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The DynoMax tech section has some good information about calculating exhaust diameter based on C.I,D. and horse power
seems like muffler selection is also a factor in their calculations
http://www.dynomax.com/assets/2012catalog_pdf/2012_Dynomax_techsection.pdf

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As per DynoMax tech piping chart about horsepower.
It is brake horsepower or rear wheel horsepower.
Big difference...
My engine is a 451 with 373 RWH and I use 3.00 may be It's too big.