Yeah the straight thru mufflers w/ and X-pipe make the most power but don't sound as good Flows w/ and H-pipe. Zero, you can't hear the Am. Thunder system just the 2-chamber (40 Series) mufflers it includes. Go here
http://www.flowmastermufflers.com/we...PerfMuff4.html to hear the mufflers. The recording is actually on a 5.0 but they sound very similar to the 4.6. The American Thunder kit is just the 40 series mufflers plus tail pipes and flow tubes.