What set the engine apart and made it such a competitor was the magic in its aluminum cylinder heads. The heads featured a "crescent" shaped, semi-hemi combustion chamber with huge canted valves (the Boss 429's 2.3-inch intakes were the largest ever used in a Ford production engine) and huge round ports that could nearly swallow a tennis ball. Another unique feature was the "dry-deck" method that replaced the use of traditional head gaskets. Instead, O-rings were installed to seal the cylinders and the water and oil passages. Riding on top was a 735-cfm Holley, parked on an aluminum high-riser intake manifold. For driveability, a hydraulic valvetrain was employed. In full street dress, with air cleaner and emission equipment in place and closed exhaust, the Boss 429's output was advertised at a ridiculously underrated 375 horsepower. In race form on the NASCAR track, the Boss 429 put out nearly double that amount of horsepower. It was a 200-mph-at-Daytona, full-tilt racing engine toned down for the street.
The Mustang would never compete in a NASCAR race, and while the production Torino would have seemed to be the logical choice for the Boss 429, Knudsen wanted to give the Mustang an extra pump on the performance meter. Interestingly enough, a well-tuned 335hp 428ci Super Cobra Jet Mustang could kick a stock Boss 429 to the curb. Magazine tests of the period revealed the 428SCJ would run nose to nose with the Boss 429. That wasn't because the Boss 429 wasn't able to kick ass, it only served to show how undertuned the engine really was.
Each Boss 429 Mustang began as a partially completed Mach 1 Sportsroof that was delivered to Kar Kraft. These cars were already equipped with heavy-duty suspensions. Kar Kraft would add a .62-inch rear stabilizer bar and the modifications to the brakes and suspension they had first engineered for the Boss 302. Kar Kraft had reworked the platform's basic suspension geometry, and between computer analysis and plenty of track time, stiffer springs and revalved Gabriel shocks were specified for use with the wide 15x7-inch wheels and sticky F60 tires (the front fender wheel openings had to be cut and rolled under to clear the fat Goodyear Polyglas 60-series rubber). The rear shock placements were staggered to handle the increased lateral loads induced by the better adhesion levels generated by the wider tires.
Unlike the Boss 302, the Boss...
Unlike the Boss 302, the Boss 429's flanks were devoid of stripes, graphics and other visual nonsense. It was all about performance.
Kar Kraft had their engineering hands full trying to drop the King Kong Boss 429 between the Mustang's narrow shock towers. To shoehorn the big "Shotgun" 429 into the Mustang's narrow engine compartment, Kar Kraft had to cut the shock towers and move them outwards two inches. That required a wider strut tower brace. The front suspension was moved forward 1 inch, and special spindles, relocated upper and lower control arm mounting points, stiffer springs, and a .94-inch front stabilizer bar were added to improve the car's steering and handling characteristics due to the additional mass of the heavier engine. A slimmer power brake booster was required to clear the engine's left-hand valve cover. The battery was moved to the trunk for clearance and to improve weight distribution.
The remainder of the Boss 429 package consisted of Ford's Top Loader close-ratio, four-speed gearbox spinning a 3.91:1 rear axle with a Traction-Lok differential. The front shovel spoiler had to be shortened to clear the pavement since the Boss 429 sat 1 inch lower than stock height.
On the hood was a large scoop designed by Shinoda, driver controlled to allow cold air to the carburetor. Also included in the Boss 429 package were color-keyed, dual outside racing-style mirrors, engine oil cooler, and chromed 15x7 Magnum 500 wheels with those jumbo F60 Goodyears. The interior received he Mustang's Black Deluxe Dcor group, as well as a standard AM radio. An 8,000-rpm tach was included in the instrument package.