A friend just rebuilt a 1997 Merc. 454 Mag and kept it completely stock except for the below changes. Can anyone estimate what the horsepower is?

1. Rebuilt at .030 over
2. Compression ratio 10:1
3. BG SeaDemon 750 CFM Carb

All else is stock.

Thanks.

400

What kind of exhaust?

Agreed

I was thinking about the same....380-400hp.

Don't you first need to know what the factory hp rating was on the motor or was there only 1 option that year, I know my boats older and there were 3 different hp ratings depending on the heads

There was only one "454 Mag" motor (with a carb) in 1997. The other motors were referred to the "7.4" and the "454 Mag MPI "(or possibly EFI instead of MPI). I believe the "454 Mag" was rated 350HP at the prop that year and the MPI variant at 385HP.

If all he did was bore it and up the CR and change the carb, he maybe added 30hp with the need to run high octane fuel.

Should have changed the cam as well. That could bring another 30-50 hp.