RE: cat back and down pipe
 

Yeah! Your ECU won't like that very much and you'll loose your ULEV rating. IN other words....don't do it. It is a cheap Chevy Camero mod. not for a modern car.

RE: cat back and down pipe
 

f&ck ULEV, i don't care about the atmosphere. No just kidding, maybe not. The CAT is where alot of the HP will be gained in the exhaust system because its right after the turbo and restricts the flow. You can get a test pipe but you will get DTC's and a CEL but it doesn't affect your performance or driveability, its just a light thats on your spedo, not the thong lookin thing. Some people run them and have never gotten a CEL but most people do. The other alternative is a high flow cat, but they can be pretty pricey. Milltek makes one for the b5 A4 and it is like $650. Search on audiworld's forums, alot of stuff is documented over there about them.

RE: cat back and down pipe
 

No it will be fine. i have done it. you just need to put a second O2 sensor down the line. and in order to keep the computer calm you need to use sparkplug spacer to get the sensor up so it doesnt see much turbulence

RE: cat back and down pipe
 

does this modification increase the hp´s ?

RE: cat back and down pipe
 

are you talking about removeing the cat? if so yes more hp and more mpg