Department of Natural Resources
Vehicle muffler noise and the use of spark arresters has been a matter of concern regarding the operation of Off-Road Vehicles (ORV) for many years. The State of Michigan 's ORV Law has required them since 1975. At Silver Lake State Park a static or stationary test for this equipment is used on all types of vehicles. Noise levels will be checked at a twenty-inch distance from the exhaust pipe at half the vehicle's rated revolutions per minute (RPM) or as detailed in the automotive specifications SAE J1169, March 92 or SAE J1287, June 86. The maximum noise level permitted under these testing conditions will be 94 dBa which is the current limit set by the Off-Road Recreation Vehicles Act of Part 811 of 1994 PA 451 (MCL 324.811). All vehicles are required to have a muffler and spark arrester containing baffles meeting established United States Forest Service (USFS) specifications.
Generally speaking, the guidelines for testing at "half the rate RPM" will be based on some vehicle similarities. In most cases, full-sized vehicles with "V" configured engines will be tested at 2,500 RPM and all others at 2,000 RPM, which was a method developed and used while testing these vehicles at Silver Lake in 1997. Diesel powered, full-sized vehicles are tested at 1,500 RPM. Most reverse flow mufflers will meet the spark arrester requirements, as will most turbochargers. However, straight-through mufflers, such as glass-pack designs without baffles and mechanically driven superchargers, do not qualify as adequate spark arresters.
Motorcycles and ATVs will be tested like the full-sized vehicles by placing a sound meter at a twenty inch distance from the muffler exhaust pipe. In most cases, 80 cc engines will be tested at 6,000 RPM, 125 cc at 5,000 RPM, 250 cc at 4,000 RPM and 500 + cc at 3,000 - 3,500 RPM. Annually, the Motorcycle Industry Council produces a manual detailing the standards ATVs and motorcycles must meet. A muffler that meets the Environmental Acoustic Reduction (E.A.R.) standards may be considered as a viable solution for motorcycles. Many ATV and motorcycle mufflers are stamped as being USFS approved and will meet Silver Lake State Park standards, if they have not been altered. Those vehicles that have been manufactured for "closed track" use only, such as many motocross or "dirt-bike" motorcycles and competition style ATVs, will most likely not meet the requirements. This is also often true where after market installations have altered the original exhaust system in attempts to gain additional horsepower.
Testing for compliance with these regulations will be done in a random fashion either within the ORV area parking lot, at the base of the ORV area entrance ramp roadway by the safety check station, or within the ORV area sand dune complex itself. Park rangers or other law enforcement officials may insert a plastic rod into the vehicle's exhaust pipe to determine whether there is a spark arrester or baffle present. Most often home-made baffling devices, such as steel wool or metal door screen material, do not meet the USFS requirements.