Quote Originally Posted by 1project2many View Post
I've seen custom systems built which place a pipe inside a pipe to reduce heat loss. Maybe this is why some GM trucks have a similar configuration on the Y pipe pre-cat.
at the very least, on 1st/1.5 generation W cars with 3100s, this happened at the factory as well. the downpipe looks to be in the 2.25" range, but that's actually just an "over" pipe. the actual downpipe is somewhere in the 1.75" range(I think even 1.5" at portions), then it hits the cat and comes out at 2" post-cat.

different theories have been thrown around and I think it's a good mix of the 3 big ones: lower emissions, less heatshielding necessary and more quiet engine operation.