[Christ]

Simple rule of Physics - All fluids will follow the path of least resistance.

When watching the flow of a fluid through different paths, one thing is clear - it will always try to flow straight.

If it begins to flow in a straight line, unobstructed, and you put something directly in front of the flow, the flow will first attempt to push that object out of the way. When that can't be achieved, the next option is to flow around it.

When you put a bend in a pipe, you're essentially blocking optimal flow - causing it to change direction. When you do that, you're slowing it down. When you slow it down, pressure increases. When pressure increases, pipe diameter must increase to lower pressure. This means more weight to the system, more expansion area for the gasses, and more surface area to leech heat from the exhaust gasses, cooling the exhaust.

Ideally, the exhaust's path will be straight as possible, with as little restriction to flow as possible, and only the correct diameter to flow exactly the necessary amount of fluid (gasses are fluid) from A to B without creating as much pressure. This means that the pipe can and should be a smaller diameter when dealing with low volumes of fluid, so that it can't expand outward as much, and thusly, can't cool as fast. This creates (again) the largest pressure differential across the valve, which aides in scavenging.

Harmonics also apply to scavenging, in exactly the same manner that they apply to intakes.

Tuning the length of the pipe so that the pressure waves are pulling away from the valve as it opens will create a lower pressure gradient at the valve seat, further facilitating exhaust flow with less engine power required. The exhaust tuning doesn't mean you have to make the pipe shorter or longer, it means you have to make a shorter or longer primary tube, which opens to a different mass.