I have a 10.15 compression 509 was efi 502 . Has alluminum heads with 320 cc intake runners 2.30 intake and a bullet cam 234 intake duration @50 with .590 lift and 242 duration and .598 exh at 113 lobe seperation hydraulic roller . The boat is a 22 dcb with xr drive with shorty lower. I have been told I can't use the stock efi and it would need a bunch of upgrades so we were going to use an aftermarket intake and carb. Need some recommendations. Thanks.

I have a performer RPM on mine, somewhat similar setup

505", 335 runners with 278/284 duration .567/.578 lift

Put up good low end numbers on the dyno and I'm happy with it. Dynoed with both a carb and Holley sniper without issue

What EFI is currently on it? MEFI 1,2,3,4 or ??? Your existing EFI can be used but would need some tweaking. If its a 555 ECM that would be the toughest to get programmed. All you really need to work on are the fuel tables, a little bit of timing and scaled the injectors to the appropriate size.

It's the factory efi. It's a 98 dcb 22 foot tunnel . It was originally a 502 mag.

Salt or fresh water application?

PM Arcticfriends (Smitty/Steve). He probably has more experience than anyone when it comes to the Merc Mag MPI setup.

Fresh. Norther California lakes and rivers.

The EFI can be used, but the intake may be restrictive. I'm guessing you are going to be 570ish hp??
If you go carb route, and 850cfm and RPM Air Gap would be my choice.

A carb will be less hassle.

I went with Merlin single-plane intakes and 4150-style 1050s on my 509s.

You didn't say what heads.

so its a 98, most likely a mefi1, does it have v belts on engine? Do ecm connectors plug in the ends of ecm and its rectangle shaped or ii the face and its square shaped? It will definitely need "tuned" to run, idle and drive with that cam and other mods, the intake should be chopped down and a bored out TB, better FA put on it.. If its a mefi1 id be inclined to get rid of the intake, if its a mefi3 its a coin toss depending if you have someone local that can tune it on the dyno after you mod intake or on water vs swapping to a regular intake and carb, Smitty