06/17/2026
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The 351 Cleveland V8 in the Boss 351 Mustang was not a regular 351 Cleveland. It shared the displacement and the basic architecture. Everything else was different.
The Cleveland V8, Ford's 335 series, used a canted-valve cylinder head design. Valves tipped 9.5 degrees from vertical and splayed 4 degrees from the cylinder axis. This allowed larger valves with a more compact combustion chamber. The same design philosophy as the Boss 302, the Ford 385 series big blocks, and the Chevy Mark IV. Canted valves were the performance technology of the late 1960s, and the Cleveland was Ford's middle-displacement answer.
From that starting point, the HO version in the Boss 351 received special attention at every critical point.
Four-bolt main bearing caps in the cylinder block instead of the standard two-bolt. Magnafluxed crankshaft and connecting rods, inspected for invisible cracks that could cause failure under high loads. Uprated valvetrain components throughout. And a solid-lifter camshaft with nearly .550 inches of lift and 250 degrees of duration. Solid lifters meant manual valve adjustment and a lumpy idle. It also meant the valvetrain could handle higher rpm without the float that hydraulic lifters suffered at the top end.
Compression ratio: 11.0:1. Some sources report 11.7:1. Either way, this was an engine that demanded premium fuel and rewarded it with power that had no business coming from 351 cubic inches.
An Autolite 4300 spread-bore carburetor with manual choke sat on top, fed fresh air by a standard hood induction system. The factory rated the engine at 330 horsepower at 5,400 rpm and 370 lb-ft of torque. Paired with a standard four-speed manual transmission and a 3.91:1 rear axle ratio that kept the engine in its powerband through every gear.
The results were stunning for a middle-displacement V8. Car and Driver's Patrick Bedard wrote that the Boss 351 was "quicker than a number of super cars with an extra 100 inches of displacement." The magazine recorded quarter-mile times around 14 seconds flat at 100 mph.
Motor Trend's comparison test told the story even more clearly. The Boss 351 ran 13.80 seconds at 104 mph in the quarter mile. The 429 Cobra Jet Mach 1 with its automatic and 3.25:1 rear axle managed 14.61 at 96 mph. A 351 beating a 429 by nearly a full second. The displacement disadvantage didn't matter. The Boss 351's combination of compression, cam, gearing, and a four-speed manual put the power to the ground more effectively than 78 extra cubic inches in the Cobra Jet could overcome.
The Boss 351 existed because the Boss 302 and Boss 429 were cancelled. Ford had slashed its racing budget from $12 million to $2 million for 1970 and pulled out of motorsports entirely in November of that year. Kar-Kraft was shut down. There was no longer any need to homologate a 5.0-liter engine for Trans-Am or a hemi for NASCAR. The decision to carry the Boss name forward using the new Cleveland V8 came in the spring of 1970, development didn't begin until June, and the car didn't arrive at dealers until the first week of November.
Only 1,806 were built. The short model year, the dying muscle car market, and the fact that the Boss 351 arrived months after the rest of the 1971 Mustang lineup all contributed to the low number. The HO 351 Cleveland continued in detuned form with lower compression, but the Boss 351 as a model was discontinued after one year. The Boss name wouldn't return to the Mustang until 2012.
The 351 Cleveland HO in Boss trim was Ford's last great naturally aspirated performance V8 of the muscle car era. Four-bolt mains, solid lifters, 11:1 compression, a cam that idled like it had somewhere to be, and quarter-mile times that embarrassed engines with a hundred more cubic inches. One year. 1,806 cars. And an engine that proved displacement isn't everything.
