probably a stupid question but can someone tell me the difference?

Remember the old Half Price store ad ?

........."It's the same thing"

Efi and Mpi are the same motors, just marketing on Mercury's part.

thanks guys.
mercury list the 502 efi as a gen V block and the 502 mpi as a gen VI block but both have the same hp , 415.
is there much difference between these blocks?

Gen VI is a roller cam engine.

When Mercury says EFI, they usually mean throttle body injection (TBI). MPI is multi-port injection where the fuel is forced directly into each cylinder individually.

In the case of the 502 efi from years ago, it probably was an mpi motor.

I'm no expert but this doesn't sound quite accurate. Does the MPI just provide for "closer to the intake valve" metering of the fuel flow that still must flow thru the intake runners and intake valves. The quotes are because I don't really know what I am talking about....

You're description is accurate, Reddog. My point was that fuel is injected into a specific PATH for each cylinder, unlike TBI and carbs.

how bout 93' motors?

Those are definitely MPI. The new decals are a nice update touch.