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Our events have many GT4s on track and most pass the sound test. Those that don't, are very aggressive on the throttle to the red line. Sound check is at the tower on the front straight (wired directly to city hall) and as mentioned in T6. Try and see, if there is a problem with the GT4 a short shift here and there will still allow you to enjoy the track.
Sounds like the simplest solution is to cut the wire to city hall.....
I passed sound with a 991.2 cup, put the road car OEM side mufflers and center exhaust on the cup and close the valves shut with metal wire. not the best solution but makes the car quiet as a whistle.