These are a couple quotes from the "blue book" - which is the California Emissions handbook.
www.arb.ca.gov
Don't know if this helps - I think they right these damn things so they can interpret them as they see fit later.
§ 27151. Modification of exhaust systems
(a) No person shall modify the exhaust system of a motor vehicle in a manner which will amplify or increase the noise emitted by the motor of the vehicle so that the vehicle is not in compliance with the provisions of Section 27150 or exceeds the noise limits established for the type of vehicle in Article 2.5 (commencing with Section 27200). No person shall operate a motor vehicle with an exhaust system so modified.
(b) For the purposes of exhaust systems installed on motor vehicles with a manufacturer's gross vehicle weight rating of less than 6,000 pounds, other than motorcycles, a sound level of 95 dbA or less, when tested in accordance with Society of Automotive Engineers Standard J1169 May 1998, complies with this section. Motor vehicle exhaust systems or parts thereof include, but are not limited to, nonoriginal exhaust equipment.
§ 27156. Air pollution control devices
(c) No person shall install, sell, offer for sale, or advertise any device, apparatus, or mechanism intended for use with, or as a part of, any required motor vehicle pollution control device or system which alters or modifies the original design or performance of any such motor vehicle pollution control device or system.
(h) This section shall not apply to an alteration, modification, or modifying device, apparatus, or mechanism found by resolution of the State Air Resources Board to do either of the following:
(1) Not to reduce the effectiveness of any required motor vehicle pollution control device.
(2) To result in emissions from any such modified or altered vehicle which are at levels which comply with existing state or federal standards for that model year of the vehicle being modified or converted.
