[AzCivic]

here's another way to put it, read it slowly and let it sink in:

"In order to understand how the merge collector works, you must first know why one exhaust system can generate more torque and horsepower than another. The ability of a header/exhaust system to maintain exhaust gas velocity has a tremendous impact on realizing maximum torque and horsepower. Any significant changes in the system such as baffles (mufflers) or collectors will have an impact on exhaust gas velocity. The large area change seen at the transition from the primary tube to the collector causes an immediate reduction in exhaust gas speed. This reduction in speed decreases the efficiency of the system thereby reducing scavenging ability. Scavenging ability is the key factor to which an exhaust system is evaluated. The more efficient the system, the more air/fuel mixture that can be pulled into the combustion chamber. Obviously, the greater quantity of air/fuel mixture that can be burned, the more torque and power potential the engine has."

copied and pasted from dynatech's website.