Light truck usage[edit]

For the 1977 model year, Ford replaced its FE big-block 360 and 390 engines in its light truck line with its new 351M and 400 engines. For light-truck use, a manual tranmission could be ordered for the first time with these engines. As a result the block was strengthened in the main bearing supports, in particular the #3 support to better handle the loads imparted by the clutch. The truck engines had unique parts including pistons for different compression ratios from the car engines, truck specific intake and exhaust manifolds, camshaft with more lift, and timing set that did not retard the camshaft timing. The strengthened engine block was introduced to the Ford cars for the 1978 model year.[20]

Replacement[edit]

The 351M and 400 were last offered in a Ford passenger car in 1979. They remained available in Ford light-trucks until 1982. Reduced demand for larger engines due to tightening CAFE regulations led to the abandonment of the 351M/400 and the Cleveland production line. By 1980, mid-sized V8's had disappeared from the option list for almost all Ford cars. Only the fullsize panther platform Fords had anything larger than 302 ci available, and this need was filled with the 351W. With low demand for engines in the size range of the 351M/400, the 335-series V8's no longer had a need to be produced.

In addition, there were difficulties adapting the M-block to the second generation of emissions controls. Unlike previous Ford engines, Thermactor and exhaust gas recirculation features had already been built into the 351M and 400 engine, rendering adaptation to electronic feedback fuel/air systems difficult.[29] One requirement of the second-generation equipment was an oxygen (O2) sensor in the exhaust, which had to be placed before the Thermactor air was added. Since Thermactor air was injected right into the block's exhaust ports in the M-block, there was nowhere for the O2 sensor to go.