i just want to clarify that it isn't backpressure that affects an exhaust. its the diameter and length of the pipe. as each cylinder fires, it ejects hot gas into the exhaust pipe. at a given engine speed and pipe length/diameter, this 'pulse' will reach the end of the exhaust at the same time as the another pulse is leaving the cylinder. hence the vacuum created by the pulse 'pulls' the exhaust gas out of the cylinder.

in other terms, its like blowing through a straw or a toilet roll: each will have a certain optimum airflow where any more just creates back pressure and any less makes for low pressure at the other end.

so when you pick your exhaust, you have to pick what size of pipe you want to use, as this will determine at what revs the pipe will flow most efficiently