What is the primary diff from a 454MPI and the 454mag MPI?
is the 454MPI cast, not harden crank ect? are the rods cast?

I think you mean piston being cast ,,not rods ,welcome to the board
pistons are cast and cams are different ,I think the crank is cast not forged also
see ya

Thanks bobby
thats what i was afraid of. probably an excellent engine, but not a good foundation to build on.

heads different ,cast crank ,rods, and pistons

Let's add different injectors, plenum, e.c.u., cam, etc. It would be easier to list the parts that are the same---block, rods and accessories.

As far as I can remember Mercruiser has called the lower HP engines with cast crank, cast pistons, small rods and small port heads the 330 or 7.4 except fpr one engine. The 330 horsepower in front of the Alpha drive was called a 454 Magnum, somewhere around 1986 or 1987. All Mercruisers that have the good parts are called 454's, they have the steel crank, big rods, forged pistons and rectangle port heads.
Dennis Moore