M-Block 351M/400 History

Demise of the M-Block

Copyright 1999-2003 Dave Resch
All rights reserved.


Unfortunately, the M-block was first developed at the beginning of the 1970s, when automotive emission control regulations became more stringent, low-lead and eventually unleaded fuels were mandated, and the microprocessor technology that would someday allow high power output and low emissions to coexist was still many years away in the future.

Almost from the beginning, M-block performance was crippled with primitive, first-generation emission control devices (AIR and EGR systems) and crude mechanical “de-tuning” (retarded cam timing and low compression ratios) to meet emission control regulations. As a result of these unfortunate circumstances, the M-block’s real power potential was never developed by the factory, and worse yet, the M-block itself came to be perceived by the public as merely a low-power “smog” motor. Consequently, the M-block has been woefully overlooked by many Ford performance enthusiasts, as well as the aftermartket performance industry.

The ultimate cruel irony in the M-block’s history was that it was so well adapted to first-generation emission control systems, it was not easily updated to work with more modern electronic engine management systems that emerged in the late '70s and early '80s, and it was dropped from production at the end of the 1982 model year.

All M-block intake manifolds provide internal plumbing for both the Thermactor air injection reaction (AIR) system and the exhaust gas recirculation (EGR) system with no modification to the cylinder heads, exhaust manifolds, or other engine components. Unlike other small-block and the big-block engines, the M-block required no cumbersome and unsightly external tubes to feed Thermactor air into its exhaust and feed exhaust gas from the manifolds to the EGR valve. The M-block intake manifold simply used the cylinder heads’ exhaust gas cross-over passages to inject fresh air from the Thermactor pump into the exhaust gas stream and feed exhaust gas to the EGR valve through a spacer/adapter below the carburetor.

By the early 1980s, the new generation of automotive emission controls depended on electronic management systems with “feedback” control of the fuel/air mixture. These systems require an oxygen sensor in the exhaust gas stream to measure the amount of O2 (unused atmospheric oxygen, which indicates a lean fuel mixture). For these systems to work, the (AIR) fresh air injection point must be located downstream from the oxygen sensor to prevent false readings. With its air injection right inside the cylinder heads’ exhaust ports, the M-block was not easily adapted to a feedback mixture control system.

Actually, it would have been a simple matter to install a more complicated managed Thermactor air injection system (just like the ones used on other Ford V8 engines at the time) and pipe the fresh air to ports located just in front of the catalytic converter. However, with less demand for the 350-400 class engines (which were by then used almost exclusively in full-size trucks and Broncos), Ford apparently decided it could meet those production needs with the 351 Windsor engine alone, and since a few components were shared between the 351W and the 302, it probably made economic sense to discontinue the M-block, which was by then an “odd duck” in terms of manufacturing adaptability.

Both M-block engines were used in passenger cars through MY 1979. After MY 1979, the biggest engine available in a Ford passenger car was the 351/5.8L Windsor, and it was available in the US only in the Crown Victoria police package. In Canada, the 351 Windsor was available in both the Crown Victoria police package and in a Canada-only Crown Victoria trailer-towing package.

Ford began to gradually phase the M-block out of trucks from MY 1980 through MY 1982. Beginning in MY 1980, the 400 was dropped from the engine lineup for Broncos and F150 pickups. The 351M and 400 were dropped from all but the F250HD and F350 models by the end of MY 1981, and they were dropped completely at the end of MY 1982.

By MY 1983, the 351W replaced the 351M, and the 6.9L Navistar diesel engine (and the 460 in HD 4x4 trucks) replaced the 400.

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