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.
Next:
Big-Block Comparisons
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