I have a 2000 A4 1.8tm and I ordered a magnaflow 14815 muffler and and magnaflow resonator that I want to put on once they both get here. My question is what size piping should I go with??? I've seen most setups with 2.5" piping but I just want to get some opinions before making a final decision.

 

2.5 is good if you are mostly stock or mild tune. if its possible, go 2.5 from cat and taper down to 2.25 at the muffler. this would help with keeping the exhaust gasses moving because the exhaust will be a little cooler as it travels down the pipe.

the size of the pipe will also change the pitch of the exhaust. too big and it will sound deep and too small will be high pitch.

 

Yes ok with 2.5 but 2.25 it is better. I know it will not affect much on that I have seen the custom exhaust with the 2.25 pipe fitting. This is suggestion and you can do what you want.