MG ZR exhaust.

It's a BMW trick and it does increase power output.

How? As I understand it, the tuned length of an exhaust pipe effectively finishes at the first box. After that it's just a question of getting the gasses away without further restriction. Or is the idea that adding backpressure alters the pressure wave pattern in the tuned part of the exhaust? Mike.

I think the increased back pressure at low revs aids low down torque, then opening the second pipe at higher revs helps top end power.

On the ZR I looked at, the butterfly valve was open at rest, so it would have to close as soon as the engine was started. Not unlikely I suppose as there would be lower pressure in the inlet manifold, than at WOT. Can't understand how more backpressure would increase torque though. Surely by restricting the exhaust gases, you're restricting the fuel/air intake of the engine? Mike.

It's also to reduce noise at lowish engine speeds.