None.
North Dakota Century Code
§ 39-21-37. Muffler -- Prevention of noise and smoke.
1. Every motor vehicle must 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 may 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 must be so equipped and adjusted as to prevent the escape of excessive fumes or smoke.
Antique
Available for vehicles that are at least 40 years old and not used in the routine functions of business or farming operations.
Collector
Available for vehicles that are at least 25 years old. A signed affidavit stating that the vehicle will be operated solely as a collector's item and not for general transportation must be submitted when registering.
YOM
Vehicles registered as "Antique" may display YOM plates with DMV approval.