If we had the EcoModder chassis dyno we could test your car under load.Your decibel reading might be different if the engine was really 'working.'
Some messengers have reported a need for back pressure for scavenging at low load,and less restriction as you climb the power curve.
Some motorcycles have dyno-tuned 'Smart' exhaust systems which incorporate a butterfly valve which is computer driven,always holding the valve in the 'sweet-spot' according to test cell development mapping.
I raced a Honda in high school and it's power fell off without the factory muffler.A reverse-cone megaphone exhaust provided a nice balance between low and high end performance.Some of my friends fathers were rocket scientists and they could explain the physics involved.Kind of tricky.
