Jerry Heasley
December 12, 2011

You could describe this pair of SportsRoofs as the top in the '71 Mustang class. One is a Boss 351 and the other a 429 Cobra Jet Mach 1, both in factory Light Pewter Metallic paint.

Kelly Schultz did not pick these cars by plan. At any one time he is liable to have 10 to 20 classic Mustangs in his collection. He tends to lean toward high-performance Bosses and Mach 1s, and by chance ended up with this dynamic duo.

Almost the minute I started snapping photos, Kelly and I started playing the "same" game by pointing out the similarities between the two '71 Mustangs. The identical color is obvious. Both have ram-air and four-speeds with Hurst shifters. Both are equipped with Traction-Lok differentials, 9-inch rear ends, and Magnum 500 wheels. Both have black tape stripes and blackout hoods.

"The Boss has more black," Kelly said of the hood. He also noted how the Mach 1 came with a black honeycomb rear panel, while Ford painted the panel on the Boss 351. These Mustangs also share the same rear spoiler.

On each car, Ford centered a Mustang tri-color running horse in the black ABS plastic honeycomb grille with amber driving lights. Ford catalogued them as "Sportlamps." Because the front spoilers are also similar, the two front ends are eerily close in appearance. The easy-to-spot difference is the Boss 351's chrome bumper compared to the body-color Polyurethane plastic over the Mach 1 bumper.

The functional ram-air induction on both cars uses a pair of "NASA" (not NACA) scoops to collect and channel air to the air cleaner via an assembly on the underside of the hood. However, ram-air was an option on the 429 Cobra Jet; it was standard equipment on the Boss 351. The two hoods also share another feature--hood lock buttons, which were new for '71.

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The Magnum 500s measure 15x7 inches. For driving purposes, Kelly mounted modern B.F. Goodrich radial tires on both cars.

I wondered about the rear end gears. Kelly explained that every '71 Boss 351 came with a 3.91:1 Traction-Lok differential. Although the 429 CJ utilized the same 9-inch differential, the gear set in this particular car is a highway cruising 3.25:1. With the extra torque of the big-block, the 429 didn't need deeper gears, but with the 3.91s the Boss 351 has the reputation for quicker 0-60 and quarter-mile times.

The Boss and CJ also share the optional Instrumentation Group. This cool extra features oil, amp, and temperature gauges in the center of the dash and an 8,000-rpm tachometer to the left of the speedometer.

Just as I was running out of "sames," Kelly pointed out the power brakes with front discs. There are also rev limiters, both set to restrict engine rpm at 6,150 rpm.

Inside, I noticed both cars came with deluxe interior and three-spoke steering wheels. However, the Mach 1 bucket seats were a little sportier, highlighted by vertical stripes on each side.

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Performance was the remaining issue. Could the Boss 351 outrun the 429 Cobra Jet Mach 1? The Boss 351 has the better gears (3.91 versus 3.25), but the 429 CJ has 450 lb-ft of torque, 80 more than the Boss 351 at the same 3,400 rpm. Horsepower numbers are a little closer. The Boss 351 was factory rated at 330 horsepower, 40 shy of the 429 CJ.

The 429 CJ looks faster on paper, but what are the numbers? When new, the Boss 351 jumped from a standing start to 60 mph in 5.7 seconds and ran the quarter-mile in 13.6 seconds. In a Motor Trend test from 1971, the bigger cube 429 CJ accelerated from 0 to 60 in 6.4 seconds and ran the quarter-mile in 14.7 seconds. However, in this test the Mach 1 used a Cruise-O-Matic, not a four-speed, so the question remains unsettled.

Generally speaking, enthusiasts give the Boss 351 the nod as the faster of the two. At some time, we'd like to pit the cars against each other on the quarter-mile. The one caveat is for the 429 CJ (or SCJ) to have a set of 3.91:1 gears, same as the Boss 351.

One last similarity--both cars are rare. Ford built 1,806 Boss 351s, all SportsRoofs. For the 429 Cobra Jet, production was spread across all three body styles. In the SportsRoof body style, the number is 1,737 for both Cobra Jet and Super Cobra Jet, nearly identical to the Boss 351.

Too bad the number wasn't the same.

Challenge Issued
Which car is faster, the 429 Cobra Jet or the 351 Boss? I believe, contrary to old road tests, that the 429 CJ will beat the Boss 351 in both 0-60 and the quarter-mile.

My reasoning is simple. The 429 CJ has more torque and horsepower. I don't believe the 429 Cobra Jet's weight disadvantage is enough to negate this car's power advantage. The Boss 351's faster claim is due to the difference in rear axle gears in most of the magazine test cars. Every Boss 351 came with 3.91:1 gears. For a fair test, we need to pit a Boss 351 against a '71 Mach 1 fastback with the 429 Cobra Jet and 3.91:1 gears.

If anyone is up to the challenge, send an email to jerry@jerryheasley.com. Maybe we can match a Boss 351 against a 429 CJ.