If I understand correctly, the Turbo has open front and rear differentials and a viscous centre differential. Would it be possible to substitute a limited slip differential for the rear differential? Has anyone tried it?

I just did this on my Audi S4 and I am very pleased with the outcome. I would also think it would transform the handing characteristics of the Turbo.

Stephen

 

Manthey Racing offer a rear LSD for the 996 turbo as did the Porsche factory in '05 in the ROW market. I know cjv is changing the front, center and rear diffs in his car but that might cost alot of money. You can always be the first.

 

Khalid, Thank you for replying and thank you for so much useful information!

First, I had no idea that PAG ever offered an LSD on the Turbo. Do you have any information on how it changed the handling characteristics of the car? Or what affect it had on the PSM?

I thought Chad had removed the FWD portion of his car? And I also understand an LSD on FWD can be quite a handfull. In any case, I shall have to ask him what he is up to.

And I guess I also need to talk to Manthey.



Stephen

 

Stephen,

I wish I did know more on how it affects the handling. It was an option on the turbo S in the ROW market but all we got here were three tiptronics, no 6 speeds and none with either X73 or LSD options.

Good luck with Manthey and keep the board up to date with what you find out.

 

Porsche makes two LSD's for the rear along with two ratio's. There is one used in the Cup cars that is a forged unit. It costs approx. 2K. There is another LSD made for the RSR which is a billet unit. It costs approx. 5K. The difference between the two mainly being strength. Each of these units can be bought in different ratio's, the 40/60 and the 50/80 (it may be 60/80). I used the RSR 40/60. Traction is much improved on medium to long turns and straight line. In tight turns she wants to twitch/crab (not sure I'm using the correct description) a bit and the car is a little more tricky in wet conditions. This setup is also used with great success in hill climb races.

If you are going to change the front LSD it is very expensive. The 996tt diff. will not work at the splines are too small. You must use a 993tt front diff with the larger splines and thicker axles. The front wheel carriers etc must also be changed.

Brian Copan at BMC did the rear LSD work on my car. He can be reached at 513-779-3302

FixedWing,

My car is awd abeit without PSM as most know it.

 

Guard makes an indestructable unit for these car. It is also selectable as to the braking / acceleration ratios. You can get it as a 40/60 - 50/80 or a 50/80 - 80/80.

 

Yes it is a knock off of the RSR unit. My RSR unit spent some time of there.

 

Hi Bob, Chad & Khalid,

Glad to see so many responses on this topic.

The unit I have in the back of my Audi was made by Guard. It is a 40/60 unit. It has only been in there a short while so I am still testing it out. But so far at least, I absolutely love it. The car handles much more like a RWD car. In my opinion, it transforms the Audi.

Chad, I know exactly what you mean when you define the twitchiness and crabbing. In the dry an overly aggressive LSD will cause understeer as it fights your attempt to turn in. In the wet it can cause faster oversteer as both rear wheels let go together. But I’m surprised that you are getting these issues with a 40/60 LSD. I notice more instability and ability to control on the throttle but non of the problems in slow corners where it fights your attempt to turn the car in.

I’m not surprised at your comments that traction is much improved.

I wonder what the factory put in the 2005 Turbo? I would assume that they properly sorted whatever they put in. I would think the solution would be to install a factory unit. Then the only issue would be whether one also needed to upgrade the PSM.

Thanx guys!

Stephen

p.s. nice to see you posting Chad.

 

Stephen,

We are using a mechanical (gear) 40/60 in the rear and a biased 40/60 up front. I believe this is why it is more pronounced.

 

"40/60 - 50/80 or a 50/80 - 80/80"

Can someone explain to me what these various ratios refer to?

 

The numbers refer to the percent of slip before the LSD engauges/activates (locks the two wheels together) in braking and acceleration.

 

Thanks for the reply. So are you saying with a 40/60 - The rear dif will have to be slipping 40% of the time during acceleration for the unit to lock and 60% of the time during braking for the unit to lock?

 

Zippy,

You got me a little there. It is something like that, but not exactly. The reason I say this is a 40/60 is fairly drivable on the street while a 50/80 will give some problems while trying to parallel park. In other words it takes less slip to lock the wheels with the 50/80 than it does with the 40/60.

 

You're reaching the limits of my knowledge also.

Take a look at the photo below (from Audii-Dudii on AudiWorld's site). Do you see the "V" cut in to it? Something is lodged in that V and as there is a twisting action it rides up the V. By varying the angle of that V it is possible to vary the leverage on the clutches. That makes it possible to adjust the amount of leverage in each direction.

So now it is possible to have a torque split (and it is torque split and not rotational speed split) that varies based upon whether the LSD is under power or whether it is in the overrun (throttle closed).

That's useful because you might not want the LSD to act as aggressively when you lift the throttle as when you apply it.

So an 80 LSD is a very aggressive LSD that probably only belongs on the track. It will split torque almost evenly in all situations. A 40/60 LSD is still quite aggressive but will allow a lot more variation and especially allow more variation on the overrun. More normal family sedans will often have even less aggressive LSD's.

Some reading:

Limited Slip Differential (LSD) Lock % (Pros/Cons)

And also, another thread on Rennteam that I started:

Limited Slip Differential in the Turbo

Stephen

 

Actually, if you follow the torque path through the diff, you'll see that it's the thrust plates that drive the pinion shafts (the "somethings" that rest in the "v") and not the other way around. Same end result, though...

The additional clamping pressure provided by the thrust plates (as compared to that provided by the belleville washers) is dynamic -- i.e., it only comes into play under load -- whereas the pressure provided by the belleville washers is static -- i.e., it's present at all times. By playing around with the combination of these two, you can vary how the LSD interacts with your suspension and specifications such as "40/60" or "50/80" only tell you the TOTAL locking percentages, not how they are split ... in short, understand that two differentials marked "40/60" may behave somewhat differently in the car, depending upon how they were tuned. (Also understand that unless you're a tinker like me -- I used to call myself a "tweaker" but that has an entirely different connotation these days! -- differential tuning is really racecar stuff and you can probably plug-and-play any rear LSD and be happy with its performance).

BTW, Stephen, due to the layout of the 911-based Porsches (and even to a lesser extent, the 951 chassis cars as well), they benefit from having more lockup under braking than your Audi does ... what makes your car seem a bit nervous and unstable helps those cars rotate into the corner. Half of the reasons I've been tinkering with the setup in my A4 -- besides "just because"! -- is to optimize its performance in an application other than what it was designed for ... were I driving a Porsche (someday, I swear!), it should work quite a bit better out of the box than it does in my Audi.

Anyway, if you search the archives, I've discussed this stuff in a few threads -- although he didn't contribute, Paul Guard contacted me privately and we had a few interesting discussions about his diffs and the tuning options he can offer -- so if this subject interests you, there's some more info available...