Engines need back pressure to operate at max output which is supplied by mufflers. Catalytic Converters for EPA reasons and mufflers are required by law although many backyard mechanics punch out the insides of the converters (illegal ) but growing up with the muscle cars of the 50,60 and 70s where many states outlawed glasspack mufflers for noise reasons, the best legal muffler we found and used were Corvair mufflers which were just as good as glasspacks but legal in all 50 states and gave the back pressure to our huge HP engines without loss of power. The Corvair mufflers worked great with Header exhaust manifolds
