Originally Posted by Jeff@TopGearSolutions
I think the only reason people want a catted version is for emissions purposes in some states or to reduce the smell/noise/raspy tones.
I agree the catted versions should be avoided, they tend to fail over-time, and really defeat the purpose of replacing the downpipes, which is to remove the biggest restriction, the cats.
That reminds me of a couple years ago when I was still somewhat into the Audi scene and I was helping my brother diagnose an issue with his B5 A4 QT.He was having low boost issues and blowing white smoke. Pulled the turbo several times and found nothing wrong with it. It was a brand new ATP turbo that we installed a couple thousand miles prior to the issues. We checked several other things including changing injectors etc. and could not diagnose the issue. Even enlisted the help of some of his local friends who were supposed to Audi gurus. I mentioned to them that I thought the HFC may be the problem and they all dismissed me. Well after pulling the turbo a final time we decided to remove the HFC and inspect it. Upon a visual inspection we found the honey comb material completely FUBARD. It pretty much became dislodged and clogged the exhaust path and melted. This caused the back pressure which ion turn resulted in low boost and seals blowing oil.
I'm going to have to bookmark these bms dp. I love the quality of the tig welds but I'm not ready to go catless just yet bc of the smell and I'm also still under warranty. I have heard allegedly thought that with a quality protune you can minimize the obnoxious gasses.