The Mustang Boss 429 is one of the most unique Mustangs made. Its uniqueness lies in its rarity, its engine and simply for the fact that so much effort went into modifying the basic Mustang to make the Boss 429 engine fit.
In 1969, muscle car fans, and specially Ford fans, thought that the 1969 Ford Mustang Boss 429 would be the Mustang to finally rival the best of the Corvettes. They were disappointed when it wasn’t. But Ford never intended the Boss 429 as a street dominator, or as any kind of drag-racing threat.
The Boss 429 was a hand-built muscle car intended solely to satisfy the homologating rules of NASCAR. The coupe was longer than previous models and sported convex rather than concave side "lines". Ford also introduced a luxury Grande model equipped with interior wood paneling, a quartz clock, and a 351 cid Windsor engine.
Only available from 1969 and 1970, the Boss 429 came standard with a Mustang SportsRoof (the new corporate name for the fastback) and the new Mach 1 muscle car version’s deluxe interior. It sported none of the garish decals and paint schemes of the day; only a hood scoop and 15 in (380 mm) "Magnum 500" wheels fitted with Goodyear "Polyglas" tires, with a small "BOSS 429" decal on each front fender.
Holding a big block with a huge bore and hemispherical combustion chambers, the motor had staggering potential for power. However, the brainchild of this car, the late Larry Shinoda, was disappointed with the finished product. He was quoted as saying that he wanted a 10-second capable car in factory form. For several reasons, the actual production Boss 429 certainly wasn’t capable of such times. The rev limiter, a small carburetor (the Boss 302 Mustang had a larger one), restrictive intake manifold, a mild solid lifter cam, and restrictive exhaust corked up the motor and kept it from revving.
Furthermore, all of the smog equipment choked it down. The finished product was still strong, rated at 375 horsepower at 5200 RPM, but the powerband was narrow for an engine of this size, a result of the restrictions. Stoplight drag racing was prevalent in the day, and owners of these Mustangs, as well as other cars such as Chrysler’s street Hemi, could be surprised by "lesser" cars of the day that produced broader powerbands and more low-rpm torque. 100+ horsepower can easily be added with the right cam/intake/carb/exhaust selection, along with a broader powerband. While power steering was a "mandatory option" on the Boss 429, neither an automatic transmission nor air conditioning was available. In the case of the latter, there simply wasn’t enough room under the hood.
The 429 engine was unlike any other Ford motor, being much wider in the cylinder head, thanks to its semi-Hemi combustion chamber design, and this meant strut towers needed widening and the battery relocating to the trunk. The Boss used the ’Top Loader’ close-ratio, four-speed manual because the autos couldn’t handle the torque. The suspension had uprated springs and shocks plus an anti-roll bar.
Each Boss 429 Mustang came with a KK sticker placed on the inside of the driver’s door above the Ford Warranty Plate which signified Kar Kraft’s production number. The first Boss 429 was numbered "KK NASCAR 1201" while the last 1969 is numbered 2059. Some Boss 429s may have this silver tape stripe missing; a small brass plate was substituted by Kar Kraft on a small number of cars.
1969 Boss 429 Mustangs were available in five colors: Wimbledon White, Royal Maroon, Raven Black, Black Jade, and Candy Apple Red. 1970 versions were painted Grabber Blue, Grabber Green, Grabber Orange, Calypso Coral and Pastel Blue. All 1970 Boss 429s came with a gloss black painted hood scoop.