I have a pair of 1999 502 mpi's and would like to get reliable 650-700 HP out of them, I was thinking of throwing a pair of blowers on them but not sure about size and if I need to do any internal work.
If I do a 420 blower on a stock 502 what would be my HP rating
Thanks for your input

I'd do vortech or procharger if you go the blower route.. Old roots style make to much heat. Get someone(that knows what they're doing) to tune your ecm in the boat or dyno.

The 420 is what the Merc. 600SC used as OEM. It was generally considered to be fairly low-heat. It was reliable as an OEM setup with no chiller/intercooler (merc. used a reduced compression ratio and carbs). I'm far from expert though. I'd be curious how you would mount the 420 and retain the MPI arrangement and how you'll tune the EFI to supply the correct amount of fuel. Sounds like a great project! I would guess that with chillers you could make around 700hp and without about 600hp if you run 93 octane or better.

ProCharger's, just dont follow their instructions, LOL.

Just dont forget to factor in all the other expenses like fuel pumps,regulators, all new fuel line, making the fuel system have a return line to the tank, injectors, tuning the ecm, etc.

Im not 100%, but I wanna say the fuel rails on those dont like a certain type of injectors and you may need to change them as well ? Ask Youngs Performance about that one.

I have Vortechs system on 502 mag and am very happy with it. You might want to consider head gaskets and new valve springs if you have any kin d of hours on the motor. My stock springs were whipped at around 250hrs.