FYI for those in CA:
"Exhaust systems which meet smog requirements but are still too loud. Exhaust equipment that states it is, "Legal in all 50 states," refers to smog, not noise.
Note: New law (27150.2 VC) does not require law enforcement to use sound level meters to test for excessive noise. Citation is based on officer's judgment. Cited violators may have testing done at smog referee stations or may be directed by the court to have testing done. Vehicles in violation must be brought into compliance. A fine may also be imposed."
http://www.chp.ca.gov/community/streetlegal.html