So far the best info is what i’ve found from SEMA’s network
Oregon Revised Statutes
§ 815.250 Operation without proper exhaust system; exemptions; penalty.
(1) A person commits the offense of operation without proper exhaust system if the person drives or moves on any highway or owns and causes or knowingly permits to be driven or moved on any highway a motor vehicle that is not equipped with an exhaust system that meets the requirements under this section.
(2) An exhaust system only meets the requirements of this section if all of the following apply:
(a) The exhaust system must be in good working order.
(b) The exhaust system must be in constant operation.
© The exhaust system must meet noise emission standards determined by the Department of Environmental Quality to be substantially equivalent to the following standards based upon a stationary test conducted at a distance of 25 feet in accordance with procedures established by the Department of Environmental Quality:
I. Vehicle type: Motor vehicles required to establish a registration weight under ORS 803.430 and commercial buses
Maximum level, dBA: 94 dBA if model year before 1976; 91 dBA if model year 1976 and after
III. Vehicle type: motor vehicles not described under I or II
Maximum level, dBA: 92 dBA if model year before 1976; 88 dBA if model year 1976 and after
(3) This section does not apply to the following vehicles:
(a) Vehicles of special interest that are registered under ORS 805.020 and that are:
(A) Equipped with original manufacturer’s equipment and accessories, or their equivalent, and that are maintained in safe operating condition; or
(B) Street rods that conform to ORS 815.107.
(b) Road machinery, road rollers or farm tractors.
© Antique motor vehicles that are maintained as collectors’ items and used for exhibitions, parades, club activities and similar uses, but not used primarily for the transportation of persons or property.
(4) The court in its discretion may dismiss a citation issued for violation of the offense described in this section if evidence is presented that the exhaust system complies with or has been repaired or modified to comply with the requirements under this section.
(5) The offense described in this section, operation without proper exhaust system, is a Class C traffic violation.