The small diameter opening at the tail of the header is a "hot tube" restrictor which acts sort of like a cheap catalytic converter to burn off some remaining emissions before they get out into the air.
This is very restrictive for performance, and most people remove it by using the right size hole saw to core it out from the back end. This leaves a full size pipe all the way thru, and then you add a free flow silencer, and a free flow air filter and re-jet the carb.
This gives a nice boost overall, and is worth the effort.