The issue of sound control is really affecting the hot rodders who autocross. SCCA (and many road courses) is having to enforce noise limit (dB)standards. When I was autocrossing, I had to pedal the car when in proximity to the dB meter. If I were to really pursue autocross, I imagine my fix would be (either to run a stock Miata in stock class, or) to mount a huge (like 4' long) RV muffler cross-wise from the rear bumper mounts external to the car. Something I could install with v-bands the night before the race. I don't know if it would be better to direct the outlet straight toward the sky, straight back, or straight at the ground.... but getting scolded for having a car that sounded as good as it looked and ran (perhaps with a better driver) really sucked the fun out of hanging with the SCCA crowd. It's not their fault-they are just doing what they gotta do to keep the facility.