I have 2 motors, both with less than 100 hrs. The first is a 1997 600 SC, basicaly a Gen 5 502 7.5 to 1 compression with a flat tappet cam and a 420 mega blower. The second is a 500 HP Carb with 8.5 to 1 compression and roller cam. What I need help with is which motor is the best choice for making the most power with the 420 Mega Blower set up. I live in Utah and run the boat at 3-5000 feet in elevation. The motor will be set up with a Super Chiller and CMI E top headers. What cam would make the best choice? I would like to make about 700 HP if poosible. This setup is all going into a 1997 Hallett 270 V hull. Any help or suggestions are greatly appreciated. Both motors are Stock with them Mercury Racing locks still attached.

Give RMbuilder Bob of Marine Kinetics, he is a cam expert and will get you what you are looking for. Do a search many of us here have used him.

Thanks, I will do that. Any input as to which motor I should use? They are both on engine stands and one will be extra. I just want to make as much power with what I have rather than spend a ton on a new blower setup.

The only real difference between the long blocks is the cam and comp ratio. You could make more power with the blower on the 500HP. It should be about 700hp with 6# or so of boost.

Any more input? Is the cam in the HP 500 a good cam to use with a blower? Do I need to swap in a different cam? Does anyone have any top pulleys for the Merc 420 blower?

If you are putting a blower on the HP 500, I would switch cams. The HP500 carbed cam has a 110 lob sep. You would be better off with a similar cam with a 112-114 lobe sep. Even the 500EFI cam would be better choice and give a better idle.