Audi RS6 4.2ltr This page contains pictures and details the exhaust system including fitting the Remus exhaust system upgrade,

the twin 2 1/2" exhaust system has four catalytic converters, at least that's what i could see, two behind the firewall and two behind the gearbox, [there maybe pre-cats off each the four turbo's but very hard to see] the flex bellows absorb any harmonic vibrations from the exhaust while the oxygen sensors analyze the exhaust gases

The "X" Pipe
The exhaust gases from left and right engine pipe mix in the x section,this evens out the exhaust gas back pressure, giving the engine
a much smoother note. A slight reduction in overall diameter in the x area, the exhaust gas speed increases through the x which helps [slightly] by creating a scavenging effect .

 

 

The Remus rear mufflers sound
even better than it looks