GOALS OF AN ENTIRE EXHAUST SYSTEM
The 2 goals
of a header-cat-exhaust
system (notice I didn't just say exhaust only) is to: a) to efficiently remove as much of the combusted inert exhaust gases out of the cylinder.
Remember that burnt exhaust gas is inert or does not combust twice (EGR & fuel economy is another story) and therefore cannot make power if it is in the cylinder...it takes up space in the cylinder and prevents fresh air and fuel from coming into the combustion chamber to make power....