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Back pressure is a commonly misused term when people speak of exhaust systems.... Velocity and flow are the important things in an exhaust.
The purpose of an exhaust system is to simply provide a way of exit for exhaust gasses. In order to get the maximum performance you want an exhaust system free of as many restrictions as possible. Obviously having a larger pipe diameter can increase flow capability...However you can go too far! An engine can only put out so much air. If the pipe is too large you loose velocity of the exhaust gasses. When that happens the exhaust cools and condenses. The engine literally has to push the exhaust out of the pipe. This pushing creates a differential which people refer to as "Back pressure".
Ideally you want a smooth pipe of the proper size that flows the perfect amount of air at the engine's most efficient operating RPM. This keeps the velocity of the exhaust gasses high which helps efficiency.
But wait... there's more! Just putting a pipe on there is going to make 5-0 unhappy... so you have to account for a muffler which controls noise and a resonator which controls drone. Placement of the muffler and resonator is very important. If you are in the USA you will also need a catalytic converter to make the EPA happy...
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