The Federal Government doesn't set exhaust noise standards---those are created and enforced by state and local jurisdictions. The hot rod advocates at SEMA (Specialty Eequipment Manufacturing Association) introduced a measure into California legislature in 2002 that would set exhaust decibel limits at 95 dB (decibel) for every motor vehicle. California and several other states adopted the proposal as law, but many other states still use their own standards for exhaust noise levels.
Other People Are Reading
As stated, several states have adopted SEMA's 95-dB standard. This sound reading is taken 20 inches from the vehicle's exhaust pipes with the "A-weighted" (a specific type of calibration) decibel meter placed at a specific orientation to the exhaust pipe tip. When tested, the car should be in neutral with engine speed held at 3/4 of the engine's maximum rated revolutions per minute (RPM). These testing procedures are outlined by the Society of Automotive Engineers under Test Standard J-1287.
One universal law in any state is that road-going vehicles must use some sort of muffler or noise-reducing device, and that said device must be in working order at all times. However, states do not, for the most part, say anything about what kind of muffler you need to use. Low-restriction "cherry bomb" style straight-through mufflers are acceptable for most applications.
Variable Noise Tolerances
Many states have different criteria for vehicles of differing type and year of manufacture. Florida statute 316.293 is one good example of variable tolerances, and is fairly representative of such. Motorcycles manufactured before January 1, 1979, are limited to 82 dB at 35mph or less and 86 dB at anything more than 35mph. Newer bikes are limited to 78 dB at or below 35mph and 82 dB at higher speeds. Other vehicles under 4536kg and built before January 1, 1979, are limited to 76 dB at or below 35mph and 82 dB for speeds faster than 35mph. Newer cars are limited to 72 dB at or below 35mph and 79 dB for speeds more than that. Almost all states that haven't adopted the SEMA standard use such variable noise tolerances.
The loudest exhaust systems hardly qualify as exhaust systems at all. Many enthusiasts like to install either electric or manual exhaust cutouts, which are essentially large butterfly valves mounted in the exhaust system. When open, these valves vent exhaust gases into the air without sending them through the muffler or (possibly) the catalytic converter. Many states, such as Texas, state that "[a] person may not use a muffler cutout, bypass or similar device on a motor vehicle." (Texas Transportation Code 547.604). However, there is some room for interpretation here. Texas says that you can't USE (open) the exhaust cutout on the street---there is nothing said about whether or not you can install one. If you're going to use a cutout, make sure to get the number of a lawyer with a minor in English.
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- American Motorcyclist Association: Florida State Motorcycle Laws
- SEMA Action Network: SEMA's California Exhaust Noise Testing Program Legislation Becomes Law
- California Bureau of Automotive Repair: Vehicle Exhaust Noise Level Certification
- Law Server: Florida Statutes 316.293
- GM High-Tech Performance Magazine: Exhaust System & Nitrous System Laws in U.S. - You Have the Right to Remain an Enthusiast?