Quote:
RC said:
Regarding the advantages of LSD on a AWD car, this has been discussed in some older threads, searching the archives may be a good idea. If nothing comes up, we could start a new discussion about LSD on the Turbo. And yes, it was available for the 996 Turbo and Turbo S but I'm not sure if a simple retrofit is possible.
Hi Christian!
The big problem I see on retrofit would be PSM and whether it had to be adapted to work with the LSD or not. I would guess not as I cannot see PAG investing the resources in adapting PSM for the last model year on a very low production option. But I might be wrong. If I am wrong, could PSM also be upgraded? Surely this is something that could be found out?
As to the advantages of LSD on an AWD car, I did search before posting but didn't see anything. It might be that my search was not wide enough. Maybe this is a timely topic given the upcoming 997 Turbo?
In any case, it would have tremendous advantages - especially on a rear engined car such as the Turbo.
The central differential on the 996 Turbo is a viscous coupling. It splits power between the front and rear based upon the different rotational speeds of the wheels at the front and rear. It is reactionary. As the rear wheels begin to slip it moves more power to the front and vice versa.
But this does nothing for the distribution of power side-to-side. At the front and rear these cars have classic open differentials. An open differential will not manage the power side to side. Traction is limited to whichever of the two wheels has minimum traction.
On the Turbo the power tends to be predominantly at the rear. So what is actually happening is that as one rear wheel (often the inside wheel in a turn) starts to slip, more power is sent to the front by the viscous coupling. As the slippage at the rear increases so even more power is sent to the front. Ultimate traction is limited by the limit of traction at the front and since that is also an open differential, that too is limited by whichever of the two front wheels has the least traction (again, probably the inside wheel in a turn).
So is it even fair to call this an AWD car? I think not. What it really is is a two wheel drive car. The difference is that the two wheels include one on the rear and one on the front (the one at the rear with the least traction and the one at the front with the least traction). How is this better than a RWD car which drives both rear wheels through a LSD? I'm not sure it is. It will be more stable because of the way that the power is split back-to-front but that's about it. The RWD car probably can put down more ultimate power - especially if it is a rear weight biased car such as the Turbo.
So the solution is to add a LSD at the back of the Turbo. Now the power is first split between the two rear wheels. Before the viscous coupling comes in to play, there needs to be slippage on
both rear wheels. Only then will the viscous coupling move power to the front of the car. It isn't practical to use an LSD on the front. So the car will ultimately become a 3-wheel drive car. It will also predominantly be a RWD car since the Turbo tends to send most of its power to the rear at first. So it would handle and feel much more like a RWD car - and this has always been the complaint against the Turbo - that it doesn't have that classic RWD feel of most Porsche 911 cars. As traction became limited at the back, some power would be shifted to the front so it would still have more traction than a RWD-only car and would also be more stable.
So in my opinion, the LSD option on the Turbo is a most important option. And if I can, I would really like to retrofit an LSD to my own 996 Turbo.
Stephen
p.s. I would not object to moving this thread to the 997 Turbo board or possibly it is time to merge the two boards?
