i did something once on my civic that probally won't do you any good on an audi. I had a 92 civic si with 2.5 stright pipe no cat and how i got though emissions was i cut the sheild off the cat and slide it over the exhaust pipe and put it about where the cat was on the car and welded it in place went though 3 emissions tests with it like that and no one ever said anything about it

 

That's what I'm talking about. Wrap the old cat housing around the test pipe, that way it will look like the old cat.

 

It'd be much easier to hollow your cat, put another pipe inside, and then weld it up, rather than trying to peel the heatshield off the other one without making it look obvious what you did... any good exhaust guy would be able to tell.

 

If I want to build my own test pipe, what size pipe do i use? what is the diameter on the back of the turbo?