California Vehicle Code 27156 states it is illegal to modify a vehicle's emissions system(s).
We were unable to find a provision which allowed for the exemption of non-smog check required vehicles, i.e. pre-1975. It seems the law applies to all vehicles being operated or parked on public highways. Section 3340.41.5 of the California Code of Regulations explains what is considered an Emissions System Tamper. See Tampering with Emissions Control Systems.
California Vehicle Code 27156 reads as follows:
(a) No person shall operate or leave standing upon a highway a motor vehicle that is a gross polluter, as defined in Section 39032.5 of the Health and Safety Code.
(b) No person shall operate or leave standing upon a highway a motor vehicle that is required to be equipped with a motor vehicle pollution control device under Part 5 (commencing with Section 43000) of Division 26 of the Health and Safety Code or any other certified motor vehicle pollution control device required by any other state law or any rule or regulation adopted pursuant to that law, or required to be equipped with a motor vehicle pollution control device pursuant to the National Emission Standards Act (42 U.S.C. Secs. 7521 to 7550, inclusive) and the standards and regulations adopted pursuant to that federal act, unless the motor vehicle is equipped with the required motor vehicle pollution control device that is correctly installed and in operating condition. No person shall disconnect, modify, or alter any such required device.
(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, a required motor vehicle pollution control device or system that alters or modifies the original design or performance of the motor vehicle pollution control device or system.
(d) If the court finds that a person has willfully violated this section, the court shall impose the maximum fine that may be imposed in the case, and no part of the fine may be suspended.
(e) “Willfully,” as used in this section, has the same meaning as the meaning of that word prescribed in Section 7 of the Penal Code.
(f) No person shall operate a vehicle after notice by a traffic officer that the vehicle is not equipped with the required certified motor vehicle pollution control device correctly installed in operating condition, except as may be necessary to return the vehicle to the residence or place of business of the owner or driver or to a garage, until the vehicle has been properly equipped with such a device.
(g) The notice to appear issued or complaint filed for a violation of this section shall require that the person to whom the notice to appear is issued, or against whom the complaint is filed, produce proof of correction pursuant to Section 40150 or proof of exemption pursuant to Section 4000.1 or 4000.2.
(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 a required motor vehicle pollution control device.
(2) To result in emissions from the modified or altered vehicle that are at levels that comply with existing state or federal standards for that model-year of the vehicle being modified or converted.
(i) Aftermarket and performance parts with valid State Air Resources Board Executive Orders may be sold and installed concurrent with a motorcycle’s transfer to an ultimate purchaser.
(j) This section applies to motor vehicles of the United States or its agencies, to the extent authorized by federal law.