Here's everything you want to know about emissions laws and regulations. The bold type and underlining are mine. The rest is the verbatum text of the applicable statute (RCW) and regulation (WAC):
West's RCWA 46.37.390
Title 46. Motor Vehicles (Refs & Annos)
Chapter 46.37. Vehicle Lighting and Other Equipment (Refs & Annos)
46.37.390. Mufflers, prevention of noise--Smoke and air contaminants-- Standards--Definitions
(1) Every motor vehicle shall at all times be equipped with a muffler in good working order and in constant operation to prevent excessive or unusual noise, and no person shall use a muffler cut-out, bypass, or similar device upon a motor vehicle on a highway.
(2)(a) No motor vehicle first sold and registered as a new motor vehicle on or after January 1, 1971, shall discharge into the atmosphere at elevations of less than three thousand feet any air contaminant for a period of more than ten seconds which is:
(i) As dark as or darker than the shade designated as No. 1 on the Ringelmann chart, as published by the United States bureau of mines; or
(ii) Of such opacity as to obscure an observer's view to a degree equal to or greater than does smoke described in subsection (a)(i) above.
(b) No motor vehicle first sold and registered prior to January 1, 1971, shall discharge into the atmosphere at elevations of less than three thousand feet any air contaminant for a period of more than ten seconds which is:
(i) As dark as or darker than the shade designated as No. 2 on the Ringelmann chart, as published by the United States bureau of mines; or
(ii) Of such opacity as to obscure an observer's view to a degree equal to or greater than does smoke described in subsection (b)(i) above.
(c) For the purposes of this subsection the following definitions shall apply:
(i) "Opacity" means the degree to which an emission reduces the transmission of light and obscures the view of an object in the background;
(ii) "Ringelmann chart" means the Ringelmann smoke chart with instructions for use as published by the United States bureau of mines in May 1967 and as thereafter amended, information circular 7718.
(3) No person shall modify the exhaust system of a motor vehicle in a manner which will amplify or increase the noise emitted by the engine of such vehicle above that emitted by the muffler originally installed on the vehicle, and it shall be unlawful for any person to operate a motor vehicle not equipped as required by this subsection, or which has been amplified as prohibited by this subsection so that the vehicle's exhaust noise exceeds ninety-five decibels as measured by the Society of Automotive Engineers (SAE) test procedure J1169 (May, 1998). It is not a violation of this subsection unless proven by proper authorities that the exhaust system modification results in noise amplification in excess of ninety-five decibels under the prescribed SAE test standard. A court may dismiss an infraction notice for a violation of this subsection if there is reasonable grounds to believe that the vehicle was not operated in violation of this subsection.
This subsection (3) does not apply to vehicles twenty-five or more years old or to passenger vehicles being operated off the highways in an organized racing or competitive event conducted by a recognized sanctioning body.
CREDIT(S)
[2001 c 293 § 1; 1977 ex.s. c 355 § 33; 1972 ex.s. c 135 § 1; 1967 c 232 § 3; 1961 c 12 § 46.37.390. Prior: 1955 c 269 § 39; prior: 1937 c 189 § 36; RRS § 6360-36; RCW 46.36.050; 1927 c 309 § 17; 1921 c 96 § 21; 1915 c 142 § 20; RRS § 6362-17.]
HISTORICAL AND STATUTORY NOTES
Severability--1977 ex.s. c 355: See note following RCW 46.37.010.
CROSS REFERENCES
Washington Clean Air Act, see § 70.94.011 et seq.
LIBRARY REFERENCES
2001 Main Volume
Automobiles 5(2), 115.
Westlaw Topic No. 48A.
C.J.S. Motor Vehicles §§ 26, 44, 56.
NOTES OF DECISIONS
Validity 1
1. Validity
Statute which requires cars to be equipped with muffler to prevent "excessive or unusual noise" was not unconstitutionally vague and, thus, police officer was justified in stopping defendant's vehicle for excessively loud noise emanating from vehicle's exhaust; what is loud and excessive noise for vehicle is matter of common knowledge and is as capable of ascertainment as other circumstances in which law enforcement officers must depend upon their senses to determine whether violation of law has occurred. State v. Olsson (1995) 78 Wash.App. 202, 895 P.2d 867.
These are the regulations which are applicable. The first sets forth acceptable exhaust noise levels and the second applies to the location of exhaust.
WASHINGTON ADMINISTRATIVE CODE
TITLE 173. ECOLOGY, DEPARTMENT OF
CHAPTER 173-62. MOTOR VEHICLE NOISE PERFORMANCE STANDARDS
Current with amendments adopted through November 3, 2004
2WAD Req. by T. Hanson
Current with amendments adopted through 4-6-1999
173-62-030. Standards.
(1) No person shall operate any motor vehicle or any combination of such vehicles upon any public highway under any conditions of grade, load, acceleration or deceleration in such a manner as to exceed the maximum permissible sound levels for the category of vehicle in Table I, as measured at a distance of 50 feet (15.2 meters) from the center of the lane of travel within the speed limits specified, under procedures established by the state commission on equipment in chapter 204-56 WAC, 'procedures for measuring motor vehicle sound levels.'
Table I
IN-USE MOTOR VEHICLE NOISE PERFORMANCE STANDARDS Measured @ 50 feet (15.2 meters)
Maximum Sound
Level, dBA
Speed Zones
Vehicle Category (type) Effective 45 mph (72 Over 45 Stationary
Date kph) or less mph Test
(72
kph)
Motorcycles July 1, 78 82 N/A
1980
Automobiles, light trucks July 1, 72 78 N/A
and all other motor 1980
vehicles 10,000 pounds
(4536 kg) GVWR or less
35 mph (56 Over 35
kph) or less mph
(56
kph)
All motor vehicles over June 1, 86 Reserved 90 Rese- 86 Reserved
10,000 pounds (4536 kg) 1977 1986 rved
GVWR and after
(2) Every motor vehicle operated upon the public highways shall at all times be equipped with an exhaust system and a muffler in good working order and constant operation to prevent excessive or unusual noise.
(3) No person shall operate a motor vehicle in such a manner as to cause or allow to be emitted squealing, screeching or other such noise from the tires in contact with the ground because of rapid acceleration or excessive speed around corners or other such reason, except that noise resulting from emergency braking to avoid imminent danger shall be exempt from this provision.
(4) No person shall operate any motor vehicle upon any public highway if the vehicle exhaust system exceeds the maximum permissible sound levels of Table II for the category and year of vehicle, as measured at a distance of twenty inches (0.5 meter) from the exhaust outlet under procedures established by the state commission on equipment in chapter 204-56 WAC, 'procedures for measuring motor vehicle sound levels.'
(5) No person shall sell or offer for sale a new motor vehicle except an off-highway vehicle, which produces a maximum noise exceeding the noise levels in Table III at a distance of 50 feet (15.2 meters) under acceleration test procedures established by the state commission on equipment in chapter 204- 56 WAC, 'procedures for measuring motor vehicle sound levels.'
Table IIIN-USE MOTOR VEHICLE EXHAUST SYSTEM NOISE PERFORMANCE STANDARDS Measured @ 20 inches (0.5 meters)
Vehicle Category (type) Model Year Maximum Sound
Level, dBA
Motorcycles before 1986 1986 and 99 (reserved)
after
Automobiles, light trucks and all other before 1986 1986 and 95 (reserved)
motor vehicles 10,000 pounds (4536 after
kg) GVWR or less
Table IIIMAXIMUM SOUND LEVELS FOR NEW MOTOR VEHICLES Measured @ 50 feet (15.2 meters)
Vehicle Category (type) Date of Manufacture Maximum
Sound
Level,
dBA
Any motor vehicle over 10,000 before January 1, 1978 after 86 83 80
pounds (4536 kg) GVWR excluding January 1, 1978 after January
buses 1, 1982
All buses over 10,000 pounds (4536 after January 1, 1980 after 85 83 80
kg) GVWR January 1, 1983 after January
1, 1986
Motorcycles after January 1, 1976 after 83 80
January 1, 1986
Automobiles, light trucks and all after January 1, 1976 80
other motor vehicles 10,000
pounds (4536 kg) GVWR or less
Statutory Authority: Chapter 70.107 RCW. 80-14-041 (Order DE 80-29), S 173- 62-030, filed 9/30/80; Order DE 77-2, S 173-62-030, filed 6/1/77; Order DE 75- 17, S 173-62-030, filed 8/11/75; Order DE 74-33, S 173-62-030, filed 1/30/75, effective 7/1/75.
WA ADC 173-62-030
WASHINGTON ADMINISTRATIVE CODE
TITLE 204. STATE PATROL (COMMISSION ON EQUIPMENT)CHAPTER 204-90. MINIMUM REQUIREMENTS FOR CONSTRUCTION AND EQUIPMENT OF
SPECIAL MOTOR VEHICLES--SPECIAL MOTOR VEHICLES
Current with amendments adopted through November 3, 2004
2WAD Req. by T. Hanson
Current with amendments adopted through 4-6-1999
204-90-130. Exhaust system.
A special motor vehicle shall be equipped with a leakproof exhaust system that includes the exhaust manifold(s), headers, the piping leading from the flange of the exhaust manifold(s), the muffler(s), and the tail piping.
Exhaust systems on property-carrying vehicles shall discharge the exhaust fumes to the rear of that part of the vehicle designed and normally used for carrying the driver and passengers.
Exhaust systems on passenger vehicles shall discharge the exhaust fumes at a location to the rear of the vehicle body or direct the exhaust fumes outward from the side of the vehicle body at a location rearward of any operable side windows.
No part of the exhaust system shall pass through any area of the vehicle that is used as a passenger compartment, nor in close proximity to the fuel system without being properly shielded.
Statutory Authority: RCW 46.37.005. 83-11-028 (Order 83-05-01), S 204-90- 130, filed 5/13/83.
WA ADC 204-90-130
) since putting in my set up without any problems. 
