[djone101]

You will always have back pressure in an exhaust system. It depends where you measure it as to how great it is. Also it depends how you measure it. A single pressure tapping can often give an inaccurate reading due to the gas dynamics. We prefer to use a piezo ring which has a series of small holes in the circumference of the pipe with a chamber running around it feeding the pressure transducer.

Haven't read the original thread but I have spent a lot of time working on exhaust systems usually trading off minimising back pressure whilst optimising noise...