What does the M in 351M ford engine stand for Does anyone really Know Modified Michigan Midland or just a designation to differ it from the 351 Cleveland or 351 Windsor.?

Original answer:

it really doesn't stand for anything, its just an "m" people think that it stands for modified or some of the others and that's fine, but in reality its just an "m" nothing more nothing less

Additional answer:

There is a casting code of MCC common to most 351M/400 engines. The code stands for "Michigan Casting Center." Many who know this believe the M stands for Michigan. I agree with the previous poster that Ford Motor Company does not officially call the M code engines by any other name; however, the M likely originates from the casting foundry name, as it does with the 351C and 351W engines.

Note that many refer to the 351M as a 351 "modified" presumably because the M engines share components with the 351C and 351W. Of course, the designation "modified" could apply to many engines produced by any of a number of automakers, as many new engines are derived through some modification of an existing engine. So it is extremely unlikely that engineers at the Ford engine group would call a new 351C/W hybrid "modified."