High Performance Options for the 351M/400 Engine

By David Resch

High Performance Options for the 351M/400 Engine

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.

 

History

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 335 Series Engines

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.

 

The Demise of the M-Block

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.

 

Big Block Engine Swaps

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.

 

Comparing M-block Power to Big Block Power

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.

 

Interchangeable Components in the 335 Series

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.

Engine Component

Interchangeability

Comments

cylinder heads

100% interchangeable

You can use any cylinder head from any 335 series engine on any 335 series engine block.

All 335 series engines share the same bore diameter, bore spacing, head mounting bolt locations and sizes, and block to head water jacket passages.

camshaft

100% interchangeable

You can use a camshaft designed for any 335 series engine in any other 335 series engine.

timing set (crankshaft and camshaft sprockets and timing chain, or gear drive system)

100% interchangeable

Timing sets are interchangeable between 335 series engines because they all share the same crankshaft to camshaft dimension and block front design.

valve train (lower)

varies by component

Valve lifters are interchangeable between all 335 series engines.

Push rods are not interchangeable between 351C and M-block engines because of different block deck heights.

valve train (upper)

varies by component

For non-adjustable, hydraulic tappet valve trains, almost all components mounted to the cylinder head are interchangeable.

All 335 series engines use the same rocker arm ratio (factory 1.73:1).

Rocker arms are not interchangeable between adjustable (solid tappet) and non-adjustable valve trains, unless the head is machined to accept the other type of rocker arm.

Some adjustable valve trains require pushrod guideplates and hardened pushrods.

valves

some interchangeable

The 351C 2V heads and M-block heads use the same size valves (2.04/1.66).

The M-block head can be machined to use 351C 4V valves (2.19/1.71).

pistons

partially interchangeable

The 351C and 400 pistons have the same compression height (dimension from wrist pin to top of piston), but the M-block wrist pin is slightly larger than the 351C wrist pin.

The 351M piston is unique, with a taller compression height than the 351C and 400 pistons, and it is not interchangeable with any other engine.

distributor

100% interchangeable

Distributors for 335 series engines also interchange with the 385 series (429/460).

water pump

100% interchangeable

All 335 series engines use the same front cover and water pump attachment.

fuel pump

100% interchangeable

Mechanical (cam driven) fuel pumps on all 335 series engines use a unique vertical bolt attachment pattern, and they are not interchangeable with any other Ford engines.

thermostat housing/water outlet

100% interchangeable

There were several factory designs used in different applications. The main difference between the various factory designs was the number and location of pipe-threaded bosses for PVSs and other temperature sensors.

oil pan

100% interchangeable

Even though the gasket surface and bolt patterns are identical among all 335 series engines, there are different designs (e.g., front sump, rear sump, etc.) used in different applications.

oil pump

100% interchangeable

All 335 series engines use the same oil pumps and pickup tube mounting. Pickup tube designs vary by oil pan applications.

 

 

M-block Performance Components

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:

Intake manifolds  Both Edelbrock and Weiand manufacture aluminum intake manifolds that allow you to use a 4V carburetor on an M-block engine. Edelbrock offers both EGR and non-EGR versions of its Performer 400 intake manifold. The Edelbrock EGR manifold has all the necessary provisions for both Thermactor air injection and the EGR system, making it completely emissions legal, even on later model M-blocks in California. Both the Edelbrock non-EGR and the Weiand manifolds have provisions for the Thermactor air injection system.

Because the M-block has a taller deck height than the 351C engine block, the M-block’s cylinder heads are farther apart than the 351C’s and intake manifolds designed for the 351C will not fit on the M-block engine. However, Weiand and Moroso both make adapters that will allow you to use a 351C manifold on an M-block engine. Since intake manifolds are made specifically for the M-block, the only reason you’d need the adapters is if you wanted to use the 351C 4V cylinder heads (which have much bigger intake ports than the 351C 2V or M-block heads) and a 351C 4V intake manifold.

Exhaust headers  Several companies that make exhaust headers offer headers for the M-block engine in Ford truck (and Bronco) applications. You can get both 4-into-1 type headers and Tri-Y (2-into-2-into-1) headers for the M-block.

Actually, since cylinder heads are interchangeable between all 335 series engines, any header designed for the 351C would bolt up to an M-block. The suitability of any particular header might be determined by its fit in a particular vehicle application (i.e., clearing the starter, engine mounts, frame rails, crossmembers, transfer case, etc.).

Pistons  Interestingly, the only performance component made specifically for the 351M engine is a TRW forged piston (part # L2466F). Federal Mogul claims this piston raises the compression ratio with stock M-block heads from the original 8.0:1 to approximately 8.6:1.

The following sections describe performance options for each of the major M-block engine components:

  • Pistons
  • Main Crankshaft Bearings
  • Valvetrain (camshaft, timing set, valves, etc.)
  • Fuel Induction (intake manifold, carburetor, etc.)
  • Exhaust (headers)
  • Accessories (water pumps, etc.)
  • Dress-Up (chrome valve covers, etc.)

 

Pistons

In 1971, the 400 was equipped by the factory with flat top pistons that produced a 9.0:1 compression ratio, but I have not been able to find any manufacturer that reproduces this piston now. In fact, the only mass produced piston I have been able to find that is specifically designed to improve compression in the M-block engine is the TRW L2466F forged piston, and it is a 351M piston.

Other than using a healthier cam, increasing the compression ratio of an M-block engine is the most significant performance improvement you can make. Fortunately, the compression height (distance from wrist pin centerline to piston top) of 351C pistons is compatible with 400 pistons. Stock compression heights of the various 351C pistons range from 1.631" to 1.657". The compression height of the stock 400 piston is 1.650". (Remember, the 351M uses the same rod as the 400 with 0.50" less stroke, so its piston compression height is much taller than the 400 piston to make up for the different stroke with the same rod. The 351M piston compression height is 1.947".)

WARNING: Pistons are not interchangeable between the 400 and 351M. If you used 400 pistons in a 351M, the piston top would reach TDC at more than 0.3 inch below the deck surface and you would have a compression ratio of only about 5.5:1. If you use a 351M piston in a 400, it will smash into the head because it’s top surface is about 0.3 inch above the deck surface.

The following table is sorted by engine application. In some cases, a single piston design can be used in more than one engine. Compression ratios are estimated by the piston manufacturers based on the average combustion chamber volume in the factory stock (unmodified) cylinder heads for each engine application. For example, the Keith Black KB148 piston is estimated to produce an 8.5:1 compression ratio when used with 351C 2V cylinder heads (average combustion chamber volume 76.2cc) and 9.8:1 compression ratio when used with early type (’70-’71) 351C 4V cylinder heads (average combustion chamber volume 62.8cc).

Engine Application

Material

Estimated CR

Brand & Part No.

351C 2V OEM-type

Cast

7.9:1

Sterling 425P

351C 2V

Forged

8.4:1

TRW L2416F

351C 2V

Hypereutectic

8.5:1

Keith Black KB148

351C 2V

Hypereutectic

8.8:1

Sterling H555P

351C 2V

Forged

8.9:1

TRW L2379F

351C 2V

Hypereutectic

9.5:1

Keith Black KB177

351C 2V 1/16-1/16-3/16

Hypereutectic

9.5:1

Keith Black KB108

351C 2V 1/16-1/16-1/8

Forged

10.2:1

TRW L2348F

351C 2V 1/16-1/16-1/8

Forged

11.4:1

TRW L2408F

351C 2V 1/16-1/16-3/16

Hypereutectic

10.6:1

Keith Black KB149

351C 4V 1970-72 (early heads)

Forged

10.2:1

TRW L2379F

351C 4V

Hypereutectic

9.8:1

Keith Black KB148

351C Boss 1971-72 with 66cc

Hypereutectic

12.1:1

Keith Black KB149

351C 4V 1973-74 (late heads)

Forged

9.4:1

TRW L2416F

351C 4V

Hypereutectic

10.1:1

Sterling H555P

351C 4V

Hypereutectic

11.1:1

Keith Black KB177

351C 4V 1/16-1/16-3/16

Hypereutectic

11.1:1

Keith Black KB108

351C 4V 1/16-1/16-1/8

Forged

12.0:1

TRW L2348F

351C 4V 1/16-1/16-3/16

Hypereutectic

12.7:1

Keith Black KB149

351M 1975-82 OEM-type

Cast

7.9:1

Sterling 451P

351M 1975-82

Forged

8.0:1

TRW L2436F

351M 1975-82

Forged

8.6:1

TRW L2466F

400 1971-82 OEM-type

Cast

7.9:1

Sterling 427P

400 1971-82

Forged

8.2:1

TRW L2414F

400 1971-82

 

9.5:1

Ohio Piston

 

Crankshaft Bearings

The 351C crankshaft has smaller main bearing journals than the M-block crankshaft. Therefore, crankshaft main bearings are unique to the M-block engine.

The following table lists crankshaft main bearings by engine application (as identified by the bearing manufacturer).

Engine Application

Main Bearing Type

Brand & Part No.

351M 1975-76

stock

Clevite MS981P

351M 1975-76

stock

Federal Mogul 5107M

351M 1975-76

stock full groove

TRW MS981P.std

351M 1975-76

competition full groove

Federal Mogul 123M

351M 1977-82

stock

Clevite MS1432P

351M 1977-82

stock

Federal Mogul 5078M

351M 1977-82

stock half groove

TRW MS1432P.std

351M 1977-82

competition half groove

Federal Mogul 122M

351M 1977-82

competition 3/4 groove

Federal Mogul 130M

400 1971-76

stock

Clevite MS981P

400 1971-76

stock

Federal Mogul 5107M

400 1971-76

stock full groove

TRW MS981P.std

400 1971-76

competition full groove

Federal Mogul 123M

400 1977-82

stock

Clevite MS1432P

400 1977-82

stock

Federal Mogul 5078M

400 1977-82

stock half groove

TRW MS1432P.std

400 1977-82

competition half groove

Federal Mogul 122M

400 1977-82

competition 3/4 groove

Federal Mogul 130M

 

All 335 series engine crankshafts use 2.311" rod bearing journals. The following table lists crankshaft rod bearings by bearing type (as identified by the bearing manufacturer).

Rod Bearing Type

Brand & Part No.

Stock

Clevite CB927P

Stock

Federal Mogul 8-3400CP

Performance

Clevite CB962P

Competition

Clevite CB927H

Competition with dowel hole

Clevite CB927HD

Competition

Federal Mogul 8-7175CH

Competition

Clevite CB927V

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