This all sounds pretty severe, but a careful read of the wording reveals that enthusiasts may still be protected by there being no actual decibel limit in existence for pure exhaust noise in Michigan. In fact, back in 2006 SEMA helped defeat proposed legislation that would have imposed an 80- decibel limit to exhaust noise and also ban vehicles equipped with replacement exhaust systems that differed from the design specifications of the original muffler.
Nitrous Systems
Michigan Compiled Laws 752.272a makes it unlawful to sell or distribute nitrous oxide (or devices for dispensing it) "for the purpose of causing a condition of intoxication, euphoria, excitement, exhilaration, stupefaction, or dulling of the senses or nervous system." Although the "excitement" and "exhilaration" language seems fairly ironic in light of the goals of using nitrous on a vehicle, the intent of the legislature was probably only to prevent actual inhaling of the substance itself. Further, the statute "does not apply to nitrous oxide that has been denatured or otherwise rendered unfit for human consumption," which seems to account for the addition of sulphur dioxide fairly well. First offenders are subject to imprisonment for up to 93 days and/or a fine of up to $1,000.
New Jersey
Exhaust Systems
New Jersey Statutes Annotated 39:3-70 provides: "Every motor vehicle having a combustion motor 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 cut-out, bypass or similar device upon a motor vehicle on a highway," which is pretty much the boilerplate language used in most other states we're looking at here.
New Jersey Administrative Code 13:20-32.20 describes the standards to be used at official inspection facilities. Aside from stating that improperly mounted exhaust systems and systems that pass through, or hat can send gas into, the passenger compartment (which presumably applies to those that end before the rear axle) are no good, systems where "the muffler is missing, defective, or not in proper operating condition" and those with "a muffler cut-out, muffler bypass or any similar device, or any change or modification to the exhaust system which causes excessive noise" will be refused.
In other words, we weren't able to find anything on decibel limits in New Jersey, save to say that SEMA helped strike down a measure a few years back that would have instituted a noise restriction for all aftermarket mufflers sold in the state, because it was vague and stated no actual decibel limit.
Nitrous Systems
New Jersey Statutes Annotated 24:6G-1 disallows persons except "a duly licensed physician, dentist, veterinarian, nurse, hospital, sanitarium or other medical institution ... to have under control or possess nitrous oxide in any form unless the person obtains a written permit issued by the Department of Health." That person must be at least 19 years old before he or she can get such a permit, but this author can tell you that they are relatively inexpensive and easy to obtain. Though the law on possession is clear, we unfortunately were unable to find laws on point regarding actual nitrous systems on vehicles, so you should probably assume the usual applies (i.e., disconnecting or removing the bottle while on the street).