My 2 penneth. We have a 120db max sound level for 9.00 cars / classes and slower. They are regularly checked at race meeting but there no sound level in place at a RWYB's . I ran a true dual (no H or X) 4" system with bullet muffler to the axle with turn downs. No noticeable effect on ET or MPH, db dropped from 124.9 (open headers) to 120.4 close enough for the tech guys to pass. I later made an X pipe using the same 4" dyno max mufflers, again no noticeable effect on ET or MPH db dropped to 119.8 when tested. I've since welded V clamps to the pipes to make swapping stuff out easy, we've ran with the x pipe but no mufflers, again no noticeable effect on ET or MPH. I've yet to be tested for sound level in this configuration but to the ear it is a little louder. For the street we have a pair of old super trapps which are v clamped on to the end of the bullet mufflers in place of the turn downs. Never ran on the strip with these, I'm guessing they woukd really hurt the performance. Last time I ran with just open headers, I burnt all the candy paint off the trans crossmember