RB TII exhaust?
 

Do you need a FCD for the old style RB Turbo exhaust? The one that is not the polished aluminum? And are they interchangeable for an S4 and an S5?

if you keep your cat then no. They are all the same exhaust. Yes.

So if you have the turbo-back with the pre-silencer you'll need the FCD?

not if you like fuel-cut.

Why not just install one? For the money it sure beats guessing whether or not you need it and it ensures you peace of mind:)