History and development
Like many British manufacturers, AC Cars had been using the Bristol straight-6 engine in its small-volume production, including its AC Ace two-seater roadster. This had a hand-built body with a steel tube frame, and aluminium body panels that were made using English wheeling machines. The engine was a pre-World War II design by BMW which by the 1960s was considered dated. In 1961 Bristol decided to cease production of its engine.
In September 1961, American retired race car driver and automotive designer Carroll Shelby wrote to AC asking if they would build him a car modified to accept a V8 engine. Bristol engines for the AC Ace two-seater sports car had recently been discontinued so AC agreed, provided a suitable engine could be found. Shelby went to Chevrolet to see if they would provide him with engines, but not wanting to add competition to the Corvette they said no. However, Ford wanted a car that could compete with the Corvette and they happened to have a brand new engine which could be used in this endeavor: the Windsor 3.6-litre (221 cu in) engine – a new lightweight, thin-wall cast small-block V8. Ford provided Shelby with two engines.
AC Ace 3.6
In January 1962 mechanics at AC Cars in Thames Ditton, Surrey designed the "AC Ace 3.6" prototype with chassis number CSX2000.
AC had already made most of the modifications needed for the small-block V8 when they installed the 2.553-litre (156 cu in) inline 6 Ford Zephyr engine, including the extensive rework of the AC Ace's front end bodywork. The only modification of the front end of the first Cobra from that of the "AC Ace 2.6" was the steering box, which had to be moved outward to clear the wider V8 engine.
The most important modification was the fitting of a stronger rear differential to handle the increased engine power. A Salisbury 4HU unit with inboard disc brakes to reduce unsprung weight was chosen instead of the old E.N.V. unit. It was the same unit used on the Jaguar E-Type. After testing and modification, the engine and transmission were removed and the chassis was air-freighted to Shelby in Los Angeles on 2 February 1962, By this time the small-block's displacement was increased to 4.7 L (289 cu in).
Shelby's team paired this engine along with a transmission into CSX2000, in less than eight hours at Dean Moon's shop in Santa Fe Springs, California, and began road-testing.
A few changes were made to the production version:
The inboard brakes were moved outboard to reduce cost.
The fuel tank filler was relocated from the fender to the center of the trunk. The trunk lid had to be shortened to accommodate this change.
AC exported completed, painted, and trimmed cars (less engine and gearbox) to Shelby who then finished the cars in his workshop in Los Angeles by installing the engine and gearbox and correcting any bodywork flaws caused by the car's passage by sea. A small number of cars were also completed on the East Coast of the US by Ed Hugus in Pennsylvania, including the first production car; CSX2001.
The first 75 Cobra Mk1 models (including the prototype) were fitted with the 4.3 L (260 cu in). The remaining 51 Mk1 models were fitted with a larger version of the Windsor Ford engine, the 4.7-litre (289 cu in) V8.
In late 1962, Alan Turner, AC's chief engineer completed a major design change of the car's front end to accommodate rack and pinion steering while still using transverse leaf spring suspension (with the leaf spring doubling as the upper suspension link). The new car entered production in early 1963 and was designated Mark II. The steering rack was borrowed from the MGB while the new steering column came from the VW Beetle. About 528 Mark II Cobras were produced from 1963 to the summer of 1965 (the last US-bound Mark II was produced in November 1964).