3inch down pipe into 2.5inch catback ? 2.0t b7 a4
 

I'm getting ready to put an exhaust on my car and my downpipe is larger than my catback, both are advertised as 2.5 inch but I believe the downpipe is more like 2.75". I can put them together with a welder, my question is should I be concerned about a power loss or some sort of flow issue ? The car is ready for stage 2 post the exhaust, once that is on I have the flashed ecu.

Thanks in advance

Nick

No input on this ? My downpipe is 3inch at the turbo and drops down to 2.75 at the flange, is there any danger of "bottle necking" if I use a 2.75 to 2.5 adapter to get on to my 2.5 inch cutback ??

Depending on the rest of your setup, this shouldn't really be an issue, if you just have or are looking to get a basic tune (stage 1 or 2) it should not make much of a difference, unless the smaller pipe protrudes far into the larger one. but even that probably won't make a noticeable difference. but the turbulence it would make is technically "bad". all the major tuning companies recommend a 3 inch down pipe for tunes, and most classify it as required for anything past stage 1. but until you get to injectors, hp fuel pumps etc... they make no mention of anything past the downpipe. in all honesty, it probably isn't so much the diameter of the downpipe they are worried about, they are most likely concerned with the inlet and outlet diameters of the cat (which are limited to the diameter of the pipe) to make the cat efficient while not being restrictive.