Ford had been aggressively updating its engine lineup. Existing engines grew in displacement; the 352 grew to 390 ci, then 406, 427, and 428. A modern and compact small-block replaced the obsolete 292 Y-block in 1962 and was immediately upsized to a 260, then a 289, and finally a 302 in 1968. Also in 1968, a new big-block engine family utilizing similar weight-reducing designs appeared as the 429. It would be radically transformed into the Boss 429 the following year, and into the wedge-head 429 Cobra Jet for 1970. Ford's engine development was on the move.
In concept, the 351W was an extension of the 289/302 design, though the block was larger and not much was directly swappable; even the firing order was different. The new 351, nicknamed the Cleveland after Ford's Engine Plant Number 2, where it was produced, was yet another completely new engine from a completely new engine family Ford called the 335. While the 351W was obviously an extension of the 289/302, the new 351C was its own thing.
To put the new design into production, Ford spent big. "Ford Motor Company will release this fall a brand new $100-million engine designed for the 1970s," read an August 15, 1969, press release. "A 510,000-sqare-foot addition to the company's Cleveland Engine Plant #2 was built to produce the new 351 engine."
The biggest changes were in the heads, which, save for the water jacket openings, were verbatim pickups of the Boss 302 heads introduced in 1969. The heads had a lot in common with the heads introduced on the 429 in 1968. Initially, two different castings were used: 351Cs with two-barrel carbs had big ports and valves, while 351Cs with four-barrel carbs had huge ports and valves. Two-barrel engines got 2.04/1.65-inch valves, intake/exhaust, and four-barrel engines got 2.19/1.71-inch valves. For perspective, 2.19 is also the size of intake valves on Chevrolet's revered 427 L88. Intake and exhaust ports were also substantially larger in the four-barrel engine.
Another key upgrade was the operating angle of the 351C's valves. Instead of having all the valves aligned in the same plane, as with the 351W, the 351C tilted the valves slightly. From the cylinder centerline, intake valves leaned around 9 degrees toward the intake port and around 4 degrees to the side. This allowed for slightly larger valve diameter and smoother bends in the port contours. Because each cylinder's valves operated at different angles, each valve had its own pedestal. Cams for both engines were hydraulic, and the four-barrel cam was slightly hotter than the two-barrel.
Airflow through the heads was a quantum leap forward from Ford's traditional smallish ports, though the four-barrel heads and valves may have been too much of a good thing, at least for responsive street performance. NASCAR engine builders improved port efficiency by using epoxy to raise the port floor and reduce the opening of both intake and exhaust ports by around 30 percent.
Other four-barrel upgrades over the two-barrel engines were stainless-steel head gaskets, higher valve lift (an increase of 0.025-inch intake/exhaust), a different combustion chamber with quench area yielding an 11.0:1 compression ratio, and dual exhaust. Horsepower ratings were 250 at 4,600 rpm for the two-barrel, and 300 at 5,400 rpm for the four-barrel, 10 more than the 1969 351W four-barrel. Interestingly, the 351C four-barrel's rated torque output of 380 at 3,400 rpm was down from the 351W four-barrel's rating of 385 at 3,200.
The 351C found lots of applications, including as an intermediate engine option on many base models, as well as being the standard engine on step-up models like Mach 1 and Cougar XR7. Road tests of the day showed 351C four-barrel Mach 1 Mustangs running 15.2 e.t. 's, about a tenth or two quicker than a similar 351W Mach 1 from a year earlier.
Like the 429, which got a performance makeover one year after first appearing as a passenger car engine, the 351 had a hot new version for 1971. Original plans were to continue the Boss 302 into 1971, but that was scratched in the last stages of development. Likewise, the Boss 429 was dropped. The 429 Cobra Jet and 429 Cobra Jet Ram Air replaced the 428 Cobra Jet as the top engine option for 1971, but the only Boss Mustang offered would be the Boss 351, built and badged around the new 351C engine.
Ports and valve size were unchanged from the 1970 351C, though the valvetrain was extensively upgraded in ways that might escape the casual glance, starting with a hotter mechanical-lifter cam with higher lift and extended duration. Boss 351 pushrods were the same length as standard production 351s, but were hardened and ground, and kept in place with guide plates. Stronger valvesprings had 315 pounds of pressure open, up from the 285 pounds for non-Boss 351 engines, and they got a stamped-steel seat to prevent them from wandering at high rpm. Boss 351 heads were machined for threaded 7/16-inch rocker pedestals (production 351s used pressed-in 5/16 studs), and the valves were fitted with hardened, single-groove valve keepers.
The Boss 351 intake was cast aluminum and featured a spread-bore pattern. The carburetor looked like any other Autolite 4300, but the Boss used a 4300-D with the spread-bore pattern—smaller primaries and larger secondaries. All Boss 351s came with ram air induction, comprised of an open-element breather mated to functional, dual-inlet hood scoops. Exhaust manifolds, having been designed well in the first place, were standard 351 production.
Factory rated at 330 hp at 5,400 rpm, this new engine had the highest output the engine family would see. Other 351 performance engines would follow, but none would match the Boss 351. In Ford's advance press materials it was called the 351 H.O., though in other Ford literature it is referred to as the Boss 351, which it has come to be known as today.
Now things get confusing. In May 1971, a third version of the 351 four-barrel appeared, called the 351 Cobra Jet. It was, in essence, a warmed-over 351 four-barrel, but with lower-compression (9.0:1) open-chamber heads and only a few of the Boss 351's goodies—four-bolt mains, a slightly upgraded hydraulic-lifter cam (0.480/0.488-inch lift and 270/290 degrees duration intake/exhaust), dual-point distributor, and the Autolite spread-bore 4300-D carburetor. The rest of the engine was standard 351 four-barrel.
As if the waters weren't muddy enough, 1972 brought yet another shuffling of the 351 landscape. The 351 Cobra Jet was gone, at least in name. The 1972 351 four-barrel engine had all the same specs as 1971's 351 Cobra Jet, but it was no longer called a Cobra Jet, at least by Ford. (Mercury literature retains the CJ reference.) The sole difference is that the 1972 version's cam was retarded 4 degrees. Horsepower rating for this engine was 266 at 5,400, and torque was 301 at 3,600.
While the 1972 351 H.O. retained most of the Boss 351's superb hardware, three key revisions had a big impact on the engine's output. The heads with the large ports and valves were retained, as were the mechanical lifters and valvetrain, but the new heads had the larger, open combustion chamber. That, with new flat-top pistons, dropped the compression ratio 2.5 points to 8.8:1. Ford also dialed back cam duration from 290 degrees to 275. Horsepower fell 55, from 330 to 275, and torque plunged 84 lb-ft, from 380 to 296. Even though 1972 ratings were net, this was a sharp detune. More cuts were on the way.
For the record, Ford also introduced the 400 two-barrel in 1972, a higher-displacement (half-inch-longer stroke) but low-revving, low-octane, low-compression, low-performance spinoff of the 351. It had a taller block to accommodate the longer stroke, but there was never a performance version of the 400.
Because of the move to net horsepower standards, ratings changed by car. The November 11, 1972, Lincoln-Mercury Product News Bulletin explained: "…the new rating procedure results in minor power rating differences when the same engine is used in different car lines. For example, the 351-2V engine has a different horsepower rating for Mercury, Montego and Cougar. This is caused by differences in the exhaust, air intake and emission control systems. NOTE: All ratings are for engines equipped to meet the California emission standards. There are slight variances where the California equipment is not installed."
Horsepower and torque specs were unpublished in both the 1973 and 1974 Ford Car Facts dealer albums. There were almost two pages of discussion about emissions systems, but not a single line about horsepower and torque, save for this note in the section on Power Teams: "Engine compression ratios, horsepower and torque data to be furnished at a later date."
Connecting rods on 351C engines, both two- and four-barrel versions, were forged steel. Main bearing caps for both had extended width to allow for the possibility of four-bolt mains. Neither engine had four-bolt mains for 1970 but beginning in 1971 they would become standard on four-barrel 351C engines.
The 1971 and 1972 351 H.O. engines got many valvetrain upgrades, including larger-diameter, threaded rocker studs; hardened pushrods; guide plates; hardened, single-groove valve keepers; higher-rate valve springs; steel spring base; and hotter, solid-lifter cam. These changes, teamed with the Cleveland's ample ports and valves, really gave the Boss 351 a mean streak.
Exhaust manifolds, long a bottleneck on Ford engines, were addressed on the 351C. These castings may not have flowed as well as the 427's long-tube manifolds, but they were a big improvement over the 289 and 390 manifolds. Four-barrel 351 exhaust manifolds had larger ports and passages than the two-barrel castings, but both had 2.0-inch outlets. The exception was Boss 351 manifolds, which had larger 2.25-inch outlets.
You can tell a 351 Cleveland engine immediately by its rectangular valve covers. Like the 289/302 predecessors, 351 Windsor heads have a little angle at the front and rear of the valve covers. Ford invested more than $100 million to bring the 351C to the market, and while it added a bright spot to the automaker's engine lineup early in the 1970s, it closed out the decade as an unimpressive, long-stroke 351M and 400 two-barrel.
This content was originally published here.
