Compare the Audi A8 S8 4.0 V8 Turbo - [2011] vs Porsche 911 Carrera S Cabriolet 997 - [2008]. Find out which car is the fastest, most powerful, lightest or has the biggest engine in our top trumps style comparison page. Find out which car has the highest top speed or is the fastest accelerating, is it the Audi A8 S8 4.0 V8 Turbo or the Porsche 911 Carrera S Cabriolet 997. We'll leave it up to you to decide which is your favourite.