I've never changed more than described in S4wiki

So usual rear o2 deletes and FKHABMN to 0

I think there is more possible, to also fix up BTS model but i'm not sure if its of any use

Thats the reason why I asked. I just passed inspection, so I am good for another year. I am saving up for a race CAT, so next year this wont be a problem. I ve just driven the car and yea no real performance, but it does speed up and slow down a lot smoother. Throttle response feels also way better.

Placebo is a wonderful thing. It does absolutely nothing apart from increasing noise. Your car with stock turbo 180hp? Is fine even with a 2.25" downpipe. 2.5" is enough for 380-400 hp. 3" is simply retarded on this engine. It's same way retarded as putting a giant 1 meter cone filter via a reducer onto a turbo with 2" inlet, except in the opposite way.

Helps if you are familiar with the ideal gas law and how gas volume reacts to temperature drop.

Well written but in the wrong order. You wrote what accounts for 5% (max) first and 95% second.
Exhaust gas post turbo is up to 850C, exhaust post downpipe tends to be already 150 deg cooler. Very easy to see on modern Diesel engines with three or more EGT sensors at different places in the exhaust.

You need someone who understands how an engine works to make the project.
Just because you can weld an exhaust or rebuild an engine does not mean you have any clue about the combustion process.

Let me guess, you have no idea how to get it tuned afterwards either, in fact, that will probably be the last thing you think about and will be very surprised when nobody who is any good at this is interested, as they did not build it.

Find new mechanic or company. This one is clueless.
Also, my comment was extremely relevant. You have no clue about anything. You really expect that you are going to do a EFR7163 project with this level of knowledge?
Once you bolt all the parts together and nothing works, nobody will want to tune it. You clearly won't be able to do it yourself, so before wasting a ton of money, then selling the car for parts, go and find a company who can do the whole project from back to finish including the tuning.

Not a "mechanic", a tuning company.

Also, you are nobody to moan on here. Contribute 0.1% of what I have, then talk.
So far you have just asked a bunch of questions, most of which can be answered by search.

Hello gentlemen, I am not experienced in tuning because I have been playing and learning for 1.5 years, but I have 20 years of experience in mechanics.
From my experience with the exhaust in my car a3 1.8 t aum it comes out that the exhaust of the catalyst and the 3-inch pipe descending gradually to the original exhaust gives better flexibility in revolutions below the operation of turbo k03s, I do not know about larger turbines. My configuration is 3 inch pipe then factory center muffler then end pass with branching to blank silencer above it. Modified flight installed side water intercooler and whole installation cooler tank pump all under bumper. 320 injectors factory fuel pressure regulator 4bar.
The engine is mainly powered by propane-butane gas, boost in peak 1.2 bar, down to 0.8 at 6600 rpm, max flow from maf 164 g / s, max inlet temperature at external temperature 27st / c = 55st / c ignition +3 degrees in in relation to the series. I wrote through the translator, sorry and greetings.