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Rules of Thumb for Boat Noise Levels
Bob Brown of Hot Boat magazine authored the Power Talk High Performance articles. In the January 2004 issue he wrote about the California noise law to be implemented in January 2005. The article provided the following rules of thumb giving boat owners an idea of what noise levels to expect at idle from engines with through-transom open exhaust. The numbers given in the article indicate how much a system might exceed the 88 dba limit. In the listing below the numbers have been converted to noise level ranges instead of over the limit ranges.
Bob Brown's Rules of Thumb
A stock naturally aspirated big block V-8 Engine (454, 496, 502, etc.) is 90 to 94 dba.
A modified or high performance engine package is 93 to 98 dba.
A supercharged engine is 96 to 100 dba.
Bob wrote that twin engine setups are louder but didn't note how much louder. Twin engine packages are 3 dba louder than single engine packages so, if you have a twin engine setup, just add 3 dba to the numbers shown above. Add 5 dba for triple engine packages.
Also if you have headers that are nearly dry such as CMIs or dry such as Stellings then add another 2 to 4 dba. Aqua Power mufflers will usually take this added noise out provided water is available as in the CMI and Stellings headers. That means the muffler delivers more quieting than shown in its rating when attached to through-transom header tailpipes.
I'm not absolutely sure but I believe the 502 engine should have been quoted 2 or 3 dba louder.
You may know what noise level your boat creates at idle with through-transom open exhaust. If so please send an e-mail with the noise level and specifics about your engine and exhaust system.