1971 Ford Mustang Boss 351 Specs

1971 Ford Mustang Boss 351 Specs

Just 1,806 were built in 1971, their lone year of production, and they remain among the hottest performers in the world of small-block Fords. The Boss 351's main claim to fame was its R-code V-8, whose 330hp at 5,400 rpm and 370-lbs.ft. of torque at 4,000 rpm would outgun a number of big-blocks. But the engine was just the beginning of the Boss's goodness. The 351 was bolted up to a Hurst-shifted four-speed manual transmission, with a Traction-Lok 9-inch rear end equipped with 3.91:1 gears. All came with the beefed-up Competition Suspension package and power front disc brakes. Externally, they were marked by their twin, functional "NASA" hood ducts, Mach 1 honeycomb grille, lower body paint treatment, bodyside stripes and 351 decals. The original Boss Mustangs, the 302 and 429, were introduced in 1969, developed for Ford by Kar Kraft Engineering of Brighton, Michigan. The Boss 302 duked it out with Chevrolet's hot Z/28 Camaro in Trans-Am racing, while its big brother made it possible for Ford to legalize the big-block engine for NASCAR racing. By 1970, the handwriting was on the wall for the muscle-car era, as rising insurance premiums and ever-tighter safety and emissions requirements took effect.

1,000 above a base Mustang SportsRoof. The critics loved the new Boss. Hot Rod in its February, 1971 issue. Super Stock and Drag Illustrated in its March, 1971 issue. At first glance, the Boss 351 bears a resemblance to its stablemate, the Mach 1, but with "Boss 351 Mustang" decals on the front fenders and tail and a larger blacked-out area on the hood. The main way to determine whether the car you're looking at is a genuine, bona-fide Mustang is to look at the VIN: the fifth character should be an "R," designating the high-output Boss 351 engine. At the heart of the Boss's heavily massaged 351 Cleveland was a crankshaft cast of high nodular iron, specially tested for hardness, and held in place by four-bolt mains. Connecting rods were forged, shot-peened for strength and magnafluxed for cracks, and held to the crank by exceptionally strong 3/8-inch bolts. At the other end of the connecting rods were eight pop-up, forged aluminum pistons with three rings apiece. The free-flowing 4V heads were nearly identical to those used on the earlier Boss 302, except for their revised cooling passages.

Generous, smoothly contoured intake passages led to the large, canted valves--2.19 inches for the intakes, 1.71 inches for the exhausts--helping the engine breathe freely, allowing "excellent performance potential," as the brochure said. The polyangular combustion chambers created a relatively high compression ratio of a high 11.7:1. Hardened and ground pushrods, solid lifters and special valve springs and keepers beefed up the valvetrain. Like other members of the Cleveland family, the HO 351 had a bore and stroke of 4.00 inches x 3.50 inches. A cast aluminum over/under intake manifold was capped with a 4-bbl. Autolite model 4300-D carburetor with a spread bore pattern with small primary bores and oversized secondaries. These carburetors were built just for the Boss 351 engine, and are "extremely rare," according to Prochot. Carol Jennings, business manager for Pony Carburetors, a Las Cruces, New Mexico supplier of carburetors, carburetor parts and restoration services. The 4300-D draws breath through the Dual Ram Induction hood with its vacuum-actuated ducts.

A dual-point, dual-diaphragm distributor fired the Autolite ARF 42 plugs, and cast, finned aluminum covers with special Boss 351 insignias sat over the rocker arms. Ford advised that buyers who wanted an oil cooler could adapt the oil cooler from a 1970 Boss 302; none were installed on the HO 351 at the factory. The Boss 351 was equipped with a four-speed Toploader with a floor-mounted, T-handle Hurst shifter. You could have any differential you wanted on your Boss 351, so long as it was a Traction-Lok 9-inch rear end equipped with 3.91:1 gears. No other ratios were available. Great for zero-60 times, the low gearing made for a busy 3,200 rpm at highway speeds of 55 mph. Ford differentials are identified by a code stamped to a tag attached to the front differential cover, with the top line revealing the differential's identification number and the lower line stating the gear ratio.