I have 2 1974 GTOs. One with a 350, 9.5:1, 288/302 with 408lift, headers and 2.5 in. pipes with super 40s. The other has a 455, 9.0:1, 301/313 with 470 lift, headers, and 3 in. mandrel bent pipes with super 40s (that I just put on this weekend). It seems as though the 350 is louder than the 455. Also my dad has a 1967 gto with a 400, 11:1, 301/313 with 470 lift (same cam as 455), stock manifolds and 2.5 in. flowmaster super 40s and its even louder than both mine. Anyone know why this could be? compression maybe? Thanks for any info.