It really depends where you check the diameter at because its not really very round in most areas.
From what I've seen the stock Y pipe section is about 2.5" O.D. and 2.375" I.D., the wall is roughly 1/16" of an inch thick.
I've checked it with a dial caliper before I bought two sections of stainless pipe to replace my cats with and the peices I bought are 2.625" O.D. and 2.495" I.D.
The tubing is about .065" thick wall for the sections I bought.
http://www.burnsstainless.com/304tub...raight304.html
The section that used to attach to my muffler inlet is about 2.75" O.D. from what I remember when I cut it off to install the turbo. The outlet was slightly larger, about 2.812' for the tailpipe.
I guess GM could not decide on which size to go with?!?!?
Seems a little odd that all these different diameters were used.

 

Speaking of flay spots....has anyone welded in a round piece of SS where that flat piece runs parallel with the crossmember? Looks like there could be some restriction there....

 

Not yet but thinking about that too!

 

Yeah, I'm thinking that while I'm in there....

 

You may as well do it, it can't hurt anything.
I wish I had a damm welder man!!

 

all i can add is on stock style exhaust is it goes to 3 inch before the muff.

 

Yeah, but pull apart the flange (where a cat-back bolts up) and measure the diameter of the piping in there....I think it's 2.75", or maybe even less.

 

as stated earlier
the exhaust piping right before the cat, and after the manifold flange is 2.5" ID.

 

Does this nullify the positives of having 3" piping before and after?