351C, 351M & 400 Section
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M-Block 351M/400 History
The 335 Engine Family
Copyright © 1999-2003
There were two engine types in the Ford engine family known as the 335 series, the 351 Cleveland (351C) and the “M-block.” There were several minor variants of the 351C engine (including the basic 2V and several high-performance 4V versions), but there were only two M-block variants — the 351M and the 400. From 1971 to 1982, Ford manufactured millions of M-block engines for use in mid-sized cars, full-sized cars, luxury cars, and light-duty trucks.
The 400 featured a unique engine block design with the big-block (429/460) bell housing bolt pattern, and the same bore diameter (4.00") and bore spacing (4.38") as the 351C and the later Windsor-class small blocks (i.e., 289, 302, and 351W).
Compared to the 351C block, the 400’s deck height is over 1" taller (10.297" vs. 9.206"), the 400 has larger crankshaft main bearing journals (3.00" vs. 2.75"), and the 400 uses longer connecting rods (6.58" vs. 5.78"). The 400 achieves its additional 49 cubic inches of advertised displacement with a 4.00" stroke of the crankshaft (compared to the 3.50" stroke of the 351C). In fact, the 400 has the longest stroke of any Ford pushrod V8 engine!
Both engine types in the 335 series (351C and M-block) share many design characteristics, but the only major component that is interchangeable between 351C and M-block engines is the cylinder head. Several internal components and accessories interchange, including camshafts, timing sets, water pumps, fuel pumps, oil pans, distributors, and thermostat housings (see the interchange table below for more details). All 335-series engines are externally balanced with a 28 oz-in imbalance on the crankshaft (the same as 302 and 351W engines up to 1982).
Development of the 400
When it was introduced in the fall of 1970 (MY 1971), one year after the introduction of the 351C, the 400 was available in Ford’s Custom, Galaxie, and LTD car lines, and in the Mercury Monterey, Marquis, and Brougham car lines. Billed as the 351C’s big brother, the 400 was designed to provide brisk acceleration for big, heavy, full-sized cars.
Big torque at low engine speeds was a major priority of the 400’s design, and the 400’s long stroke helped to assure that performance in a package that was both smaller and lighter than its predecessor, the 385 series (429/460) big block.
Unfortunately, the 400’s design had one major flaw that was not resolved before production began. When the 351C 2V cylinder heads were installed on an engine with 15% more stroke (15% more displacement), the compression ratio produced by flat-top pistons similar to the 351C 2V pistons was 10.2:1! To get the compression down to a more reasonable level in the 400, Ford engineers developed a piston with reduced compression height and a small dish (8 cc), and a new 400-specific cylinder head with a slightly larger combustion chamber (77.8 cc). With the relatively high-octane leaded fuel available in the early 1970s, this was an acceptable compromise, and it produced a compression ratio of 9.2:1. However, when unleaded fuels were mandated a few years later, the increased deck clearance of the 400 piston would cause problems.
Beginning in MY 1973, Ford retarded camshaft timing by 6 degrees on all 335-series engines (except the 351C 4V) to meet emission control regulations. Camshaft timing was retarded by changing the keyway offset from the alignment mark on the crankshaft timing sprocket.
Production of the 351C ended at the end of MY 1974. To keep up with the market demand for engines in the 350 cubic inch class (which apparently Ford couldn’t do with its production of the 351 Windsor alone), the 351M was developed. The 351M was a simple de-stroked adaptation of the 400, and it allowed Ford to offer both 351 cid and 400 cid engines with fewer different parts and more common packaging than the 351C allowed. Since the 351M and 400 use the same block, the engine mounts and bell housing bolt patterns are common to the two engines. The 351M was introduced in passenger cars in MY 1975.
The 351M uses the same block and heads as the 400, with a shorter stroke crankshaft (3.50"), the same connecting rods, and a unique piston with a taller compression height to compensate for shorter stroke with the same rods in the same block. In fact, the only functional difference between the 351M and 400 is the crankshaft and pistons. Ford did use slightly different components in the 351M and 400, such as harmonic balancers, camshafts, and valve springs, but functionally, the engines were identical except for the crankshaft and pistons. The 351M’s unique piston was a dished type that produced a compression ratio of 8.6:1 when it was introduced.
The 351 Cleveland built such a strong performance reputation and market appeal that Ford referred to the 351M as a “351 Cleveland” in their own marketing literature for at least the first couple years after the 351M was introduced. Ultimately, this led to some confusion, and even a mistaken belief among some pickup owners that their trucks had 351 Cleveland engines. In fact, the only US market trucks ever equipped with a 351 Cleveland engine by the factory were '70-'74 Rancheros, which shared the Torino/Montego platform and powertrain options.
In MY 1975, the USEPA mandated catalytic converters and unleaded fuel for all passenger cars, and the 400’s main design flaw came back to haunt it. From the beginning, the 400 had an unusually large deck clearance (0.067"). With the low octane of unleaded gasoline, and no unleaded premium available for a few years, Ford had to make drastic changes to the 400. To avoid detonation problems, Ford introduced a lower-compression piston with a 15 cc dish, and specified static ignition timing at just 4 degrees BTDC. Advertised horsepower dropped from 170 in 1974 to 158 in 1975.
Even with retarded ignition timing and lower compression, the 400’s excessive deck clearance was simply incompatible with low-octane unleaded fuel, so the 400 acquired a reputation in the mid-to-late-'70s for persistent pinging and detonation problems. Ford continued to tweak the 400 specs for cars, switching to even lower compression pistons with larger and larger dishes (up to 32 cc), until by 1978, the 400’s compression ratio was down to only 7.33:1 in car applications.
Though not as bad as the 400, the 351M had its own detonation problems, even with a better deck clearance (0.020"). Attempting to solve the 351M problems, Ford changed 351M pistons every year from MY 1977 to MY 1979. In 1977, they dropped the compression ratio from the original 8.6:1 to 8.3:1, then in 1978, they dropped the compression to 7.74:1. In 1979, Ford went back to 8.3:1 pistons in the car 351Ms.
As Ford planned a MY 1977 makeover for their light-duty truck line, they decided to replace the aging FE 360-390 engines with the M-block 351M and 400 engines. Before MY 1977, the M-block had only been available in cars, and only with an automatic transmission, so Ford redesigned the M-block for its truck debut.
Main bearing support webs were revised, particularly the #3 main (thrust) bearing support web, which was strengthened to withstand the force imparted by a clutch. This redesign for truck applications was the only major change in the M-block’s engine block design throughout its production life.
Several unique parts were developed for truck M-blocks, including flywheels for manual transmissions, harmonic balancers, and truck-specific intake and exhaust manifolds. The original “non-retarded” crankshaft timing gear was also re-introduced on truck engines.
Other than the strengthened D7TE truck block, the truck 351M was basically the same as the car engine, with truck-only 8.0:1 pistons and a slightly different camshaft. The truck 400 also used a truck-only piston that produced 8.4:1 compression ratio, and a slightly different camshaft with more lift at both intake and exhaust valves. In MY 1978, car M-blocks received the D7TE engine block enhancements in their own D8AE engine block.
In the late 1970s, American
manufacturers began using metric designations for engine displacement. The
351 cubic inch engines (both 351 Windsor and 351M) were referred to as 5.8
liters, and the 400 cubic inch engine was referred to as 6.6 liters. The W
(Windsor) and M suffixes were attached to the 351s’ metric designations to
distinguish the two types (i.e., 5.8W and 5.8M).
Even though the M-block was relegated to smog oblivion by the factory, and even though the M-block has been largely ignored by the aftermarket performance industry, the 351 Cleveland is reasonably well supported and the M-block has just enough in common with the 351C that it can share several key performance enhancing components. Even though you might have heard otherwise, there are several high-performance options for Ford’s M-block engines.
When looking for M-block performance components or (more likely) performance components that can be adapted to the M-block, you need to keep in mind which of the 335-series (351C/351M/400) engine components are interchangeable.
335-series cylinder heads
When discussing interchangeable components of the 335-series engines, one of the first thoughts that comes to mind for many people is the performance potential of interchangeable cylinder heads with the 351 Cleveland.
You might even wonder, “Since the 351C and M-block share the same cylinder head design, couldn’t I improve my M-block’s performance by swapping in a higher compression (smaller chamber) 351C cylinder head?”
Unfortunately, the answer is, “Maybe.”
The 351C 2V heads have essentially the same size ports and the same size valves as the M-block heads, so they would be compatible with the runners in an M-block intake manifold, but the combustion chamber size difference will not produce a significant change in the M-block compression ratio, and in some cases, the 351C 2V cylinder heads actually have larger chambers than M-block heads!
The 351C 4V CobraJet heads and later 351C 4V heads have combustion chamber volumes close enough to M-block chamber volumes that they are not worth swapping to significantly increase the compression ratio. The only 351C 4V heads with small enough chambers to make a significant change in compression ratio are the early 351C 4V heads or the rare Boss 351 heads. However, the main disadvantage of using the big port, big valve 351C 4V heads on an M-block is that the power band of the M-block is not compatible with the port volume of the 351C 4V heads. The 351C 4V heads do not achieve optimal airflow velocity until engine speed reaches about 3500 rpm, and M-blocks in a typical (i.e., truck or large car) application really need to produce a lot of torque and power below that speed. To realize the true benefits of the 351C 4V heads, you’d have to rev your engine up to 6000 rpm (or higher) routinely.
There is one more point to consider about the 351C 4V cylinder heads. The exhaust ports on these heads are a notoriously poor-flowing design. In fact, the stock 351C 4V exhaust port flows less than a stock 351C 2V exhaust port and less than a stock M-block exhaust port. (That’s the reason all OEM camshafts for the 351C 4V engines had such long exhaust lobe duration and high exhaust valve lift.)
The only other factory cylinder head options for the M-block are the Australian Cleveland cylinder heads. There are two major variants of the Australian Cleveland heads with the same size valves and ports as the US-made 351C 2V heads, the 302C and the 351C 2V. The Australian 351C 2V heads have a chamber size comparable to US-made 351C 2V heads, but the Australian 302C head has a very small 58 cc chamber (with some reportedly 62 cc). Obviously, the 302C heads would substantially raise the compression ratio on an M-block, and probably by too much unless you use custom pistons.
The following table lists the specifications of the Cleveland-type cylinder head variants for comparison. Compression ratios are original advertised ratios on the original engines equipped with those heads.