351m or 400?.

Is there anyway to tell the difference from just looking at 351m & 400 from the out side to tell it's a 400.
or would you need to pull the spark plug to check stroke.
And how much compression does flat top pistons give you in a 1978 bronco 400.

res0rli9 wrote:Is there anyway to tell the difference from just looking at 351m & 400 from the out side to tell it's a 400.
or would you need to pull the spark plug to check stroke.
And how much compression does flat top pistons give you in a 1978 bronco 400.

They look identical from the outside ... same block, heads and manifold.

Use the calculator.

http://www.wallaceracing.com/cr_test2.php

I come up with 9.58/1 with a 78cc head 3cc valve relief .030" in in the hole and a guessed at 4.1" head gasket volume. It goes up to 10.17/1 with a .000" deck with the same numbers.

I have heard that some 400's have a casting mark or maybe it was a stamping, somewhere on the front of the block that 351M's don't. I never seen it though myself but, it was from a reliable Ford source.

Richard the problem with the 400 was that dam piston being .070 in the hole from the factory .If you could get a piston closer to the top it would run like a stroked cleveland .To the op just have a rod of some type pull a spark plug mark you r rod turn over the engine from top to bottom measure and it will be 3.5" or 4.00" I used to do that all the time checking 360-390s.Doug