Mine too....how does THAT work with an open diff??

Chris

 

I am informed that all 996TTs came stock with an open differential (as opposed to a LSD), but a LSD was available as an option in Europe only for a year or two. This is strictly hearsay and could be wrong. That being said, I have never heard of a 996TT coming from the factory with a LSD (the GT3 Cups cars do come with LSDs).

Craig

 

The LSD was an option on the '05 turbo S in the ROW market only. Funny enough, the LSD was ordered on my turbo S while it was a stick shift. the guy who was supposed to buy the car changed his mind in the last minute and changed it to a TIP. For some reason he had a change of heart and I ended up buying it. I was there at the dealership when it was first ordered and suggested the LSD option.

 

so, if you do this rwd conversion, what LSD options are available?

 

There is a few, stock gt2/3 diff (find one used), Guard trans unit, or a quaife. Might be some missing. Do a search I think Joetwint did a great diy on this. I also did it to my car and would never look back.

 

thanks, I'd only go RWD if I swapped in an LSD! That's the first thing that made me want to get rid of my 135.

 

I did the RWD conversion after appr 10 months ownership. I regretted only 1 thing. That I waited so long!!!! The car became: GREAT!! Whether you have actually changed the suspension or not, the car finally starts to behave consistent and predictable. And initially I didn't even have an LSD (the LSD drop-in was schedule for about 1 week later).

With NO LSD (which is the case for 996TT unless you do for 100% sure have the option in your list of options) you have to drive very carefully though in case you switch off PSM. Read: VERY carefully. But there's no harm if you use PSM and want to try just driving about and figure out the overall feel until you do the LSD install.

If you have tons of power the car will still make 2 black lines with NO LSD. But only til 1 wheel grips. And if (when) one wheel suddenly gets good grip you might find ourself turned anything from 30-90 degrees off from the direction you used to be going and it will most likely not end up pretty. That's why you leave PSM on until you get an LSD, even on track.

Many guys like Cup car LSD. Downside is maintenance as I understand it. Most ppl say noise isn't too bad though - I don't know myself.

I went with Carbonetic LSD since I had tested it on other brand car and loved it since. No chattering, seriously huge traction, and completely seamless engagement which was key for serious tracking and corner entry. Note to this is that I daily drove the car.

 

is anyone on here running an OS Giken LSD? They have one listed on their site for the gt2 and gt3. Those cars come with an LSD though so I'm guessing that's just a replacement for the OE dif.

 

Got friends who ran OS Giken with great success in Redline Time Attack. Would be interesting to test or hear any feedback from a Pcar owner.

 

Yes, the stock 996tt front differential with it's viscous coupling is slower than the same car with the differential removed or simply disconnected.

However ........ replace the stock differential with a front ramp type LSD and the car will be far quicker than a 996tt (front differential removed or disconnected) or a GT2.

The problem with the stock front differential is half the motor force is directed to the front and only 5% of this force is directed to the ground. Direct all the force to the rear and again a majority of this force simply never sees the ground.

The ramp LSD is better in that it transfers the power to the front wheels ........ the problem being the front is so light, when it starts to lift, you again lose some power.

 

So what exactly did everyone remove to do the rwd conversion on there 996TT, I plan to do this to my car in the near future.
And what needs to be added and from what variant?

Thanks.
-V

 

Hi Chad! Who makes a bolt-on replacement for the 996tt front differential,
and/or viscous coupler, or am I misunderstanding what you're suggesting we
can replace?

Also, can you explain the power math more? If half the motor force is
sent to the front, yet only 5% is directed to the ground, are you saying
that 45% of the motor's power is somehow lost by the front differential?
Unless the power delivered goes to the wheels, it would have to be
dissipated by heat. That sounds like *a lot* of heat...

regards,
Joe

 

Joe,

I am not doing a bolt in differential. I am using the existing differential case, removing the guts (differential interior ie. gears, vsicous, etc.) and replacing them with a ramp guts.

As for the power, (I am simplifying things now) the 996tt tranny sends power to both rear wheels and the front wheels. The front routes through the tranny to the front shaft and then to the front differential. The rear of the front differential is basically divided into two sections. The front section has the differential. The rear section has the viscous coupling. The shaft (from the front of the tranny) connects to the viscous section and the viscous section transfers to the differential section. The viscous section basically uses an oil film to turn (connect) discs rotating at different varying speeds. The slippage between the discs is how Porsche compensates for the varying radius between the turning radius along with the difference between the front and rear rolling diameters. The rear discs basically are turning a lot faster than the front discs. The oil friction allows for the slippage. And yes, the differential of the 996tt do generate a lot of heat.

The Porsche 996tt was designed to transfer only about 5% of the power to the front tires. This transfer or limitation mostly takes place at the viscous coupling.

 

I've been looking around and haven't found an answer yet. Does the '05 Turbo S have an LSD? or were the gt2/3 the only 996 models that came with one? And was it ever a factory upgrade on regular 996tt's? TIA

 

Yep, thanks, I understood that, but got caught up in the power numbers.
If the viscous unit transfers only a little bit of torque, then the front isn't
getting 50% of the power, only a little bit. (the percentage it transfers
to the wheels and the amount (not 45%!) that is lost to heat because of
the minor slippage due to tire radius differences. I think that for turn-radius
differences, the front differential itself covers that.