Lukas is absolutely correct. It is in the product information guide (the one that's intended for dealers).

Quote:
From the 2005 C2S Product Information Guide, page 29:
The clutch system in the two models is different.



Quote:
From the 2005 C2S Product Information Guide, page 30:
Clutch on the 911 Carrera:
The new 911 Carrera with 3.6 litre engine uses the familiar single-plate dry clutch with lead-free clutch lining.

Clutch on the 911 Carrera S:
The 911 Carrera S with 3.8 litre engine has been upgraded with a self-correcting X-Tend clutch. The X-Tend clutch constantly registers the decrease in pad thickness and compensates the difference in distance caused by pad wear by rotating an adjusting ring. This has the advantage of halving the growth in force that increases over the operating period until the limit for wear is reached in comparison with a conventional clutch.