Well, I do have a bigger bore downpipe, circa 2.75", which should help but I doubt I have any real back pressure at all apart from what the turbo itself produces. With my exhaust having nothing in it except the pipe, the gases get out pretty quick - I never seem to have the boot sooting up issue like other cars and I can only put this down to the speed the smoke comes out the tailpipe and it never gets dragged into the aerodynamics of the car. My car was tuned to 220bhp originally with just a pd100 downpipe and the original pd100 exhaust system, minus the cat - so big power can be had even from close to stock exhaust systems.