Wayne Cook
September 1, 2001
Photos By: Mustang & Fords Archives

The stakes were high for the '69 Trans-Am season. Both Ford and GM had eyes on the championship. Prestige and sales were the prizes. The '68 Trans-Am season had been a disaster for Ford. The new-design Tunnel Port 302 engines had come unglued all season long, in part due to oiling problems at high rpm. The so-called Tunnel Port heads had been designed for the new 302 to replace the relatively restrictive 289 Hi-Po heads, and featured huge 2.12-inch-diameter intake valves. These heads were so named because the pushrods "tunneled" directly through the intake ports, allowing for a larger port and a straight shot at the intake valve. The concept looked good on paper, but Ford engineers needed to rethink their design for a winning Trans-Am engine.

Chevrolet had introduced the Z/28 package for the Camaro late in the introductory '67 model year, and the GM camp was having success with the car. Although '67 Z/28 production was limited to 602 copies, the '68 sales year closed with 7,199 '68 Z/28s sold. The Z/28 used the durable Chevrolet small-block for power, specifically the 327ci engine equipped with a 283 crankshaft, for a net displacement of 302 ci. In the '68 season, Chevrolet walked away with the Trans-Am championship, led to victory by Mark Donohue driving a Z/28 Camaro. Something had to be done by Ford to counter the success of the Z/28, both at the track and in the showroom. That something came to be known as the Boss 302.

Instead of the nodular-iron crankshaft found in the Hi-Po 289, the Boss 302 came equipped with a cross-drilled forged-steel unit. Rods were the 289 Hi-Po units with 3/8-inch rod bolts for extra strength. Standard rod-bolt size was 5/16-inch. Boss 302 pistons gave a compression ratio of 10.5:1. The biggest factor in the great performance of the Boss 302 was the cylinder heads. The units employed on the Boss engine were borrowed from the new 351 Cleveland parts bin. Completely different from the new-for-'69 351 Windsor engine, the 351 Cleveland was not scheduled for release until 1970, so in 1969 these heads were a brand new development. Unlike the Tunnel Port heads, the Cleveland units employed canted valves in an arrangement similar to the 396ci Mark IV big-block engine introduced by Chevrolet in the '65 Corvette. The splayed valve arrangement allowed for superior breathing characteristics. For the Boss 302, gigantic 2.23-inch-diameter intake valves were employed, with 1.71-inch being the size on the exhaust side. In '70, intake valve size for the Boss 302 was reduced to 2.19 inches to enhance throttle response and create more torque at low rpm.

A solid lifter cam provided the musical valve clatter for which the Boss 302 Mustang is famous. A 780-cfm Holley four-barrel carburetor atop an aluminum intake manifold was standard induction. Other types of Boss 302 induction such as the infamous "Cross Boss" intake and Autolite inline four-barrel were successfully produced but were disallowed by Trans-Am racing officials.

Ford brass wanted the Boss 302 Mustang to be the best-handling American car made, and the task of making it so was given to Ford engineer Mark Donner. Under his supervision, the Mustang suspension was studied and numerous changes were made for the Boss 302. These changes included heavier springs both front and rear, with a larger-diameter front antisway bar. A rear bar was not included until '70. The rear axle got staggered shocks to help control wheel hop. Special Gabriel shocks and 15-inch wheels with fatter F60-15 tires were included in the Boss handling package. Reinforced shock towers and heavier spindles were used on the front suspension to withstand the additional punishment expected with hard street or racetrack use. All of this added up to a package good for nearly 1 g on the skidpad.

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