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I believe nothing of the sort, at least relating to the explanation you put forth. Back pressure is one of the most commonly mis-used terms on auto discussion forums.
I believe that your engines exhaust size relative to your RPM's and desired performance is determined by volume. For exhaust pulses to flow properly, they need to stay hot. To keep exhaust pulses hot you have to have the right size exhaust. If your exhaust pipes are too large, your exhaust pulses cool which slows the flow of exhaust and causes the engine to have to work harder to push against that cooled exhaust soaking up power. This is one reason why guys who run pipes on the larger size of the spectrum for a particular engine will notice lost torque down low, because there aren't as many revolutions of the crank shaft which means less exhaust pulses, which means cooled exhaust, but when they rev the piss out of it, there are much more exhaust pulses which heats up the pipes and you get High RPM performance. There may not be one perfect exhaust size to cover your entire power band of your engine to have good power and torque at all RPM's, but I do believe there is one size that will satisfy most.
Another thing that influences performance is turbulence. An exhaust pulse is exactly that, a pulse of exhaust. In behind every exhaust pulse is an area of negative pressure, sort of like a vacuum. This vacuum pulls on the exhaust pulse behind it. This phenomenon is called scavenging. At a certain RPM's, your engines scavenging effect is so great that (read, efficiency!) that exhaust pulses passing by exhaust valves help pull other exhaust pulses out of the more downstream cylinder. When you start crush bending pipe so that you have bends that are smaller than the diameter of the straight pipe, you create turbulence of these exhaust pulses as opposed to mandrel bending which has consistent size in both the bends and straight sections.
Generally speaking I think you're going to find that smaller pipes will offer you better low end torque, and larger pipes will offer you higher RPM horsepower.
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