The Boss 429 Mustang wasn’t born from the desire to dominate the street, or even the drag strip, but simply to produce at least 500 cars with the new semi-hemi engine to qualify for NASCAR use.

The engine is instantly recognizable, with large cylinder heads featuring semi-hemispherical combustion chambers. Four bolt mains, along with forged-steel crankshaft and connecting rods, were selected for durability. Ford rated this engine at just 375 horsepower, likely due to insurance and liability issues, but most experts estimate it at well over 500.

Fitting the mammoth Boss 429 inside the relatively small confines of the Mustang’s engine compartment entailed modifying the shock towers and inner fenders, relocating the suspension mounts, and placing the battery in the trunk. Ordering a Boss 9 also meant that you would receive a Toploader four-speed manual transmission and 3.91:1 Traction-Lok rear end. Air conditioning and automatic transmission were not available.

The exterior of this Mustang was quite understated compared to other rides of the era. Devoid of wild stripes or graphics, only the large hood scoop and small ‘Boss 429’ callouts on the fenders let people know this car meant business.

Only 859 Boss 429 engines were produced in 1969—857 in the Mustang and 2 in the Mercury Cougar, making the Boss 429 Mustang one of the most collectible muscle cars of all time.

About the photo: Employees at Kar Kraft in Brighton, Michigan, celebrate the completion of the first 1969 Boss 429 Mustang. Kar Kraft handled the extensive modifications that were required to fit the large, Boss 429 engine into the Mustang.

Story By David Bonaskiewich