drive wheel. With a LSD, the differential will "lock-up" at a certain point and allow you to send power to the outside wheel as well. It's great stuff, a pity PAG doesn't think its customers drive hard enough to warrant one as standard equipment. BMW surely does with thier M vehicles.

And I have the most definite impression that at least one of us isn't aware of some of the functions of PSM and/or VSC (toyota/lexus. PSM will automatically apply moderate, modulated, braking to any driven wheel(s) that are/is slipping, to the point wherein its rotational rate closely matches the other (or others for C4) driven wheels.

Are there any comparitive figures on the track that would suggest PSM would be a detriment to the track versus LSD? And for those who do have PSM-is it that invasive on the track at all? And with respect to the GT2, I know it doesn't have PSM but does it have LSD? And would it be entirely possible to install the LSD from the MY 99 to the later model 996's without PSM but given the fact the MY02 have more power would a '99 LSD work on those at all? Regards. Mike

Hold 0n, turbo-look and wwest. I think you are both right. Lets look at a power turn, with the inside wheel losing grip since it is lifting. In a plain jane car (sorry, jane), almost all your power will go to the spinning inside wheel, the one with the least resistance. In LSD, once the inside wheel starts to slip or spin faster than the oustside wheel, LSD locks up the diff so that resistance is increased to the inner wheel, maintaining power to the ouside wheel, the wheel with traction. In a car with PSM, once the system detects that the inner wheel is turning faster than the outer wheel, brakes are applied to the inner wheel, maintaining power to the outside wheel. In addition, PSM will also detect that you are doing a power turn, due to feedback from a gyro measuring centrifugal (or is it centripedal? It's been a long time since highschool) force plus sensors indicating the degree of steering input from the steering wheel, the attitude of the tires and suspension, and the amount of pressure from the gas pedal. Based on all that data, after nanosecond calculations, it determines that you want the car to turn in more than what it is doing, it will preferentially transfer even more power to the outside wheel and less to the inside wheel. If you have an AWD vehicle, there are two more wheels that PSM can manipulate to make the car behave properly. AND, if I understand the interaction between PSM and engine management software correctly, if you need more power than what your right foot is supplying, the engine management software WILL supply more power, all in an effort to keep you from flying off the course. PSM is a safety item with a go-fast soul. LSD??? Who needs LSD??? BMW can keep it, together with their powder puff M engines .

Oh, yeah. And another thing. (sorry if this is getting too long). On the AWD sytem of the TT and the C4S, up to 45% of the power can be transfered to the front wheels. And, again, if I understand the press releases and advertising blurbs correctly, this can also be applied differentially to a single front wheel. I do not believe that this is a function of PSM, but a function of the viscous coupling of the AWD system. this has a tremendous effect on maintaining neutrality in fast corner, putting the car on rails. LSD???? What is LSD????

whoa! got more than i bargained for here. thanks for the feedback, guys. i am a "wuss" and got PSM just to have it; in the 02 eye candy book at the dealer, it still says it's defeatable if wanted.

In the 996 C4, and the C4S, the VC, viscous clutch/coupling, is in line with the drive shift to the front diff'l. VC's typically take hundreds of milliseconds to react to disparate rotational rates of the two "input" shafts, It takes a period of time for the fluid to heat up due to the turbulence created by the disparate rotational rates of the two sets of clutch plates. The actual attack rate of the increase in coupling coefficient is a function of the expansion rate of the viscous fluid due to rising temperature. LSD has now become a useless anackronisom(sp, sorry) The VC can route up to 45% of the engine torque to the front diff'l but it is left to the PSM to do the job (by modulating a front brake) if left to right or right to left disparate torque distribution is needed. Oh, palting, the "gyro" you speak of is actually a solid state "lateral" accelerometer, lateral in this case meaning mounted at 90 degrees to the normal direction of travel. Typically referred to as a yaw sensor. A yaw sensor in an airplane is used to detect that the airframe is not alligned with the current direction of travel.

Ed The instructors at PDE said the exact opposite. the said for certain maneuvers, it's very fast, like tight "boxes". If the drivers know how to use it, it's the quick way thru. I thought it would interfere, but was amazed by it. besides, you can turn it off. The cars felt very quick with it, and didn't slide the way my 993 will. M in C