High Performance Options for the 351M/400 Engine

By David Resch

From 1977 to 1982, M-block (351M/400) engines were a mainstay of Ford light truck powerplants. During this period, the M-block 400 was the largest V8 available from the factory in a 4×4 truck (Bronco or pickup). The 460 was not a factory option in 4×4 pickups until 1983.

This article provides information on the wide variety of performance components available for the often overlooked M-block engine and provides some background on the M-block engine’s origin and design.

In the 1960s there were two major developments in Ford V8 engine design for street vehicles, the lightweight thinwall casting engine block and poly-angle chambers with canted valves.

In 1962, Ford introduced its first V8 engine with lightweight, thinwall block castings, the small block 221. This engine was also the first design in the 90-degree small block family that would ultimately grow to include the 260, 289, 302, and 351 Windsor.

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 1968 Thunderbirds and the 460 introduced in 1968 Lincolns.

In 1968, Ford also 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 and introduced it in 1969. The Boss 302 cylinder head was an all new design with huge ports and valves (even bigger than the Tunnel Port’s) for maximum breathing efficiency. The Boss 302 proved successful in Trans Am racing and it was sold to the public as a high performance option in Mustangs for two model years, 1969 and 1970.

After the initial success of the Boss 302, Ford engine designers developed a completely new, larger displacement engine to take advantage of the free-breathing Boss 302 cylinder head. Introduced in 1970, the 351 Cleveland was the first in a new engine design family known as the 335 series.

The first M-block, the 400, was introduced in the 1971 model year as a longer-stroke variant of the 351 Cleveland design. The 400 was the ultimate development of the 335 series, and it was the last pushrod V8 engine that would ever be designed by Ford.

The were two engine types in the Ford engine design family known as the 335 series, the 351 Cleveland and the M-block. There were several variants of the 351 Cleveland (351C) engine, but there were only two M-block variants, the 351M and 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 and medium-duty trucks.

Compared to the 351C, the 400 featured a completely new block with the big-block (429/460) bell housing bolt pattern, over 1" taller deck height (10.297" vs 9.206"), larger crankshaft main journals (3.00" vs 2.749"), and longer connecting rods (6.58" vs 5.78"). The 400’s bore diameter (4.00") and bore spacing (4.38") match those of the 351C and the later 90-degree small blocks (289, 302, and 351W). The 400 produces its additional 49 cubic inches of advertised displacement with a 4.00" stroke of the crankshaft. In fact, the 400 has the longest stroke of any factory stock Ford V8 engine!

Both engine types in the 335 series share certain design characteristics, but the only major component interchangeable between 351C engines 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.

When it was introduced in 1971, the 400 was available in Customs, Galaxies, and LTDs. It was developed as the 351C’s big brother, and it 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.

In 1971, the 400 used a flat top piston that produced a 9.0:1 compression ratio. In 1972, compression was lowered with a dished piston that produced only 8.4:1 compression. The low compression piston was used until M-block production ceased in 1982. Beginning in 1973, Ford retarded the camshaft timing on some 335 series engines to meet emission control regulations.

Production of the 351C ended during the 1974 model year. 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 first used in passenger cars in the 1975 model year.

The 351M was a simple, de-stroked adaptation of the 400. It uses the same block as the 400 with a shorter stroke crankshaft (3.50"), the same M-block connecting rods, and a unique piston with a taller compression height to compensate for the shorter stroke with the same rods in the same block. In fact, the only difference between the 351M and 400 is the crankshaft and pistons. All other components are the same. In practice, Ford did use slightly different components in the 351M and 400 for different applications and for performance reasons, such as camshafts, timing sets, and valve springs, but functionally, the engines were identical except for crankshafts and pistons. The 351M’s unique piston was a dished type that produced 8.0:1 compression.

Interestingly, 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 a lot of confusion, and even a mistaken belief among some pickup truck owners that their trucks had 351 Cleveland engines. In fact, the only trucks ever equipped with a 351 Cleveland engine by the factory were ’70-’74 Rancheros, which shared the Torino’s platform and powertrain options.

Both M-block engines were used in passenger cars through the 1979 model year. After 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.

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.

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 microcomputer 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, the M-block was crippled with primitive, first generation emission control devices (AIR pumps 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 was perceived by the public to be merely a low compression "smog" motor. Consequently, the M-block has been woefully overlooked by many Ford performance enthusiasts.

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

All 335 series intake manifolds are designed to provide internal plumbing for both the Thermactor air injection system and the EGR system with no modification to the cylinder heads, exhaust manifolds, or other engine components. Unlike the small block and big block engines, the M-block required no cumbersome and unsightly external tubing to feed Thermactor air into its exhaust manifolds and feed exhaust gas from the manifolds to the EGR valve. The M-block intake manifold simply used the cylinder head’s existing 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 unique spacer/adapter under the carburetor base.

By the early 1980s, the new generation of automotive emission controls depended on electronic engine 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 fresh air injection point (still required for the catalytic converter) must be downstream from the oxygen sensor to prevent false readings. The M-block, with its air injection point right inside the cylinder heads’ exhaust ports, was not easily adapted to a feedback mixture control system.

Actually, it would have been a simple matter to install a more complicated Thermactor air injection system (like the ones used on other Ford engines) and pipe the fresh air to ports located just in front of the catalytic converter. However, with less demand now for the 350-400 class engines (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 302, it probably made economic sense to discontinue the M-block, which was by now an "odd duck" in terms of manufacturing adaptability.

Many owners of Ford trucks that were originally equipped with M-block engines have chosen to upgrade their vehicles’ performance by replacing the M-block with a 429/460 big block. In fact, this particular engine swap is so popular that an aftermarket industry has emerged to cater to it. (L&L Products in Dallas, Texas was originally founded specifically to manufacture and market components for Ford 385 series big block engine swaps.)

On the other hand, an M-block engine in factory trim has vast, untapped power potential. It is smaller and lighter than a big block (which is why it was developed in the first place), and when properly built, the M-block’s power output is comparable to most big blocks.*

*Big Block Lover’s Disclaimer: Please note that I am not asserting that "any" M-block can beat "any" big block, but I am proposing that for the same time and money you put into the engine swap itself, you can build an M-block that will eat most of the big blocks out there. Yes, yes, when all other things are equal, cubes = power, and you can always build a bigger engine that will stomp smaller engines. But when you factor in the costs and time (and hassles) involved in swapping engines and the "real-world" handling performance penalties of excess weight on the front end of the vehicle, it’s easier to put the benefits of the M-block engine in perspective.

In 1971, before SAE "net" power measurements, the 400 was rated at 260 hp @ 4400 rpm. By comparison, in 1971 the 460 was rated at 365 hp @ 4600 rpm. Bear in mind that these numbers compare a 400 with a 2V carburetor and 9.0:1 compression ratio to a 460 with a 4V carburetor and 10.5:1 compression ratio.

1971 is an interesting year to compare because the 400 had its original flat top pistons and a decent compression ratio. If you consider that the 1971 351C with a 4V carb was rated at 285 hp @ 5400 rpm with a 9.0:1 compression ratio, you begin get an idea of the 400’s awesome power potential. I estimate that with a 4V carb, a decent cam (like the 351C had), a 10.5:1 compression ratio (like the 460 had), and 1971-era emissions controls, the 400 could have easily put out 325+ hp.

The next year (1972), when more aggressive emissions controls were implemented and SAE net power measurements were used, the difference between the power output of the 400 and 460 was even narrower: 172 hp @ 4000 rpm (400) vs 212 hp @ 4400 rpm (460).

Comparing maximum torque figures for 1972 (SAE net measurements) shows even more dramatically how close the two engines’ power output is: 298 ft lb @ 2200 rpm (400) vs 342 ft lb @ 2800 rpm (460). At its peak torque, the 400 produces almost 90% of the peak torque of the 460, and it does it at less than 80% of the 460’s engine speed.

The 1972 comparison is a little more fair because the 460’s compression was dropped, too. Now we’re comparing a 400 with 8.4:1 compression to a 460 with 8.5:1 compression. Don’t forget, though, we’re still comparing a 400 2V to a 460 4V.

Even though the M-block was relegated to smog oblivion by the factory, and even though the M-block has been mostly ignored by the aftermarket performance industry, the 351 Cleveland is well supported and the M-block has just enough in common that it can share several key performance enhancing components. Even though you may have heard otherwise, there are several excellent high performance options available for Ford’s M-block.

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.

Note: In the following table, components listed as 100% interchangeable are mechanically interchangeable between all 335 series engines (i.e., a direct bolt-in). But keep in mind that, even though they might be mechanically interchangeable, any particular version of a component may not be suitable for use in a specific M-block application.

Note: I have attempted to document as many M-block performance components from as many different manufacturers as I could find in my own personal research. I have tried to avoid errors in this information, but I can make no warranties to that effect. By mentioning any particular product, this article is not an endorsement of that product or its manufacturer or supplier, nor is it a guarantee of compatibility, usability, or marketability of that product for any particular application.

Although the M-block engine is not well supported by the aftermarket performance industry, there are a few performance components that are manufactured specifically for the M-block. These are: