The 1971 Boss 351 was one of the quickest Mustangs produced in its day, but it remained in the lineup for only a single year.

While the 1971 Mustangs were introduced to the press on August 20, 1970, the Boss 351 model did not arrive until the first week in November. Development hadn’t begun until June, and that was due, evidently, to a decision that spring to cancel the Boss 302 Mustang. The automaker had slashed its racing budget from $12 million to $2 million for 1970, and in November Ford announced it was pulling out of motorsports altogether. Kar Kraft, Ford’s private racing contractor, was shut down and the locks on the doors were changed.

What all that corporate business meant, among other things, was that Ford was no longer involved in the SCCA Trans-Am series, so there was no longer any need to homologate a 5.0-liter Mustang for competition. With the Boss 302 Mustang cancelled (and the Boss 429 for NASCAR as well), a decision was made to carry forward the Boss brand using the new 351 CID Cleveland V8, introduced that same year. So now the Boss 302 and Boss 429 Mustangs were out for ’71, and the Boss 351 Mustang was in—even if it was a bit late.

Like the Boss 302 and Ford 385-series engines (or the Chevy Mark IV V8, you’re thinking), the Cleveland V8 (335 series) used a canted-valve cylinder-head design, with the 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, among its advantages. From there, the HO (High Output) 351C V8 in the Boss 351 departed from the standard Cleveland V8 in a number of ways, receiving special attention.

The upgrades included four-bolt main caps in the cylinder block, magnafluxed crankshaft and rods, uprated valvetrain pieces, and a racy solid-lifter camshaft with nearly .550-in lift and 250 degrees of duration. The compression ratio was an equally sporty 11.00:1 (some sources say 11.7:1). An Autolite 4300 spread-bore carb with a manual choke was fed fresh air by a standard hood induction system. This aggressive combination produced factory ratings of 330 hp at 5,400 rpm and 370 lb-ft of torque. Meanwhile, a standard four-speed transmission and 3.91:1 rear axle ratio provided drivers with easy access to the meat of the output curves.

For a middle-displacement V8 in those days, the result was rather stunning. At Car and Driver, Patrick Bedard wrote that the new Boss was “quicker than a number of super cars with an extra 100 inches of displacement.” The magazine recorded quarter-mile times of around 14 seconds flat at 100 mph, as did Fred Freel for High Performance Cars. 

In a January ’71 comparison test, Motor Trend found that the Boss 351 was far quicker in the quarter mile than the 429 Cobra Jet-powered Mustang Mach 1: 13.80 seconds at 104 mph versus 14.61 at 96 mph. We can mark  that down in part to the Mach 1’s 3.25:1 final drive and automatic transmission, but still—the Boss 351’s straight-line performance was an eye-opener. However, both the Boss and the Mach 1 used the same suspension package, which both Car and Driver and Motor Trend reported was not much to brag about.

Ford produced and sold only 1,806 Mustang Boss 351s in ’71, due no doubt to the short model year, and probably a number of other factors, too. By then the muscle car trend was wearing thin. The Motor Trend editors declared that of the three Mustangs they tested, their favorite was the base model hardtop with its 302 CID two-barrel V8. While the HO 351 V8 would continue in detuned form with a reduced compression ratio, the Mustang Boss 351 was discontinued after just one year. The Boss name would not return to the Mustang lineup until 2012.