When I had to meet a decibel rule at Sears Point many years back, we added a 3.5" exhaust w/race bullets & turndowns at the axle.
The 368" SB had a fairly low-tension ring package & the usual evac setup into the collectors.
That seemed to render the evac system useless.
Changed to a GM passenger car electric pump, which fixed it inexpensively.
Pulled about 5" or so, IIRC.