I have posted a similar reply before, but I purchased the standard Vortec castings from Summit Racing not the ones with Iconel. I removed the stock heads from a 1990 / 91 Mercruiser 260. Kept the same pistions, installed the Vortecs, an Edelbrock Performer manifold, 1.5 roller Rockers, XM256 cam, Outer Z28 valve springs, new Mercruiser manifolds and risers. This is a heavy 3300+lb Wellcraft Eclipse 21 1/2 ft. Cuddy. Before mods, ran about 52.7 on GPS with a Mirage 21P. After mods turned a Mirage 23P at 5000 - 5100 rpm and 60.3 on GPS (a lot of propeller slip on this antique hull). So all these mods together picked up around 7 mph. I can only estimate that might be 300 propshaft HP from the previous 230 propshaft HP, but only a guess. There is a bit more in it with the right prop.