First off, contrary to belief there is no such thing as backpressure. Second, it is all about the exhaust gas velocity. If you are just looking for a bigger sound go for a larger pipe,but you will hurt your low rpm velocity. Especially if your driving this car on the street. You will have to wind the engine up to get it to produce good torque. As doug 71 said, the larger the pipe you will move your power band higher. This is because when you have a smaller pipe you don't need as much air to keep velocity at a decent rate, but when you use a bigger pipe you create a dead spot in velocity which you feel as a loss of torque. When your rpm gets higher you start sucking more air which in turn makes your velocity higher for that given pipe size that's when you feel the power come on. But most 318 will be happy with a dual 2 inch if your driving on the street. I use dual 2 inch on my 360 pickup, and I get 18 mpg highway because I don't turn high rpm and I need the lower speed torque aka velocity. But exhaust pipe size also goes hand in hand with cam size as well. Obviously a low lift cam will have higher torque at a lower rpm so you will need a smaller pipe.
