None.
Montana Code
§ 61-9-403. Mufflers--prevention of noise
(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 annoying smoke, and no
person shall use a muffler cutout, bypass, or similar device upon a motor vehicle on a highway.
(2) The engine and power mechanism of every motor vehicle shall be so equipped and adjusted as to prevent the escape of excessive fumes or smoke.
§ 61-9-435 Exhaust noise limitation.
(1) Except as provided in subsection (3), a person may not operate a motor vehicle with an exhaust system that emits a noise in excess of 95 decibels, as measured by the society of automotive engineers' standard j1169 (May 1998).
(2) A person charged with violating this section may not be convicted if the person had reasonable grounds to believe that the vehicle was not operated in violation of the standard in subsection (1).
(3) This section does not apply to a motorcycle or quadricycle that is subject to 61-9-418.
Pioneer
Available for vehicles that were manufactured in 1933 or earlier and used solely as collector items and not for general transportation. Also available for "Street Rods," which are defined as vehicles manufactured before 1949, or manufactured to resemble such vehicles, that have been altered from their original design or have a body constructed from non-original materials.
Vintage
Available for vehicles manufactured in 1934 or later that are at least 30 years old and used solely as collector items and not for general transportation. Also available for "Street Rods" and some "Custom Vehicles," with "Custom Vehicles" defined as vehicles manufactured after 1948 that are at least 25 years old, or manufactured to resemble such vehicles, that have been altered from their original design or have a body constructed from non-original materials.
YOM
Vehicles that are at least 30 years old and used solely as collector items and vehicles that are at least 25 years old and used for general transportation may display YOM plates with DMV approval. The owner of a motor vehicle, trailer, semitrailer, or pole trailer manufactured in the year 1948, 1949, or 1950 may display a single original Montana license plate that is affixed to the rear of the vehicle.
* A vehicle registered as a custom vehicle or street rod may display a single rear plate.
* If a person is not able to comply with the requirement that a front license plate be displayed because of the body construction of the motor vehicle, the person may submit to the highway patrol an application for a waiver along with a $25 inspection fee. A certificate of waiver may be issued upon inspection of the vehicle by a highway patrol officer.