The Ford Motor Company developed an engine then they looked for the best place to put it for testing. The new power plant has a 429 cu in displacement and is based on the earlier Ford 385 engine. Four bolt main bearings with a forged steel crank-shaft and forged steel connecting rods are used in the engine blocks. Hydraulic lifters are in the 1969 models but for 1970 they have a camshaft with mechanical lifters and an upgraded dual exhaust system but both can still develop the same horse power. Passages for the oil and water are sealed with an “O” ring at each cylinder. The heads have a type of modified hemispherical combustion chamber that Ford calls “The Blue Crescent.” There is a Holly four barrel with a rating of 735 CFM bolted to an aluminum intake manifold. The valve covers are made of magnesium and these earliest engines are reputed to be more powerful than the later “T” and “A” code replacments. This engine is an experiment by Ford to produce a Hemi type power plant to rival the Chrysler 426 Hemi doing so well on NASCAR circuits. NASCAR homologation regulations at the time required that the engine be available in a minimum of 500 vehicles for sale on the retail market during each year.
The Mustang was the vehicle Ford chose to put the engine into and “Boss 429 Mustang” is the chosen moniker. For the two years that the 429 is in production there are a total of 859 units of the engine manufactured. Ford out-sourced the needed modifications on Mach I and Cobra Jet shells to Kar Kraft who were active in a number of Ford Mustang projects at the time. The engine compartment must be modified to accommodate the exceptionally large hemi block. Kqr Kraft shortens the front suspension mounts, widens the shock towers and extends the inner fenders to allow the block with exhaust manifolds to fit. There is no room for a battery under the hood so it is mounted in the trunk and a ridged ¾’ sway bar is added in the rear to beef up the handling of the front heavy car. The Boss 429 is the first Mustang fitted with a rear sway bar which improves the track performance greatly over any other big block Mustang model of that era. The engine has an oil cooler and the 3.9:1 rear axle is equipped with the “Traction-Lock” limited slip differential. The Hurst shifer is standard equipment in 1970 but because of the engines size no air conditioning is available for either year.
The Boss 429’s all black interior features an 8,000 RPM tachometer, the AM radio and has a hand lever to operate the hood mounted air scoop which is the largest ever used on a Mustang. There is a front spoiler with a lower profile than the Boss 302 version and the two racing mirrors are color coded. No 429 came with factory installed rear window louvers
The 1970 model features an expanded choice of paint and sports matt black scoops on all body colors. There was an extremely rare six pack complete with intake and carburetors available as a dealer installed option but only two units were installed. There were 499 units produced in 1970 and can be worth in excess of $500,000.00 before restoration.
There were three different 429 engines manufactured during its two year production. The “S” code 429 is the first version and it is assembled with a host of high performance parts but it has warrantee issues, possibly due to incorrect assembly. The 429 engine is revised and the “T” code is the replacement with a host of lighter duty internal parts. Reaching the end of production the “A” code 429 is the last variation with a new valve train and more pollution control equipment than the earlier engines. All of the 429 power plants produced during the two years were very under rated at 375 hp (280 kW) but will have no problem producing 600 plus horse power with a little tweaking. The Boss 428 engine is highly regarded with each unit revised by Kar Kraft getting a NASCAR “KK” number placed on the driver’s door. The first unit produced has the number KK 1201 and the last is KK 2558.