is the only difference between the 2 cars the turbo's?

 

hollow spoke wheels too, exhaust, engine itself (TT, GT3 and GT2 use GT1 derived, wehreas NA 996 incl. C4S do not)...
Maybe headlights (I never checked but the C2 and C4 have ribs in plastic unlike TT which does not and shows more chrome )

 

Suspension is recalibrated on the C4S for difference in weight/weight distribution.

 

Cosmetically:
There are no rear ducts for intercoolers, no rear spoiler, slightly modified front skirt (to compensate for lack of rear spoiler), red decorative strip linking the rear lights, the central intake ducts air to the brakes,

Running Gear:
Slightly softer rear springs to compensate for lack of additional engine weight of turbo. Otherwise, the running gear is identical.

Engine
There are significant differences in the drive train and engine, It's not just a matter of adding a turbo and intercoolers. Look to one of the Turbo guys to outline all of the differences but there are differences on the engine cams and bearings as well as flywheel, clutch, gearbox, etc... You get the idea. BIG DIFFERENCE. Wish I had one.

 

And the difference is:

$ 35,000

 

I belive it's a completely different engine block, even though both share the engine family designation.

 

They both have a flat 6...and that's about it.