Auspuffrückstau TDCi

Since the tdci won't have any screws, you won't be able to turn any either.

I don't know anything about backflow, but I can't imagine it being a problem. The turbo is already in the way, so it doesn't really matter what's behind it.

However, using a pipe as an exhaust system will only result in loud noise and no increase in power.
More boost pressure leads to a faster failure of the turbocharger and does not result in more power.

So, your device will just get louder and break, and it won't go any faster, not even by a millimeter per hour.

m;

hmm, so, does the diesel now need a reverse valve, or does it actually not matter

It's highly unlikely to make any difference without a catalytic converter if you're running a turbocharger.

Here's something else on the topic: https://community.dieselschrauber.org/en/viewtopic.php?t=7048

Den Kat baut man meist nur bei Extrem tuning raus, zb. Motorsport.

If extreme tuning modifications are not planned, such as increasing the horsepower from 110 to 200.
Then you can safely leave it in.
I myself didn't notice any significant improvement in performance with my AFN, so I put it back in.

And under no circumstances should you remove the OXI --cat if it's broken, just to save money .