Mercruiser used a larger cam, installed "straight-up" as opposed to retarded and the marine manifolds flow better. Along with a more agressive tune-up, and fewer parasitic accessories. Not to mention the fact that do you really think that a 350 Chevy ever REALLY only put out 160hp (as advertised in the 1984 Camaro)? That was all a smoke and mirrors game with emissions and the insurance industry and the government.