What's the difference between early and late cat delete pipes?

[Dave80GTSi]

Originally Posted by BYprodriver

Exhaust manifold outlet diameter is larger on 2000 up "S" maybe base engine also.

Yeah, I'd heard that exact same size difference anecdote myself as well.

But the early used non-cat manifolds (bought from Woody), when compared to my 2000 S USA cat manifolds, measure -exactly- the same.

Measured at their cylinder head flange openings (3 pipes) as well as their outlet flange (pictures of which I previously posted above), there are no pipe size differences between these different variations (other than maybe 1 mm or so which I attribute to manufacturer's production tolerances and/or carbon build-up).

Which makes the flange bolting pattern change, as pictured above, all the more puzzling to me.

But, even so, if you might have any photos which show my measurements to somehow be a weird fluke, please post here!

Thanks - DM