Hello. Ive just cutted the backbox off the Fabia Rs 1.9 tdi. I can feel it have some power loss. Could you guide me how much extra fuel it needs to compensate the loss of restriction? Very much appriciate.

It's normal to have some "turbo lag" after that, try to mess with N75 (Very little at the time), to get less lag.

But first of all, test and verify all vaccum tubes and N75 valve.

Regards.