Modified Mustangs & Fords
Ford Boss 302 Engine Build - Build A Better Boss - Tech
A Legendary 5.0L Upgrade
Ford originally offered the Cleveland heads in both 2V and 4V configurations, meaning essentially the engines were fitted with two-barrel or four-barrel carburetion. Since the CHI heads were sized between the original 2V and 4V heads, CHI naturally designated them the 3V. CHI offers a number of 3V head configurations, or more accurately port volumes (and attending flow rates). Since the goal for our 302 was to exceed 500 horsepower, we selected a set of the 208cc 3V aluminum CHI heads. These heads offered a number of desirable features, including a tall intake port roof design, which provided a generous short-turn radius to maximize flow with a given cross section and port volume.
The CHI heads feature a complete Ferrea valvetrain, including Competition Plus, swirl-polished, stainless steel valves (2.19/1.74), double valvesprings designed for the over 0.700-inch-lift solid roller cam(s) we were going to use, and titanium retainers. While the CHI heads offered CNC combustion chambers and an impressive 320 cfm right out of the box, they were given the once over by the gang at Dr. Js. With expert porting on the intake and exhaust, Dr. Js was able to increase the flow rate of the CHI heads to more than 345 cfm at 0.700-inch lift. The heads were now capable of supporting more than 700 hp-more than enough for our 500hp 302.
Given the port-alignment difference between the CHI 3V, and either the produc-tion 2V or 4V, neither of the factory Ford intakes will fit the CHI heads. For this, CHI manufactures its own line of matching 3V intake manifolds. The CHI single-plane intake we are using was designed specifically for use with the 208cc 3V heads on a 302W block (8.2 deck height). Like the heads, the intake was turned over to the flow experts at Dr. Js to ensure that the manifold could keep pace with our ported 3V heads. CHI also offers a number of other intakes for use on the taller 9.2-inch 351C, 9.5-inch 351W, and even the 10.3-inch 400M blocks.
Some of the CHI intakes for the taller-deck motors are available with Dominator flanges, but our 208cc intake came equipped to accept a standard 4150 Holley carburetor. For dyno use, we installed a 950 HP Holley-again more than enough carburetion to reach our goal of 500 hp. Care must be taken when installing the CHI heads, as there is a definite right and left cylinder head-the heads must be installed with the water passages toward the front of the engine so they line up with those in the intake. Also, make sure to seal the Cleveland water passages located on the deck surface before using them on a Windsor application.
Since we needed to rev the engine to at least 7,000 rpm to reach the 500hp mark, we needed a sturdy foundation, which came from Demon Engines. The short-block consisted of a forged steel 3.0-inch crank from Pro Comp, a set of factory 5.135-inch forged rods, and a set of forged aluminum pistons from Probe Racing. The pistons featured small domes and valve reliefs designed to fit the stock Boss 4V (Cleveland) heads to produce a static compression ratio of 10.5:1 using the stock 60cc combustion chambers. A stock 1970 Boss 302-spec solid, flat-tappet cam was used and offered a tad over 0.500-inch lift and 228 degrees of duration at 0.050.
The Boss was topped off with an original aluminum dual-plane intake, 750 Holley carb (they came with 780s originally) and a set of 1 3/4-inch Hooker headers. In stock trim, the original Boss 302 produced peak numbers of 373 hp and 325 lb-ft, pretty impressive numbers considering the stock Boss 302 was rated at only 290 hp and a like amount of torque. Obviously, the Boss 302 was purposely underrated by Ford, but just how much better could we make it?