Give BCS a call, they've probably got loads of OEM downpipes!
When you gutted your precat out, did you do it facing the floor so that none of the 'crumbs' fell back into the downpipe? That's the only way the main cat would get blocked really.
N75 should fault code I would have thought? Underboost errors etc.
Pre cat was done by my usual garage. They had never done one before so I explained that it needed to be insured that none of the material fell further into the exhaust, they seemed perfectly happy with this and appeared to be comfortable. Sticking my head round the corner from time to time they appeared to be doing all the right things. Still, may not be this after all.
Do you think BCS would be interested in supplying me with an old downpipe? The thought crossed my mind but I though that could be a bit cheeky, as they are in the business of selling replacements for that very component. They'd rather sell me one of their downpipes I'm sure! Still, a maybe - can't hurt to ask!
As for N75, niki said his maps work on a fixed N75 duty cycle set, presumably from a load/rpm lookup table. Though the N75 may be perfectly fine, the pressure regulator and then the mechanical linkage and waste gate further down the line, may have an issue. If it is a mechanical issue, there may not be a sensor which could pick it up. I.e the valve itself may be doing what it should be but the pieces of metal it drives could be faulty.
Time will tell! Definitely learning more about the car as a result if all of this.
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