M-Block 351M/400 History

In the 1960s there were two major engineering advances in Ford V8 engines for street vehicles:

In 1962, Ford introduced its first V8 engine with a lightweight, thin-wall block, the small-block 221. This engine was the first design in a new “small-block” family that would ultimately grow to include the 260, 289, 302, and 351 Windsor. These engines would be Ford’s longest-lived pushrod V8 engine design in the North American market, remaining in production until the end of model year (MY) 2000, the last year the 5.0L V8 was available in the Explorer SUV.

The first Ford engine design to incorporate poly-angle chambers and canted valves was the 385-series big block. The first 385-series engines were the 429 Thunder Jet introduced in the 1968 Thunderbird and the 460 introduced in 1968 Lincolns.

On the small-block scene in 1968, Ford introduced the Tunnel Port 302 engine to compete in the FIA Trans Am racing series. After some embarrassing failures of the Tunnel Port 302 engines, Ford developed the Boss 302 engine and introduced it the next year, in 1969.

The Boss 302 cylinder head was a new design, with huge ports and valves (even bigger than the Tunnel Port 302’s) for maximum breathing efficiency. The Boss 302 proved successful in Trans Am racing and Ford sold it to the public as a limited production, high-performance option in Mustangs for two model years, 1969 and 1970. In 1969, the Boss 302 cylinder head was a variation of the pre-production heads Ford was designing for their next-generation muscle car engine, the legendary 351 Cleveland.

Introduced in the 1970 model year, the 351 Cleveland (351C) was the first engine in a new design family known as the “335 series.” The 351C was designed to take advantage of the free-breathing cylinder heads that proved so successful on the Boss 302. The 351C was based on the design of the 385-series big blocks, but it was smaller and lighter. Like the 385-series big blocks, the 351C incorporated both thin-wall block casting technology and poly-angle chambers with canted valves.

There were two major variants of the 351 Cleveland:

The high-performance 351C 4V used cylinder heads very similar to the Boss 302 heads, while the more subtle 351C 2V used a similar poly-angle, canted-valve head, but with smaller ports and valves better suited to its moderate applications.

The 400 was introduced in MY 1971 as a longer-stroke variant of the 351 Cleveland. Using the proven cylinder head design of the 351C 2V on a unique block with a taller deck to accommodate its half-inch longer stroke, the 400 was the ultimate development of the 335 series engine design, and it would be the last pushrod V8 engine ever designed by the Ford Motor Company.