1971 Ford Mustang Boss 351 Specs
Packing a high-output version of the 351-cu.in. Cleveland V-8 under its broad, tapered hood, the Ford Mustang Boss 351 represents one of the Blue Oval's last true muscle cars. 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. Mustang sales, which had once exceeded 600,000, had slumped to 190,727 in 1970. No one could have been too surprised when both Boss Mustangs were cancelled in 1970, in connection with Ford's withdrawal from factory-supported racing, but the blow was softened with the introduction of the Boss 351 in the fall of that year. It would be a last hurrah for out-and-out small-block performance, a last call for the true muscle car believers.
The Mustang body introduced in 1971 was larger and heavier than the previous year's, as Ford traded the athletic grace of its Falcon-based predecessor for an engine bay large enough to comfortably swallow the biggest of the company's big-block V-8s--something that had been a challenge with the earlier cars. Ford recognized, too, that its pony car buyers were maturing and starting families, and would appreciate the extra room that the larger body offered. The body style, which lasted through 1973 until the introduction of the Mustang II, was larger in every dimension, and turned off some of the earlier car's fans. "The 1971-1973 cars do not get the respect they deserve," argues Don Prochot, the owner of our feature car, and we have to agree. That the car has not yet become a favorite among collectors has a silver lining for its fans, though, because prices of all but the most sought-after models remain affordable.
The Boss 351 was the premier performance Mustang for 1971, stickering at $4,124, or a premium of more than $1,000 above a base Mustang SportsRoof. That big pile of money bought performance: Motor Trend recorded a quarter-mile time of 13.8 seconds, while Car and Driver reported that the car "offers dragstrip performance that most cars with 100 cubic inches more displacement will envy." In fact, Sports Car Graphic found that a 1971 Mach 1 equipped with a 429 Cobra Jet engine just nipped the Boss 351 in a race from zero to 60mph, 6.3 seconds to 6.6, despite its 78-cubic-inch advantage. Add in the fact that most critics considered the small-block car to be better balanced, and it becomes clear that the Boss was the car to beat.
The critics loved the new Boss. "The '71 Mustang is a long way from the refined Falcon-based Mustang we all gaped at in 1964, and for that, present Mustang owners can be happy," said Hot Rod in its February, 1971 issue. "This one feels like it it's taking on the size of a Torino (and in some ways, it is), but no matter what its shape or size may resemble, it sure looks like a racer. The Boss 351 is going to salt away a few Z/28s before its season is up." "In the atmosphere of ordinary driving, the Boss 351 was a real joy," agreed Super Stock and Drag Illustrated in its March, 1971 issue. "Its looks and absolutely unique idle were attention-getters like those of few other cars in recent memory."
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.
ENGINES
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. "If you can find one, it's probably going to cost you upwards of $3,000 for an unrestored one," said Carol Jennings, business manager for Pony Carburetors, a Las Cruces, New Mexico supplier of carburetors, carburetor parts and restoration services. "And you're probably going to be waiting a long time." 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.
TRANSMISSION
The Boss 351 was equipped with a four-speed Toploader with a floor-mounted, T-handle Hurst shifter. The transmission could be ordered with wide ratios--0.78:1 in first, 1.93:1 in second, 1.36:1 in third, and 1.00:1 in fourth--or close ratios--2.32:1, 1.69:1, 1.29:1 and 1.00:1. Wide ratio transmissions were stamped with the part number RUG-AV1, while their close-ratio cousins were stamped RUG-AW1 or RUB-BJ. These transmissions are generally regarded as bulletproof, and because they were shared with other Fords, parts are easy to find.
DIFFERENTIAL
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. In the case of the Boss 351, this second line should read 3L91--that's 391 for the ratio, with an "L" inserted for the Traction-Lok. These numbers will be followed by the date and plant codes.
SUSPENSION
The standard suspension for 1971 Mustangs consisted of independent, unequal-length control arms, coil springs, angle-poised ball joints, tubular shocks, a link-type anti-roll bar up front, and a live axle with variable-rate semi-elliptical leaf springs and tubular shocks out back. All Boss 351s were equipped with Ford's Competition Suspension, which consisted of heavier-duty springs, shock absorbers and front and rear anti-roll bars, and staggered shock absorbers in the rear. Any car supplied with the chrome Magnum 500 wheels also required the Competition Suspension. Magnum 500s were optional on the Boss 351, with the standard wheel being a 15x7 stamped steel wheel with hubcaps and trim rings.
BRAKES
All Mustangs shared a dual hydraulic system with a dual master cylinder. While the basic setup for most models was a drum at each wheel and no power assist, the Boss 351 featured 11.3-inch discs up front with floating calipers, 10x2-inch drums in the back, and vacuum assist. The drums were a quarter-inch wider than those fitted to basic cars, and were shared with competition versions of the Torino, the Fairlane and the Mercury Cougar. Brake parts were shared with many models, and are easy to locate.
BODY
The Mustang is a unit-bodied car, and you won't be surprised to learn that it can rust. Spots of particular concern include the cowl area under the windshield, and the front frame rails. Both can be difficult and expensive to repair, as replacement panels are not made. Doors, trunk floors, rear frame rails and rocker panels can rust, too, but at least repair panels can be found.
The Boss 351, although it resembled the more common Mach 1, is differentiated by a number of details. Boss 351 Mustang callouts mark the lower front fenders and the rear panel, and most of the hood is painted either black or argent, depending on the body paint color; the Mach 1 has a broad stripe down the center. The lower-body paint accents and accent tape stripes were standard equipment, again done in either argent or black.
Up front was a black chin spoiler and honeycomb grille with foglamps, identical to those used on the Mach 1. Unlike the Mach 1, the standard front bumper was chrome, although it could be ordered in body color as well. The rear deck spoiler was a popular option, and came stashed inside the car for dealer installation. Dual racing mirrors in body color were standard equipment.
INTERIORS
The Boss 351 featured high-back bucket seats and a central console, and the optional Instrumentation Group of oil pressure, ampere and coolant temperature gauges mounted in a panel above the radio. Upholstery choices included the standard vinyl, the Deluxe interior with Comfort-Weave woven inserts, vinyl with cloth inserts and the coveted Mach 1 sports interior, as shown on our feature car. Color choices included black, white, ginger, green, red and blue. According to the Boss 351 Registry, about 7 out of 10 Boss 351s had black interiors. Our feature car is one of just 23 Mustangs built in 1971 with the ginger-colored cloth and vinyl upholstery.
The Mustang's extensive options sheet was open to Boss 351 buyers. Power steering ($115), a full-length console ($60), Select-Aire air conditioning ($368), Tilt-Away steering wheels ($103), AM/8-track stereos ($129), new-for-1971 power windows ($127), tinted glass ($40) and an AM radio ($66) were popular choices.
Generally, replacement upholstery and carpeting can be found from a number of sources. One exception is the Comfort-Weave upholstery in white; while the original had small flecks of black in it, the reproduction material is solid white.
RESTORATION PARTS
Recently, the availability of parts for the Boss 351, and for all 1971-1973 Mustangs, has been on the upswing, because enthusiasts have begun to discover the car, said Fred Glazier of Glazier's Mustang Barn in Souderton, Pennsylvania. The major exception is parts for the high-performance engine, particularly the Autolite 4300-D carburetor. For just under $600, Pony Carburetors offers a "clone" 4300-D, adapting a carburetor from a 351 Cleveland engine; though the company claims it functions just like the original, its larger fuel inlet gives it away to those in the know.
Within the past three to four years, a number of reproduction parts have become available, including bumpers, some body parts, hoods, grilles and rear quarter panels. Ram Air components are being made, too. More than 400,000 Mustangs were produced from 1971 through 1973, and parts suppliers are only too happy to cater to the car's huge fan club. These are relatively straightforward cars, and, with the exception of rust repair, are not overly tricky to fix.
PERFORMANCE PARTS
Weren't you paying attention when we told you that this was one of the hottest small-blocks Ford ever made? Seriously, though any number of modifications can be carried out, we'd guess that most Boss 351 owners will want to keep their cars in factory condition. (If we really wanted to experiment, we'd pick up a base SportsRoof car for a lot less money.) Back in 1971, Ford had a number of suggestions for buyers who wanted to go racing, everything from beefier springs and rocker arms to bigger carburetors and manifolds. Performance magazines suggested replacing the Autolite with a Holley 750 double-pumper and an Edelbrock "Torker" intake manifold, you could coax 400hp out of the engine. Because the stock exhaust manifolds were somewhat restricted, replacing them with headers could squeeze out another 10 to 15 horsepower. We should also mention that replacing the front bumper with a $150 fiberglass unit will shave off 56 performance-sapping pounds.
CHASSIS UPGRADES
This is another area where we think most folks will want to stick with stock. There is a lot that can be done, though, with bigger roll bars, polyurethane suspension bushings, shock tower braces and subframe connectors. It's also possible to swap out the rear drums for discs, using parts from a Lincoln Versailles or a 1984-'86 Mustang SVO. We wouldn't do it, though, and we bet you wouldn't, either.
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. Mustang sales, which had once exceeded 600,000, had slumped to 190,727 in 1970. No one could have been too surprised when both Boss Mustangs were cancelled in 1970, in connection with Ford's withdrawal from factory-supported racing, but the blow was softened with the introduction of the Boss 351 in the fall of that year. It would be a last hurrah for out-and-out small-block performance, a last call for the true muscle car believers.
The Mustang body introduced in 1971 was larger and heavier than the previous year's, as Ford traded the athletic grace of its Falcon-based predecessor for an engine bay large enough to comfortably swallow the biggest of the company's big-block V-8s--something that had been a challenge with the earlier cars. Ford recognized, too, that its pony car buyers were maturing and starting families, and would appreciate the extra room that the larger body offered. The body style, which lasted through 1973 until the introduction of the Mustang II, was larger in every dimension, and turned off some of the earlier car's fans. "The 1971-1973 cars do not get the respect they deserve," argues Don Prochot, the owner of our feature car, and we have to agree. That the car has not yet become a favorite among collectors has a silver lining for its fans, though, because prices of all but the most sought-after models remain affordable.
The Boss 351 was the premier performance Mustang for 1971, stickering at $4,124, or a premium of more than $1,000 above a base Mustang SportsRoof. That big pile of money bought performance: Motor Trend recorded a quarter-mile time of 13.8 seconds, while Car and Driver reported that the car "offers dragstrip performance that most cars with 100 cubic inches more displacement will envy." In fact, Sports Car Graphic found that a 1971 Mach 1 equipped with a 429 Cobra Jet engine just nipped the Boss 351 in a race from zero to 60mph, 6.3 seconds to 6.6, despite its 78-cubic-inch advantage. Add in the fact that most critics considered the small-block car to be better balanced, and it becomes clear that the Boss was the car to beat.
The critics loved the new Boss. "The '71 Mustang is a long way from the refined Falcon-based Mustang we all gaped at in 1964, and for that, present Mustang owners can be happy," said Hot Rod in its February, 1971 issue. "This one feels like it it's taking on the size of a Torino (and in some ways, it is), but no matter what its shape or size may resemble, it sure looks like a racer. The Boss 351 is going to salt away a few Z/28s before its season is up." "In the atmosphere of ordinary driving, the Boss 351 was a real joy," agreed Super Stock and Drag Illustrated in its March, 1971 issue. "Its looks and absolutely unique idle were attention-getters like those of few other cars in recent memory."
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.
ENGINES
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. "If you can find one, it's probably going to cost you upwards of $3,000 for an unrestored one," said Carol Jennings, business manager for Pony Carburetors, a Las Cruces, New Mexico supplier of carburetors, carburetor parts and restoration services. "And you're probably going to be waiting a long time." 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.
TRANSMISSION
The Boss 351 was equipped with a four-speed Toploader with a floor-mounted, T-handle Hurst shifter. The transmission could be ordered with wide ratios--0.78:1 in first, 1.93:1 in second, 1.36:1 in third, and 1.00:1 in fourth--or close ratios--2.32:1, 1.69:1, 1.29:1 and 1.00:1. Wide ratio transmissions were stamped with the part number RUG-AV1, while their close-ratio cousins were stamped RUG-AW1 or RUB-BJ. These transmissions are generally regarded as bulletproof, and because they were shared with other Fords, parts are easy to find.
DIFFERENTIAL
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. In the case of the Boss 351, this second line should read 3L91--that's 391 for the ratio, with an "L" inserted for the Traction-Lok. These numbers will be followed by the date and plant codes.
SUSPENSION
The standard suspension for 1971 Mustangs consisted of independent, unequal-length control arms, coil springs, angle-poised ball joints, tubular shocks, a link-type anti-roll bar up front, and a live axle with variable-rate semi-elliptical leaf springs and tubular shocks out back. All Boss 351s were equipped with Ford's Competition Suspension, which consisted of heavier-duty springs, shock absorbers and front and rear anti-roll bars, and staggered shock absorbers in the rear. Any car supplied with the chrome Magnum 500 wheels also required the Competition Suspension. Magnum 500s were optional on the Boss 351, with the standard wheel being a 15x7 stamped steel wheel with hubcaps and trim rings.
BRAKES
All Mustangs shared a dual hydraulic system with a dual master cylinder. While the basic setup for most models was a drum at each wheel and no power assist, the Boss 351 featured 11.3-inch discs up front with floating calipers, 10x2-inch drums in the back, and vacuum assist. The drums were a quarter-inch wider than those fitted to basic cars, and were shared with competition versions of the Torino, the Fairlane and the Mercury Cougar. Brake parts were shared with many models, and are easy to locate.
BODY
The Mustang is a unit-bodied car, and you won't be surprised to learn that it can rust. Spots of particular concern include the cowl area under the windshield, and the front frame rails. Both can be difficult and expensive to repair, as replacement panels are not made. Doors, trunk floors, rear frame rails and rocker panels can rust, too, but at least repair panels can be found.
The Boss 351, although it resembled the more common Mach 1, is differentiated by a number of details. Boss 351 Mustang callouts mark the lower front fenders and the rear panel, and most of the hood is painted either black or argent, depending on the body paint color; the Mach 1 has a broad stripe down the center. The lower-body paint accents and accent tape stripes were standard equipment, again done in either argent or black.
Up front was a black chin spoiler and honeycomb grille with foglamps, identical to those used on the Mach 1. Unlike the Mach 1, the standard front bumper was chrome, although it could be ordered in body color as well. The rear deck spoiler was a popular option, and came stashed inside the car for dealer installation. Dual racing mirrors in body color were standard equipment.
INTERIORS
The Boss 351 featured high-back bucket seats and a central console, and the optional Instrumentation Group of oil pressure, ampere and coolant temperature gauges mounted in a panel above the radio. Upholstery choices included the standard vinyl, the Deluxe interior with Comfort-Weave woven inserts, vinyl with cloth inserts and the coveted Mach 1 sports interior, as shown on our feature car. Color choices included black, white, ginger, green, red and blue. According to the Boss 351 Registry, about 7 out of 10 Boss 351s had black interiors. Our feature car is one of just 23 Mustangs built in 1971 with the ginger-colored cloth and vinyl upholstery.
The Mustang's extensive options sheet was open to Boss 351 buyers. Power steering ($115), a full-length console ($60), Select-Aire air conditioning ($368), Tilt-Away steering wheels ($103), AM/8-track stereos ($129), new-for-1971 power windows ($127), tinted glass ($40) and an AM radio ($66) were popular choices.
Generally, replacement upholstery and carpeting can be found from a number of sources. One exception is the Comfort-Weave upholstery in white; while the original had small flecks of black in it, the reproduction material is solid white.
RESTORATION PARTS
Recently, the availability of parts for the Boss 351, and for all 1971-1973 Mustangs, has been on the upswing, because enthusiasts have begun to discover the car, said Fred Glazier of Glazier's Mustang Barn in Souderton, Pennsylvania. The major exception is parts for the high-performance engine, particularly the Autolite 4300-D carburetor. For just under $600, Pony Carburetors offers a "clone" 4300-D, adapting a carburetor from a 351 Cleveland engine; though the company claims it functions just like the original, its larger fuel inlet gives it away to those in the know.
Within the past three to four years, a number of reproduction parts have become available, including bumpers, some body parts, hoods, grilles and rear quarter panels. Ram Air components are being made, too. More than 400,000 Mustangs were produced from 1971 through 1973, and parts suppliers are only too happy to cater to the car's huge fan club. These are relatively straightforward cars, and, with the exception of rust repair, are not overly tricky to fix.
PERFORMANCE PARTS
Weren't you paying attention when we told you that this was one of the hottest small-blocks Ford ever made? Seriously, though any number of modifications can be carried out, we'd guess that most Boss 351 owners will want to keep their cars in factory condition. (If we really wanted to experiment, we'd pick up a base SportsRoof car for a lot less money.) Back in 1971, Ford had a number of suggestions for buyers who wanted to go racing, everything from beefier springs and rocker arms to bigger carburetors and manifolds. Performance magazines suggested replacing the Autolite with a Holley 750 double-pumper and an Edelbrock "Torker" intake manifold, you could coax 400hp out of the engine. Because the stock exhaust manifolds were somewhat restricted, replacing them with headers could squeeze out another 10 to 15 horsepower. We should also mention that replacing the front bumper with a $150 fiberglass unit will shave off 56 performance-sapping pounds.
CHASSIS UPGRADES
This is another area where we think most folks will want to stick with stock. There is a lot that can be done, though, with bigger roll bars, polyurethane suspension bushings, shock tower braces and subframe connectors. It's also possible to swap out the rear drums for discs, using parts from a Lincoln Versailles or a 1984-'86 Mustang SVO. We wouldn't do it, though, and we bet you wouldn't, either.
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