If you want a quiet, efficient muffler purchase Spin Techs. On a 410 and 360 sprint car engine they showed almost no change in HP or torque during dyno pulls. The slight change noticed could easily have come from engine temp or atmospheric condition differences between the pulls. I am not affiliated with Spin Tech but if you have spent any time in the street performance market they do not drone, are not excessively loud, are competitively priced and are a quality part . www.spintechmufflers.com
Randy