The 1971 Boss 351 Mustang

In 1969, the Boss 302 and Boss 429 debuted. These two iconic vehicles, now associated forever with ‘60s muscle cars and “fast cars” in general, were the design of Larry Shinoda, who also accidentally coined their name when he referred to the projects he was working on as simply “the Boss’ car.”

The Boss 302 and 429 were manufactured between 1969 and 1970, when they were both retired and a new, one year only, Boss was launched for 1971 Mustang: The Boss 351.

Boss 302, 429, and Boss 351

Comparing the Boss 302, 429, and 351

Naturally, given that the Boss 351 shares its name with two well-known and loved cars, it’s often compared to the Boss 302 and Boss 429 despite being very much its own beast.

The Boss 302 was designed to be a perfectly balanced road car. With neat, precise turns, it’s a sports car in terms of handling. 8,641 Boss 302s were made between 1969 and 1970. The Boss 429 was a little bit wilder. Designed to be an acceleration machine, it revs great in a straight line. Only 1,356 Boss 429s were produced, and they’re considered one of the most desirable and collectible vehicles from this time period.

The Boss 351 in many ways is a perfect blend of these the 302 and 429s strengths. Only slightly less rare than the Boss 429, only 1,806 Boss 351s were produced, and they were all produced in 1971.

Like the Boss 302 and 429, the Boss 351s name comes from the displacement of its engine. While the 302 is a small block and the 429 is a big block, the 351 falls right in between the two, offering more displacement but with the engine block size and weight of a small block.

Boss 351: Appearance

Of the three Boss Mustangs, the Boss 351 definitely borrowed a lot of its external styling from the Mach 1. Unfortunately, a lot of the styling did turn out to be a little impractical. The reviewers at the time were very critical of the poor outward visibility. Some claimed that this made the car feel big and bulky, even though it still handled like a drag car.

The Ram Air hood scoop and spoiler gave it an appearance that screamed performance, and the 351, like its predecessors, performed beautifully.

Boss 351s came in a variety of colors, but the rarest two production colors are Grabber Lime and Medium Yellow Gold. The rarest color is Calypso Coral, which was a very rare special paint with at present only four confirmed. Only 67 Boss 351s were made in either color. Based on Marti Reports and proud “B1” owners, we can roughly establish that these are the production numbers for all the paint colors produced.

• Raven Black: 131
• Light Pewter Metallic: 297
• White: 120
• Bright Red: 186
• Bright Blue Metallic: 246
• Dark Green Metallic: 136
• Medium Yellow Gold: 67
• Grabber Blue: 131
• Grabber Lime: 67
• Grabber Yellow: 344
• Grabber Green Metallic: 77
• Calypso Coral: 4

Boss 351: Performance

Understanding the performance differences between the three Boss Mustangs comes down to understanding the differences between big and small block engines. Though the Boss 351 had more displacement and compression than the 302, it was still a small block engine, unlike the substantially larger 429.

The 351-cid engine the Boss 351 was named for was manufactured in Cleveland, and it was as different from the Boss 302’s Windsor as it could be. The Boss 302 had been built using a Windsor 302-cid engine, but then Cleveland heads had been added to the block due to their larger ports and valves. The additional space improved the engine’s breathing.

For the Boss 351, the entire Cleveland engine was used instead of simply the heads. The stroke difference between the two engines amounted to half an inch, increasing the Boss 351’s compression ratio in relation to the Boss 302’s.

In order to improve the performance of the Boss 351, it was also upgraded in several other ways, including upgrading the pistons to forged aluminum and adding four-bolt mains to the block.

The performance upgrades made to all three Boss Mustangs didn’t stop with their distinctive looks and incredible engine improvements. The Boss transmission was the first fully synchronized four-speed manual transmission that was available on a Mustang. All Boss Mustangs have this transmission, though it initially came with two optional gearings, one with a close-ratio and the other with a wide.

The year the Boss 351 was manufactured the only way to get the four-speed manual was to purchase one of the special edition Mustangs. All other Mustangs with a manual transmission were limited to three speeds. Additionally, the Boss 351 had a high-quality Hurst shifter.

To complete the package, the Boss 351 was then equipped with a competition-grade suspension and package and power front disc brakes. The end result was a car that could go from 0-60 in a mere 5.8 seconds, all with better handling and braking than either of the other two Boss Mustangs.

Last of a Generation

Many consider the Boss 351 to be one of the top muscle cars of all time, a title that has a lot of competition. It has a high rev ceiling and a lot of torque, combining the best features of big block and small block engines, in a package that’s relatively lightweight and handled better than either of the previous Boss Mustangs. It’s a car that’s designed for performance, and it came at the end of an era. The next several years would see the advent of the Mustang II, a lightweight Mustang with very little in the way of horsepower as drivers became more concerned with fuel economy rather than performance.

Image Credit: Ford | Boss 351| Valvoline