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Most Ford enthusiasts are well aware of the 351 Cleveland engine, and its short, but well known, history. However there is confusion regarding the 351M and 400 engines.

The 351M and 400 engines were derived from the 351C, and are all considered part of Fords "335 series" of engines.

The 400 engine was developed in in 1970 in response to the increase in pollution control measures placed on auto manufacturers. Big and heavy cars were still the fad, but the traditional Ford powerplants, such as the 390 and 460, could not meet the new emissions requirements.

Ford needed a "go between" engine, something with more displacement than the Cleveland, but not as much as the 385 series motors. Ford also needed an engine that could be used in trucks and large vehicles, and utilize their existing heavy-duty transmissions. The 351C would not work due to its small-block "Windsor" bellhousing bolt pattern.

Engineers took the Cleveland motor as a starting point and modified it in several ways:

400 cid.
Bellhousing pattern same as 429/460 (to allow use of already existing heavy-duty transmissions.
Main journal diameter increased by 0.25" to 3.00" for greater bearing surface area (incidentally, same as a 351W)
Crank stroke increased from 3.50" to 4.00" to gain 50 cid.
Deck height increased from 9.206" to 10.297" accommodate larger stroke
New intake manifold for wider deck.
Heads are identical in design to 351-2V, but have larger chambers to reduce compression, may also have air injection ports.
Different harmonic balancer and flexplate than 351C

Same block as 400, but crank is back to 3.50" stroke.
Pistons have thicker compression height to maintain compression a 8.2:1 in the tall deck.
Different harmonic balancer than 400
Production of the 400 and 351M engines ceased in the early 80's, as oil prices began to sky rocket and federal EPA regulations became stricter. Ford turned to its Windsor series engines as the primary V8 options for passenger cars and trucks.
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