The Boss 429 is a high performance Ford engine that was offered by Ford in 1969 and 1970.
The Boss 429 (also known as the "Boss 9" by enthusiasts) is arguably one of the rarest and most valued muscle cars to date. In total there were 1358 original Boss 429s made. The origin of the Boss 429 comes about as a result of NASCAR. Ford was seeking to develop a Hemi engine that could compete with the famed 426 Hemi from Chrysler in NASCAR's Sprint Cup Series (then known as "Grand National Division"). NASCAR's homologation rules required that at least 500 cars be fitted with this motor and sold to the general public. After much consideration, it was decided by Ford that the Mustang would be the car that would house this new engine.
The Boss 429 engine was derived from the Ford 385 engine. It used four bolt mains, a forged steel crank and forged steel connecting rods. The engine featured aluminum cylinder heads, which had a modified Hemi type combustion chamber which Ford called "crescent". These heads used the "dry-deck" method, meaning no head gaskets were used. Each cylinder, oil passage and water passage had an individual "O" ring style seal to seal it tight. The Boss 429 engine used a single Holley four barrel carburetor rated at 735 CFM mounted on an aluminum intake manifold that flowed well for its time. 1969 cars featured a hydraulic lifter camshaft while 1970 models got a mechanical lifter camshaft along with an improved dual exhaust system though rated power output stayed the same.
The cars were rated very conservatively at 375 HP and 450 lb·ft of torque. Actual output was well over 500 horsepower. Some people have claimed it to produce 600 or more horsepower, citing extensive testing to prove the results, though nothing has been actually proven. However, it is very easy to coax such power out of the semi-hemi engine due to it being factory de-tuned for street-ability purposes. These horsepower claims have been proven from many dynamometer test results over the years with factory stock engines; some rebuilt to factory specs, and others pulled directly from the cars themselves immediately after purchase, though specific output tends to vary per engine due to testing parameters. The 429 Drag Pack option from Ford at the time (which was a normal 429 SCJ and not the Boss version which made more power) had 495 HP, which was quite impressive for the time, however it should be noted that these ratings were strictly engine crank horsepower, and not actual wheel horsepower figures. The manufacturers and dealers only listed 375 HP because of legal issues and rising insurance costs so customers would be lured into buying these cars without the stiff fees from an insurance company, though no manufacturer has ever stated this to be true, assumingly so to prevent possible lawsuits. Despite the high horsepower and torque ratings, the car was at home in the high RPM range (the engine is said to be able to rev up to 9,000rpm for extended periods of time, which is impressive given the engines massive displacement, though were factory limited to 6,200rpm for insurance purposes), and suffered a poor by today's standards 0–60 time because of this. However, it quickly made up for that once it got moving. The car unrestricted is said to be able to exceed speeds of up to 175mph, though no actual claim has been proven due to the rarity, value and collectibility of the car, making anyone who owns one reluctant to try it.