California's Laws Regarding Modified Exhaust:
A Perspective

It has come to the attention of Bob's Muffler that some California police officers are issuing summonses to California drivers who have aftermarket exhausts on their vehicles. Bob's Muffler feels this unfairly targets the people who chose to purchase our products for legal use on the streets and public highways of California. Bob's Muffler wants to examine the actual law and the areas it covers. We wish to educate our customers.

CA. V.C. 27151 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 such 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) of this chapter. No person shall operate a motor vehicle with an exhaust system so modified.

This shows that there are other characteristics that the exhaust system must conform to truly show a violation. This also shows that other vehicle codes must be considered when in violation of this vehicle code.

CA. V.C. 27150 (a) Every motor vehicle subject to registration shall be equipped with an adequate muffler in constant operation and properly maintained to prevent any excessive or unusual noise, and no muffler or exhaust system shall be equipped with a cutout, bypass, or similar device.

(b) Except as provided in Division 16.5 (commencing with Section 38000) with respect to off-highway motor vehicles subject to identification, every passenger vehicle operated off the highways shall at all times be equipped with an adequate muffler in constant operation and proper maintained so as to meet the requirements of Article 2.5 (commencing with Section 27200), and no muffler or exhaust system shall be equipped with a cutout, bypass, or similar device. (c) The provision of subdivision (b) shall not be applicable to passenger vehicles being operated off the highways in an organized racing or competitive event conducted under the auspices of a recognized sanctioning body or by permit issued by the local governmental authority having jurisdiction.


Part (a) states that the muffler must suppress any excessive or unusual noise. This refers to part (b) which states that the muffler must meet the requirements of Article 2.5, which refers to noise levels. Part (a) and (b) also refer to a cut-away, bypass or similar device commonly known as a "dump."

CA.V.C. 27200 (a) The Department of Motor Vehicles shall not register on a dealer's report of sale a new motor vehicle, except an off-highway motor vehicle subject to identification as provided in Division 16.5 (commencing with Section 38000), Which produces a maximum noise exceeding the applicable noise limit at a distance of 50 feet from the centerline of travel under test procedures established by the Department of Highway Patrol.

(b) The Department of Motor Vehicles may accept a dealer's certificate as proof of compliance with this article. (c) Test procedures for compliance with this article shall be established by the Department of the California Highway Patrol, taking into consideration the testing procedures of the Society of Automotive Engineers.

Part (c) of this section says that the California Highway Patrol shall set the testing standards in the for of California Code of Regulation, Title 13, Article 9, Section 1036 as denoted in letter from the Department of California Highway Patrol file 60.A9190.A9181.062.9610186.

CCR 1036 Passenger Cars and Light Trucks and Buses

Exhaust systems for motor vehicles, other than motorcycles, gross vehicle weight rating of less than 6,000 LB (2,722 kg) shall comply with the specified noise limits when tested in accordance with the following specifications:

(a) Measuring Site. The vehicle under test shall be positioned either on outdoor pavement or on a shop floor ( but not over a hoist or pit) in a location where the exhaust outlets are near an open shop door. No sound-reflecting surface other than the pavement and the vehicle being measured shall be within 10 ft (3.0m) of any part of the vehicle.

The testing is done in an environment in which there is minimal sound contamination. Please note that there is nothing stating highways or other public roads as being an exception to this criterion.

(b) Microphone Location. The microphone for the sound level meter shall be at the same height as the center of the exhaust outlet. The microphone shall be no closer to the pavement than 8 in. (203mm) when the exhaust outlet is lower than this height. The microphone shall be positioned with its longitudinal axis parallel to the ground, 20 inches plus or minus 1 inch (508mm plus or minus 24mm) from the nearest edge of the exhaust outlet, and 45 degrees plus or minus 10 degrees from the axis of the outlet. For exhaust outlets located under the vehicle body, the microphone shall be located at the specified angle and at least 8 in., (203mm) from the nearest part of the vehicle. For exhaust outlets that make an angle of 45 degree or less with the longitudinal axis of the vehicle, the microphone shall be outboard of the exhaust axis. For exhaust outlet angles of more than 45 degrees, the microphone shall be on the side of the exhaust axis to the rear of the vehicle.

This denotes where the sound measuring device is relatively close to the exhaust outlet. Notice that it does not make any references to provisions for sound level testing done from distances greater than 21 inches. <

(c) Engine Operation. The engine shall be at a normal running temperature with the transmission in neutral. System sound level measurements conducted for certification by exhaust manufacturers shall be made at an average steady-state engine speed of three-quarters of maximum rpm. Sound level measurements conducted by licensed muffler certification stations shall be made at an average steady-state engine speed of 3,000 rpm for vehicles manufactured before 1972 for which the station does not have a maximum rpm data.

Please notice that the engine is supposed to be running and the law has no provisions for parked vehicles. Note that the certification can be done by the manufacturer of the exhaust system. There is no requirement that the manufacturer sound test each individual vehicle. Re: "licensed muffler certification stations," the letter from the California Highway Patrol file Number 60.A9190.A9181.062.9610186 states in the third paragraph that these facilities were canceled in 1978 due to budgetary cut-backs.

(d) Exhaust Sound Level Measurement. The recorded exhaust system sound level of a stationary vehicle shall be the highest reading obtained during the test, disregarding unrelated peaks due to extraneous ambient noise. When there is more than one exhaust outlet, the reported sound level shall be for the loudest outlet. When there are two or more exhaust outlets separated by less than 12 in. (305mm), measurements shall be made on the outlet closest to the side or rear of the vehicle. (Figure 4). The sound level for the motor vehicles with the manufacturers' gross vehicle ratings of less than 6,000 LB (2,722 kg) except motorcycles, shall not exceed the following limit:

The statement stating extraneous ambient noise refers to noise coming from places other than the exhaust. For this reason, testing cannot be done in a sound-contaminated environment. It would be logical that a roadside or a highway would be a sound-contaminated environment.

(1) Sound Level Limit. The exhaust noise shall not exceed 95 dB(A).


This is very straightforward and should leave nothing to interpretation.

(2) Exemption. The exhaust noise of vehicles manufactured after 1967 may exceed 95 dB(A) if the replacement parts are no louder than the original equipment or "factory duplicate" system and the total sound level of the vehicle when tested in accordance with Sections 1040 through 1049 of this title complies with the limits in Vehicle Code Section 27205 for the year of manufacture of that vehicle.

This has been an attempt to clarify California Vehicle Code 27151 for many people who do not have an adequate understanding of this code.  Bob's  Muffler does not guarantee favorable results in defending one's self against a noise ticket since extenuating circumstances can affect the outcome in any court. Also keep in mind that it is your right to apply for an appeal.

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