NHRA - National Hot Rod Association

4/29: It’s Boss 429 day—Celebrate Ford's big-bore semi-hemi engine

It's the engine Mickey Thompson used in his Mach 1 Mustang Funny Cars, the one "Ohio" George Montgomery twin-turbocharged, and the one Bob Glidden covered with his fire jacket after he rolled his Pro Stock car in 1986—the Boss 429.
29 Apr 2022
David Kennedy

The Boss 429 engine was designed to beat Chevy and Dodge and put Fords in the winner circle, but to make them legal for NASCAR,  Dearborn had to offer them in street cars. With Pontiac and Chevrolet’s former chief, Semon “Bunkie” Knudsen, at the helm of Ford Motor Company, the choice was made to put the Boss 429  in ’69 and ’70 Mustangs—even though that car would never race in NASCAR. To keep the Mustangs livable on the street, the Boss 429 was configured to make a rather disappointing 375 hp, while the NASCAR race engine was thought to put out closer to 550 hp.

The Boss 429 engine featured a semi-hemispherical combustion chamber head fit over a massive 4.900-inch bore-spacing block that allowed for huge pistons and plenty of room for valves. It shared its 4.36-inch bore with the 460, but used a shorter 3.59-inch stroke.

Production of the Boss 429 Mustang began in 1968 at Kar Kraft’s assembly plant in Brighton, Michigan. The cars were transported there from Ford’s Rouge plant to be fit with the engines on a small-scale assembly line.

Drag racing fans saw the engine used in all sorts of applications. Mickey Thompson phased them into his Mustang Funny Cars, and the Boss 429 is the engine that he experimented with his compressed-air supercharger.

The Boss 429 was also the engine that “Ohio” George Montgomery fit with twin turbocharges and reportedly made more than 1,000 hp. But perhaps most famously, the Boss 429 was the engine architecture that Bob Glidden dominated Pro Stock with. It’s the engine he innovated with so much, that when his car suffered a brutal rollover in 1986 at Atlanta, he climbed out of the car and covered his hand-made dual-quad intake with his fire jacket and helmet.