If the car is pre-1996 you can take the cat off and not experience any problems. If it is post-1996 then it is an OBD-II system which locates an O2 sensor behind the cats to monitor their health. With the cat gone the computer would see they are not working, throw a CEL, and put the car into reduced power mode. There are ways of tricking the O2's on certain cars but I don't know if you can on Audi's.
u can get an o2 simulater but they have problems wit that sometimes also my friends 04 is still knew something was rong even wit the simulater so its not realy recomended. But i took the stock resanater out and replace it wit a high flow wit out any problems.. not to noticable gain but it help u can see alittle bit of a difference nice sound also
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1997 Audi a4 2.8 Quattro Color: ORANGE Location: NJ
We have sold a couple of kits for imports and some domestics here at the shop. Sound quality is along the lines of a Flowmaster, build quality and materials are along the lines of Borla and some of the other nicer kits (all stainless, and the kits we sold were all polished, pipes and all).
On the imports the sound was smooth, had a good deep tone to it, not buzzy at all.
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Exhaust question - 
