Originally Posted by Ilia@IND
This is a common misconception. A better flowing exhaust system will increase power and torque simultaneously. Horsepower is a function of torque, and therefore a gain in horsepower can't really come with a loss in torque. Some modifications will move the powerband, causing a loss in low end power with a gain in high end power, but the claim that a loss of power can result from reduced backpressure or increased flow is incorrect.
When designing exhaust systems, it's important to balance two key factors: exhaust flow velocity, and exhaust flow volume. The intended goal is always to evacuate as much exhaust gas from the combustion chamber as possible, to provide room for a clean intake charge in the next intake stroke, and even to possibly introduce a scavenging effect that will "force" intake air into the combustion chamber.
A high flow velocity will help to evacuate exhaust gasses. A high flow volume will do this also. The issue is, to increase flow volume, you increase piping size. To increase flow velocity, you reduce piping size. As you increase piping size, flow volume increases, but flow velocity decreases. As you decrease piping size, flow velocity might increase, but flow volume will decrease. So of course there is an ideal balance for each engine, and this balance is different for each engine based on it's volumetric efficiency, target operating range in terms of RPM, etc... Of course there are other factors involved, and this is a highly simplified version of things, but that's the basic rule. At the extreme end, a 6" pipe would reduce flow velocity to such a degree that power loss would occur, while a 1/4" pipe would reduce flow volume to such a degree that power loss would also occur.
This is how the "too much flow is bad" myth got started. Too little flow velocity is bad. More exhaust flow will mean more power every time, all other factors being equal.
Of course, there are many other factors to consider, and I'm by no means an expert in exhaust systems engineering, I just know a few things.