I asked this question a few years ago and got no real definitive answer to my question.
Has anybody seen inside a Vectra petrol and diesel exhaust?
Reason being that I fitted a petrol Irmscher back box to my last Vec (2.2 DTi) and while I didn't feel an instant drop in power rolling roads seemed to tell a different story
Before fitting the exhaust I had 155bhp (can't remember the torque figure) and then after I had 143 (just before getting rid of the car)
Now there had been a good 2 years between fitting the exhaust and getting the second r/r done so i'm not sure if it was the exhaust that caused the loss in power.
Fast forward to this car (1.9cdti 150) and before getting my remap I fitted the exhaust.
My before (remap) torque figure was 316nm (which is about right for a standard cdti right?)
The after figure was 384.2nm which seems to be about 16nm down on what everyone else gets.
The only thing that I can think is that the, exhaust was fine with a standard map but is slightly restricted by the exhaust when remapped.
Would this make sense?