There may be an area calculation, I don't know.
We also got a Db rule at Sears Point in the '90s; so I got a Dr. Gas X, added a 3.5" system to my 3" collectors, Dynomax race Bullets, & turndowns before the rear axle that slightly faced each other.
Much quieter, though still kinda loud, didn't slow the car down, sounded great.
Funny thing is how it changed the pitch: In the burnout (6,000 max), it sounded like 10,000 RPM - people would back away from it
You can Google the Dr Gas X, and look, but none seem to have a huge common area.