351 Small Blocks

Everything about the 1971, 1972 & 1973 Ford Mustang

351 Windsor

The 351W (Windsor) is often confused with the 351 Cleveland, which is a different engine of identical displacement. The 351 cu in (5.8 L) Windsor featured a 1.3 in (32.5 mm) taller deck height than the 302, allowing a stroke of 3.5 in (88.9 mm). Although very much related in general configuration to the 289-302 and sharing the same bell housing, motor mounts, and other small parts, the 351W had a unique, tall-deck block, larger main bearing caps, thicker, longer connecting rods, and a distinct firing order (1-3-7-2-6-5-4-8 versus the usual 1-5-4-2-6-3-7-8, a means to move the unacceptable ‘noise’ of the consecutive firing adjacent front cylinders to the more rigid rear part of the engine block all while reducing excessive main bearing load), adding some 25 lb (11 kg) to the engine’s dry weight. The distributor is slightly different, so as to accommodate a larger oil pump shaft and larger oil pump. Some years had threaded dipstick tubes.

It had a unique head which optimized torque over high-rpm breathing, frequently replaced by enthusiasts with aftermarket heads providing better performance. The early 1969 and 1970 heads had larger valves and ports for better performance. The head castings and valve head sizes from 1969 to 1976 were different, notably in passages for air injection and spark plug diameters (1969-1974 18 mm, 1975 and up 14 mm). From 1977 onward, the 351W shared the same head casting as the 302, differing only in bolt hole diameters (7/16 inch for 302, 1/2 inch for 351W). Early blocks (casting ID C9OE-6015-B) had enough metal on bearing saddles 2, 3, and 4 for four-bolt mains, and as with all small-block Fords (SBFs), were superior in strength to most late-model, lightweight castings. Generally, the 1969 to 1974 blocks are considered to be far superior in strength than the later blocks, making these early units some of the strongest and most desirable in the entire SBF engine family including the 335-series. During the 1980s, a four-barrel version (intake manifold casting ID E6TE-9425-B) was reintroduced for use in light trucks and vans. In 1988, fuel injection replaced the four-barrel carburetor. Roller camshaft/lifters were introduced in this engine in 1994.

The original connecting rod beam (forging ID C9OE-A) featured drilled oil squirt bosses to lubricate the piston pin and cylinder bore and rectangular-head rod bolts mounted on broached shoulders. A number of fatigue failures were attributed to the machining of the part, so the bolt head area was spot-faced to retain metal in the critical area, requiring the use of ‘football head’ bolts. In 1975, the beam forging (D6OE-AA) was updated with more metal in the bolt-head area. The oil squirt bosses were drilled for use in export engines, where the quality of accessible lubricants was questionable. The rod cap forging remained the same on both units (part ID C9OE-A). In 1982, the design of the Essex V6 engine used a new version of the 351W connecting rod (E2AE-A), the difference between the two parts was that the V6 and V8 units were machined in metric and SAE units, respectively. The cap featured a longer boss for balancing than the original design.

The block underwent some changes since its inception. In 1971, deck height was extended from 9.480 in to 9.503 in (casting D1AE-6015-DA) to lower the compression ratio to reduce NOx emissions without the need to change piston or cylinder-head design. In 1974, a boss was added on the front of the right cylinder bank to mount the air injection pump (casting D4AE-A). In 1974, the oil dipstick tube moved from the timing case to the skirt under the left cylinder bank near the rear of the casting. These details made swapping older blocks from passenger cars with front sump oil pans to more recent rear-sumped Mustang and LTD/Crown Vic Ford cars more difficult unless an oil pan had the dipstick mounted therein. In 1984, the rear main seal was changed from a two-piece component to a one-piece design.

Introduced in 1969, it was initially rated (SAE gross) at 250 hp (186 kW) with a two-barrel carburetor or 290 hp (216 kW) with a four-barrel. When Ford switched to net power ratings in 1972, it was rated at 153 to 161 hp (114 to 120 kW), although actual, installed horsepower was only fractionally lower than in 1971. Around 8.6 million 351W engines were manufactured between 1969 and 1996 at the Windsor Engine Plant Number One.

During the 1990s, motor enthusiasts were modifying 351 Cleveland 2V cylinder heads (by rerouting the coolant exit from the block surfaces to the intake manifold surfaces) for use in the 351W, resulting in the Clevor (combining Cleveland and Windsor). This modification required the use of custom pistons by reason of differing combustion chamber terrain (canted valves vs. straight valves) and intake manifolds. This combination yielded the horsepower potential of the 351C with the ruggedness of the 351W small block and was possible because more 351C 2V cylinder heads were manufactured than the corresponding engine blocks (the 351M and 400 used the same head as the 351C 2V).

The 5.8 L, 351W, was changed during the ’90s from speed density to MAF; performance gains were directly affected. Before 1994, the 5.8 L was equipped with speed density. This programmed coding was placed into the vehicle’s computer to tell the motor how much air it should be getting, therefore supplying an appropriate amount of fuel. However, if modifications are made to increase air flow, the computer does not provide more fuel until the oxygen sensors register a lean burn and only then can the computer compensate fuel trim. After 1994, the engine was changed to mass air flow (MAF). This allowed the computer to directly read how much air the engine was receiving through the help of a sensor in the air intake as long as that exact sensor was used (MAF sensors cannot be upgraded for increased flow without using an aftermarket chip, reflash, or ECU. The rest of the intake system can be modified in any number of ways, but generally the MAF sensor remains stock). Because the computer reads this, it is able to increase the amount of fuel the engine gets when the air flow is increased, thus increasing performance.


  • 1968–1974 Ford Galaxie
  • 1968–1970 Ford Mustang
  • 1969–1970 Mercury Cougar
  • 1969–1991 Ford Country Squire
  • 1969-1970 Ford Fairlane (Americas)
  • 1970–1976 Ford Torino
  • 1974–1976 Ford Elite
  • 1975–1996 Ford E series
  • 1977–1979 Ford LTD II
  • 1977–1979 Ford Thunderbird
  • 1979–1996 Ford Bronco
  • 1979–1982 Ford LTD (Americas)
  • 1979–1991 Ford LTD Crown Victoria (After 1982, this engine would only be sold for police sales)
  • 1987–1997 Ford F-Series
  • 1977–1979 Mercury Cougar (station wagons only)
  • 1978, 1986–1991 Mercury Colony Park
  • 1978–1982 Mercury Marquis
  • 1986–1991 Mercury Grand Marquis
  • 1980 Continental Mark VI

351 Cleveland


Production of the 351 Windsor V8 engine began for the 1969 model year in the Windsor Engine Plant #1. Ford sales and marketing forecast that the demand for this engine would exceed the plant’s production capability, and it was decided the additional production would begin at the Cleveland engine plant. At this time, it was also decided to upgrade the design of the new Cleveland manufactured 351s to a higher-specification power plant. It was decided that a new cylinder head design was needed for improved performance. Two cylinder-head designs were developed. One cylinder head used the same basic design as the 351W, but with larger ports and valves. The second had very large ports with canted intake and exhaust valves, similar to the Ford 385 Series V8. Sales, marketing, and product planning favored the canted valve design, as it was viewed as more innovative.

Other changes to the engine were related to ease of manufacture and improved reliability. This led to elimination of coolant flowing through a ‘dry’ intake manifold eliminated a potential source of leaks and minimized unnecessary heat transfer. To perform this change, the front of the engine block was extended to include provisions for the coolant to flow through a cross over in the block. This extension also acted as an integrated timing chain housing. The timing chain housing was covered with flat steel that was easier to seal than the typical large timing chain cover used on other Ford V8s. These changes resulted in a bigger and heavier engine block than the Windsor V8s. To help reduce costs the oil system was revised, as explained above. Although the 351W began as the basis for the 351C, by the time it reached production the design changes resulted in almost no parts interchanging between the two designs. The two engines, however, shared the same bore spacing, engine mounts and bell housing pattern.

The 351 Cleveland began production in July 1969 1969 for the 1970 model year. Its actual displacement was 351.9 cubic inches (5,766 cc). A 4V (four venturi) performance version and a conventional 2V (two venturi) version were built. The 351C-4V was marketed as a high-performance engine, featuring the 4V large ports heads with closed “quench” combustion chambers. Later versions of the 351C with 4V heads continued to use the large ports and valves, but switched to open chamber heads in an effort to reduce engine emissions. The 351C-2V was never marketed as a high-performance engine. It used the small port 2V cylinder heads with open combustion chambers to produce a more economical engine that was tuned more for low-rpm torque. Only the Q-code 351 “Cobra Jet” (1971–1974), R-code “Boss” 351 (1971), and R-code 351 “HO” (1972) versions have four-bolt main bearing caps, however, all 335 series engines could be modified to have 4-bolt main bearing caps.


1973 H-code 2V 351 Cleveland

The H-code 351 Cleveland engines were low performance engines with low compression and two-barrel carburetors. All H-code engines ran on regular grade fuel. Compression ratio was 9.5:1 in 1970 and progressively dropped annually until it reached it low point of 8.0:1 compression in 1973 and 1974. H-code 351s were equipped with a cast-iron crankshaft, two-bolt main bearing caps, forged-steel connecting rods, cast-aluminum pistons, non-adjustable valve train, and cast-iron intake and exhaust manifolds. All H-code 351 Cleveland engines used the small port 2V heads with open combustion chambers. These engines were produced from 1970 through 1974 and were used on a variety of Ford models, from pony-car to full-sized. The with a 2V carburetor was also produced during this time which also used the “H-code” designation. Both the 351W and 351C H-code had the same or very similar power ratings, and were used interchangeably when a car was built with the H-code engine option.


The M-code was a high-compression, high-performance variation of the 351C, produced in 1970 and 1971. The M-code engines used the large-port 4V heads with a closed “quench” combustion chamber and large valves. These engines also included cast-aluminum flat-top pistons, stiffer valve springs, a high-performance hydraulic camshaft, and a square bore Autolite 4300-A carburetor. The 1970 engines had an advertised 11.0:1 compression ratio and were rated at 300 bhp (224 kW; 304 PS) at 5400 rpm. The 1971 version had a slightly lower advertised compression ratio of 10.7:1, and the power rating dropped to 285 bhp (213 kW; 289 PS) at 5400 rpm. The M-code 351C required premium fuel and was available in the 1970-71 Ford Torino, Mercury Montego, Ford Mustang, and Mercury Cougar.

1971 R-code (Boss 351)

The Boss 351 was the most potent high-performance variant of the 351C available only in the 1971 Boss 351 Mustang. Rated at 330 bhp (246 kW), it was fitted with a four-barrel Autolite model 4300-D spreadbore carburetor, an aluminum intake manifold, solid lifters, dual-point distributor, a six-quart oil pan, and cast-aluminum valve covers. Forged domed pistons gave an 11.1:1 advertised compression ratio which made premium fuel necessary. It had four-bolt main bearing caps selected for hardness and a premium cast-iron crankshaft selected for hardness (90% nodularity). The cylinder head was modified for better airflow, used screw-in studs with adjustable rocker arms, and except for the water passages and larger combustion chambers, were very similar to the heads used on the Boss 302. The valve train used hardened and ground push rods with guide plates and single grove-hardened valve split locks. The forged connecting rods were shot-peened and magnafluxed for strength, and used improved durability 180,000 PSI 3/8-inch nuts and bolts. The R-code Boss 351 was only installed in the 1971 Boss 351 Mustang, and it came equipped with Ram Air induction. Ford manufactured 1,806 Boss 351 Mustangs in 1971, 591 of which are registered and accounted for on the Boss 351 Registry site.

The January 2010 issue of Hot Rod reported a project in which a Boss 351 was assembled to the exact internal specifications of an original motor, but fitted with open, long tube, 1-3/4-inch Hooker headers (vs. the stock cast-iron manifolds), a facility water pump, a 750 Holley Street HP-series carburetor (vs. the stock 715 CFM Autolite unit), and minus the factory air filter assembly, engine accessories, or factory exhaust system. In that mildly modified state, it produced 383 hp (286 kW) gross at 6,100 rpm, and 391 lb⋅ft (530 N⋅m) torque (gross) at 4,000 rpm. A measurement of SAE net horsepower would be significantly lower, and represents a more realistic as-installed configuration with all engine accessories, air cleaner assembly, and automobile exhaust system.

1972 R-code (351 HO)

The 351C HO “R-code” had a number of changes to help meet emission standards for 1972 compared to the 1971 Boss 351 “R-Code”. The camshaft had less duration, but more valve lift, while the mechanical lifters remained unchanged. The forged pistons were changed to flat-top style and the heads to open chamber heads, but retained the same large ports, valves, and adjustable valve train used in 1971. This resulted in a compression ratio decreased to 9.2:1 while the cleaner-burning open-chamber heads helped meet the new emissions regulations. The Ram Air option was no longer available. The engine otherwise remained unchanged from 1971. This engine produced 275 hp (205 kW) using the more realistic SAE net system and was only available in the 1972 Ford Mustang.

Q-code (351 Cobra-Jet)

The Q-code 351 “Cobra-Jet” (also called 351-CJ, 351-4V) was produced from May 1971 through the 1974 model year. It was a lower-compression design that included open-chamber 4V heads. The open-chamber heads exhibited superior emissions characteristics and were required to meet the more stringent emissions standards for 1972 and beyond. The “351 CJ” high-performance engine included a different intake manifold, high-lift, long-duration camshaft with hydraulic valve lifters, different valve springs and dampers, a 750-CFM spread-bore 4300-D Motorcraft carburetor, dual-point distributor (with four-speed manual transmissions only), and four-bolt main bearing caps. These engines also featured induction-hardened exhaust seats for use with low-lead and unleaded gasoline. This engine was different from the 1970-71 M-code 351C having a more aggressive camshaft, a spread-bore carburetor, a four-bolt block and the lower compression allowed regular fuel to be used. It was rated at 280 bhp (209 kW; 284 PS) for all 1971 applications. For the 1972 model year, the only change to the engine was a retarding the camshaft events by 4°. The engine was rated at 266 hp (198 kW) (SAE net) for 1972 when installed in the Mustang, and 248 hp (185 kW) in the Torino and Montego. An increase in the combustion chamber size and the use of smaller valves occurred in 1973, which reduced horsepower to 246 hp (183 kW) for the four-barrel for the intermediate Fords, though it still retained the higher 266 hp (198 kW) rating in the Mustang. The 351 CJ (now referred to as the “351 4V”) was rated at 255 hp (190 kW) in 1974 and was only installed in the Ford Ranchero, Ford Torino, Mercury Montego, and Mercury Cougar.


Production of the 351C ended at the end of the 1974 model year. The engine was replaced by the 351M for the 1975 model year. This new variation used the same bore and stroke dimensions of the 351C, but used the tall deck block from the 400 V8 engine.

351C engine specifications chart

CodeEngine typeYearsCompressionCombustion ChamberCamshaft DurationCamshaft LiftTappetsMain Bearing CapsNotes
H351C-2V1970–1974LowOpen Chamber258° I/266° E 32° overlap0.400″ I/0.406″ EHydraulic2-bolt
M351C-4V1970–1971HighClosed Chamber266° I/ 270° E 34° overlap0.427″ I/0.427″ EHydraulic2-bolt
R351C-4V “Boss 351”1971HighClosed Chamber290° I/ 290° E 58° overlap0.467″ I/0.477 EMechanical4-boltRare
R351C-4V HO1972LowOpen Chamber275° I/ 275° E 35° overlap0.491″ I/0.491″ EMechanical4-boltVery rare
Q351C-4V CJMay 1971 – 1974LowOpen chamber270° I/ 290° E 48° overlap0.480″ I/0.488″ EHydraulic4-boltcam timing retard 4° in 1972, compression reduced in 1973