You are right, your six is almost 5 litres. But overall engine design has a lot to do with exhaust note. The size of the explosion, the volume of exhaust gases, the frequency of resonation in the pipes due to firing order & exhaust event timing (6 fires every 120 deg, V8 fires every 90 deg of crank rotation), header choice, exhaust port design, camshaft profile, etc. Not sure if flowmaster makes a muffler for smaller than 2 1/4" pipe, you'll have to look. There are a bunch of different mufflers, pipe sizes and routing options that will affect the sound.
Maybe next week I'll try to share what little I know when its not friday night and Im trying to get away from my work computer.
good luck
chip
