Alright so my roomate has a 2001 audi a4 1.8t. He just bought a k&n cold air, a diverter valve, and a borla catback. Im going to be installing all this for him. he is looking to buy a test pipe and downpipe. I am new to audis. i have had bmws and Rx7s. from what i understand, there is a downpipe with a cat, then a cat, then the muffler section. the borla is 2.5 in. I am looking at downpipes and test pipes. i see so that are 2.5 some that are 3 and some that are 2.5 to 3. All i am looking for is a down pipe and test pipe, that will bolt up to the Borla catback. Please help... would like to order the parts asap.
Charlie

 

Also he has an apr stage 1 and will get the stage 2 when the parts are put on, so from what i understand we dont need to worry about the CEL for the test pipe?

 

figured it out. this is not like a BMW or RX7. the downpipe IS the test pipe. the diameter they are talking about is turbo side... Ordered an ATP one.