The 1969-70 Boss 302 engine was created in 1968 for the SCCA's 1969 Trans-Am road racing series. Fitted to the factory-made Boss 302 Mustangs of 1969-70, it is a unique Ford small-block featuring a thin-wall high nickel content casting. It differed substantially from regular 302s, with 4-bolt mains, screw in freeze plugs, higher nickel content, and heads using a canted valve design being developed for the 351C, which made its debut in late 1969 Mustangs).
The high nickel content block had a thicker deck, and a taller intake manifold due to the heads. It also had a distinct harmonic balancer, crankcase windage tray, bigger diameter alternator pulley (from the 289HIPO), and bigger diameter power steering pulley all to accommodate the higher RPM capabilities of the engine vs a standard 302. The engine also came with an engine RPM limiter (to protect the owner's warranty options but was often one of the first things removed). The Boss 302 has eight valve cover bolts; in 1969 they were chrome and in 1970 they were cast aluminum, while the standard 302 has six valve cover bolts. The connecting rods are the same as used in the 289 HIPO, and have an engineering number of C3AE-D on them. They are capable of higher RPMs than standard 302 rods (up to ~8k RPM), aided by a spot face for 3/8 inch bolts with a unique football-shaped head (vs 5/16 for standard small blocks), and beefier cap. The crankshaft is cross drilled (this was changed in 1970 for better reliability) high strength steel forging. The cam and lifters are high lift, solid mechanical units. The cam featured 290 degrees duration and .477 inches of lift.
The wide and large port heads with staggered valve placement give the Boss 302 high power capabilities. Because of the pent-roof design of the heads, the Boss also had forged pop-up pistons to achieve the desired 10.5:1 compression ratio. Early units were typically characterized by very large intake (2.23 inches) and exhaust (1.70 inches) valves sitting in a small quench style combustion chamber. Exhaust valves were sodium-filled to aid cooling. Valve springs were dual units with an inner and outer spring to minimize harmonic resonance at high RPM. The heads feature steel spring seats, screw-in rocker studs, pushrod guide plates to aid in pushrod stability at high RPMs, and adjustable rocker arms. Fuel was provided by a Holly 780cfm manual choke carburetor. The taller intake required a thinner spacer. Ford used a phenolic spacer that incorporated an aluminum tube for the PCV hose, and also helped isolate the carburetor from the heat of the intake. Ignition was handled by a dual point distributor firing then unique 14mm AF 32 Autolite sparkplugs specified because of their smaller size (5/8 inch wrench vs. 13/16 inch wrench) so as to fit within the tight confines of the combustion chamber alongside the very large valves.
The motor produces a unique sound as a result of its solid lifter configuration. At idle, properly tuned, the engine has a great deal of 'chatter.'
The power output on this engine was a conservative 290 hp at 5,200 RPM with matching torque of 290 foot pounds at 4300 RPM. The Boss 302 engine competes well with other high performance 'small blocks', such as the Chevy 302, the Chrysler 340, and AMC 360. Although the factory specified maximum power at 5,200 RPM, the engine was allowed to rev well beyond that to a 6,250 RPM rev limited maximum.
The "Boss" in the Boss 302's name came from original designer Larry Shinoda's reference to Bunkie Knudsen, the CEO at Ford and an outspoken proponent of the car's development, who told his designers, "I want to design a car that's the coolest Mustang out there. I don't want somebody else's name on it, like a Shelby."
This engine was also optional in the Mercury Cougar Eliminator, with 169 produced in 1969 and 469 produced in 1970.