The six inch diameter round DynoMax UltraFlo mufflers have no more restriction than a resonator, but have greatly added sound reduction.

You can buy the stainless steel UltraFlo's in either
16 inch,
24 inch long or
and "RV" style 36 inch long.

36 inch long as the main muffler
and a 16 inch long as the exhaust tip
makes for a quiet system
that has a very low deep tone