i think its down to the decat and backbox, luckily my dad has an m3 to and this is where im getting all the other bits from to try so it doesnt cost me any money.

the o2 sensors are both still conected and the decat is a proper one, not just a pipe welded in place, its a supersprint system with resonating boxes to sort out the back pressure.

i think its going to need a map on the rollers to sort out but im selling the car so might just put the std zorst back on.