I have to add more. You say that "because of the resistance to flow will cause pressure behind the resistance..." but this is not true. Just because there is resistance doesn't mean there is back pressure. Take a 90 degree bend in the pipe. That bend is a restriction or a resistance to flow. However, it may not cause pressure in the exhaust and so there wouldn't be any back pressure. Another example is my Suburban's exhaust. It has many bends that are restrictions as well as cats and a muffler. These all cause resistance to flow. But when I hooked up the back pressure gauge only the side of the exhaust that had the plugged cat had back pressure. The side with the good cat had 0psi. So it has resistance to flow without having back pressure.
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