The pipe joining the centre section to the CAT is 2" as this is the diameter of pipe on the CAT. The rest of the system is 2.5". I was one of the first to have this system fitted on my Trophy back in 2008. Still on the Trophy and I have had no issues with it. Don't expect any noticeable power increases with any CAT back system, I chose the RS192 as it is one of the quietest after market systems say compared to a Yozza or Orbisoud system. Near enough close to standard until you put your foot down.