Looking at this Fred:
It seems that the XS DP does from 3'' straight down to 2.5''.
You're going to have two options.....
1. Fit the milltek using a 2.5 - 2.75 reducer onto the XS decat section. Simple option, but doesn't give the best flow.
2. Cut off the 2.5'' section from your downpipe, and either buy a 500mm length of 2.75'' pipe with a 3'' flare on one end and clamp the extension on, or get a 2.75'' section welded onto your 3'' Decat.
Annoying I know, but it depends how fussy you are!
On the daily, when I first fitted the decat, I left the downpipe as it came, which was 3'' > 2.5'' > 2.25'' in 3 steps. I later fitted a cat back 2.5'' system, in the way we speak of above, using a reducer and leaving the short length of a smaller size in situ.
After a few weeks I checked under the car, and found that the short 6'' section of 2.25 I'd left in was bright purple (HOT), yet the rest of the decat was still shiney silver. That seems to be like a pretty clear sign that it's a hot spot, which surely points to a flow restriction at the reduction?
I cut the 2.25'' section out and let in a 2.5'' to keep the bore all the way through, and everything then discoloured at the same rate
at leaset the top one is easier to access.
