Ford Boss 351 Mustang

Boss 351 at a glance

The Boss 351 belongs to the first-generation Ford Mustang, in the larger body introduced for 1971.

One Boss instead of two

For 1969 and 1970 Ford had two Boss Mustangs with very different jobs. The Boss 302, styled by Larry Shinoda, homologated a small-block V8 for the SCCA Trans-Am series; it was rated at 216 kW (290 hp), and 7,013 were sold in 1970. The Boss 429 existed because NASCAR required at least 500 road cars with Ford's new semi-hemi 429 V8; Ford built 859 in 1969 and 499 in 1970.

For 1971 Ford replaced both cars with one model. The new Mustang, approved by company president Semon "Bunkie" Knudsen and introduced in September 1970, was 3 inches (76 mm) wider so that the 429 big block fitted without a major suspension redesign. Its engine was the 351 Cleveland. The link to the Boss 302 was direct: according to Wikipedia, the Boss 302's cylinder heads had been designed for the planned 351 Cleveland, which arrived a year later.

Engine: the R-code 351 Cleveland

The Boss engine topped the 351 range of 1971. Wikipedia lists the alternatives (SAE gross):

Hagerty describes the Boss 351 engine as a special build of the Cleveland:

The two main sources give the same peak figures but different engine speeds: Wikipedia lists 246 kW (330 hp) at 5,400 rpm and 502 Nm at 4,000 rpm, Hagerty 5,800 rpm and 3,400 rpm. Both are SAE gross ratings, the last year before American makers switched to net figures. Supercars.net describes the R-code engine with Ram Air induction.

Design, size and chassis

Like every 1971 Mustang, the Boss 351 used the body that had grown 76 mm in width over the 1970 car. Wikipedia describes it as styled like the 1971 Mach 1, with a larger blacked-out bonnet than the Mach 1, front and rear spoilers, dual exhausts and chrome bumpers with the sport-lamp grille.

The cars documented by supercars.net and ClassicCars.com share the same running gear:

• a Hurst-shifted Top Loader four-speed manual;
• a 9-inch Traction-Lok rear axle with 3.91:1 gears;
• the Competition Suspension;
• power front disc brakes;
• 15-inch Magnum 500 wheels on the ClassicCars.com example.

Performance and acceleration

The road tests Hagerty cites recorded 5.8 seconds to 60 mph and quarter-mile runs down to 13.8 seconds. Hagerty adds that the 429 Super Cobra Jet Mustang of the same year was quicker, while the Boss 351 was lighter and better balanced.

Boss 351 in numbers

1CARS calculated the specific output of the three Boss engines from their rated power and displacement:

On these figures the Boss 351 sits in the middle: almost the specific output of the high-revving 302 with close to the torque per litre of the 429. Its 246 kW also made it the most powerful small-block Mustang of 1971 in the catalog; only the 429 big-block versions were rated higher. The comparison tool sets it against the Boss 302 and Mach 1 side by side.

The end of the Boss

Wikipedia states that tightening emission regulations ended both the Boss 351 and the optional 429 big-block engine after 1971. The 1972 Mustang Mach 1 could be ordered with a 351 HO rated at 205 kW (275 hp) in the new SAE net system. The Boss name returned in 2012 on the Boss 302, rated at 331 kW (450 PS).

Boss 351 buying guide

• Documents: check the R-code in the VIN and a Marti Report; the cars described by supercars.net and ClassicCars.com come with build sheets or a Marti Report.
• Survival: supercars.net notes that fewer than 600 cars are listed in the Boss 351 Registry.
• Drivetrain: every car should have the four-speed manual; an automatic Boss 351 is not original.
• Market references: a car sold at Mecum's Monterey auction in 2011 for US$45,000, and another failed to sell at US$47,000 at Kissimmee in 2014, according to supercars.net.