AC Cobra - History and Development

History and Development

Like many British specialist manufacturers, AC Cars had been using the Bristol straight-6 engine in its small-volume production, including its AC Ace 2-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 of BMW which by the 1960s was considered dated. Bristol decided in 1961 to cease production of its engine and instead to use Chrysler 331 cu in (5.4 L) V8 engines. Although untrue, it is commonly believed that AC was left without a future source of power and that American ex-racing driver Carroll Shelby saved the company from bankruptcy. AC started using the 2.6 litre Ford Zephyr engine in its cars. In September 1961, Shelby airmailed AC a letter asking them if they would build him a car modified to accept a V8 engine. AC agreed, provided a suitable engine could be found. He first went to Chevrolet to see if they would provide him with engines, but not wanting to add competition to the Corvette they said no. Ford, however, wanted a car that could compete with the Corvette and they happened to have a brand new thin-wall small-block engine which could be used in this endeavor. It was Ford's 260 in³ HiPo (4.2 L) engine – a new lightweight, thin-wall cast small-block V8 tuned for high performance. Ford provided Shelby with two engines. In January 1962 mechanics at AC Cars in Thames Ditton, Surrey fitted the prototype chassis CSX0001 with a 260 ci Ford V8 borrowed from Ford in the UK; the 221 ci was never sent. However, early engineering drawings were titled "AC Ace 3.6". 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. His team fitted it with an engine and transmission in less than eight hours at Dean Moon's shop in Santa Fe Springs, California, and began road-testing.

Production proved to be easy, since AC had already made most of the modifications needed for the small-block V8 when they installed the 2.6 L Ford Zephyr engine, including the extensive rework of the AC Ace's front end. The most important modification was the fitting of a stronger rear differential to handle the increased engine power. A Salisbury 4HU unit with inboard disk brakes to reduce unsprung weight was chosen instead of the old ENV unit. It was the same unit used on the Jaguar E-Type. On the production version, the inboard brakes were moved outboard to reduce cost. 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 motor.

The first 75 Cobra Mark I (including the prototype) were fitted with the 260 cu in (4.3 L). The remaining 51 Mark I model were fitted with a larger version of the Windsor Ford engine, the 289 cu in (4.7 L) V8. In late 1962 Alan Turner, AC's chief engineer completed a major design change of the car's front end and was able to fit it with rack and pinion steering while still using transverse leaf spring suspension. 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 to the summer of 1965 (the last US-bound Mark II was produced in November 1964).

Since late 1962 when the new GM Stingray was shown up briefly by the Mk1 Cobra (until hub failure intervened) the development of the Grand Sport Corvette program had continued at a pace and was thought to be going for a build series of 125 cars. This would allow GM to compete directly in the FIA GT class of racing. Just to compound this Enzo Ferrari was trying to pull another "fast one" on the FIA with the request for the homologation of the 250LM. The FIA had not forgotten the serious lack of production of the 250GTO, which it had granted homologation in advance of Enzo's assured 100 minimum per year. Just thirty six were produced over three years with two very different chassis. Neither of which were too similar to the 250 GT which was supposed to form the basis of the vehicle. In an effort to prepare for the task ahead alternative engines were considered. The 289 cu in (4.7 L) leaf-spring Cobra dominated the US domestic race series (USRRC), with only one race lost in three years. The results in the FIA GT class were different. This was mainly due to the number of circuits that had much higher sustained speeds. Aerodynamics were more important and put the roadster at a disadvantage. As a result, coupe versions were built.

A stroker 289 (325),and the larger 390/427 up to the "cammer" 427 was considered. Shelby was told at the eleventh hour to use the iron 427 cu in (7.0 L). There was little time to fully develop a competition vehicle. The coil spring Cobra production was slow and an insufficient number made to meet FIA's GT homologation. Therefore the S/C (street / competition) was produced by making available to the general production the full race options for the street. By now Enzo was having races recatergorised in Italy to prevent the almost inevitable defeat on home soil as the 250LM was not homologated as a GT and would have to run as a prototype. GM had pulled the plug on the Grand Sport and so the five chassis that were built had to run as prototypes and so were placed in a difficult position to say the least.

Shelby had earlier in 1964 fit a larger Ford FE engine of 390 cubic inches (6.4 L) in to CSX2196. Unfortunately the car was not able to receive the development it needed as resources were aimed at taking the crown from Ferrari in the GT class. Ken Miles drove and raced the FE-powered Mark II at Sebring and pronounced the car virtually undrivable, naming it "The Turd". It failed to finish with the engine expiring due to damper failure. A new chassis was required developed and designated Mark III. CSX2196 was revised for the show down at Nassau which allowed a more relaxed class division of racing. This allowed the GT cobras to run with prototype Ford GT, GM Grand Sport Corvettes and Lola Mk6. The first meeting that the GS Corvettes turned up to in 1963. It was for this event in 1964 that the Fliptop cobra was used. An aluminium 390 cubic inches (6.4 L) engine was used. However, the car failed to finish.

The new car was designed in cooperation with Ford in Detroit. A new chassis was built using 4 in (102 mm) main chassis tubes (up from 3 in (76 mm)) and coil spring suspension all around. The new car also had wide fenders and a larger radiator opening. It was powered by the "side oiler" Ford 427 engine (7.0 L) rated at 425 bhp (317 kW), which provided a top speed of 164 mph (262 km/h) in the standard model and 485 bhp (362 kW) with a top speed of 185 mph (298 km/h) in the competition model. Cobra Mark III production began on 1 January 1965; two prototypes had been sent to the United States in October 1964. Cars were sent to the US as unpainted rolling chassis, and they were finished in Shelby's workshop. Although an impressive automobile, the car was a financial failure and did not sell well. In fact to save cost, most AC Cobra 427s were actually fitted with Ford's 428 cubic inches (7.01 L) engine, a long stroke, smaller bore, lower cost engine, intended for road use rather than racing. It seems that a total of 300 Mark III cars were sent to Shelby in the USA during the years 1965 and 1966, including the competition version. 27 small block narrow fender versions, which were referred to as the AC 289, were sold in Europe. Unfortunately, The MK III missed homologation for the 1965 racing season and was not raced by the Shelby team. However, it was raced successfully by many privateers and went on to win races all the way into the 1970s. The remaining 31 unsold examples were detuned and fitted with wind screens for street use. Called S/C for semi-competition, an original example can currently sell for 1.5 million USD, making it one of the most valuable Cobra variants.

Shelby wanted the AC Cobras to be "Corvette-Beaters" and at nearly 500 lb (227 kg) less than the Chevrolet Corvette, the lightweight roadster accomplished that goal at Riverside International Raceway on February 2, 1963. Driver Dave MacDonald piloted CSX2026 past a field of Corvettes, Jaguars, Porsches, and Maseratis and recorded the Cobra's historic first-ever victory. Later, Shelby offered a drag package, known as the Dragonsnake, which won several NHRA National events with Bruce Larson or Ed Hedrick at the wheel of CSX2093. Only five Dragonsnake Cobras were produced by the factory, with three others (such as CSX2093) prepared by customers using the drag package.

An AC Cobra Coupe was calculated to have done 186 mph (299 km/h) on the M1 motorway in 1964, driven by Jack Sears and Peter Bolton during shakedown tests prior to that year's Le Mans 24h race. A common misconception is, that this incident persuaded the British Government to introduce the 70 mph maximum speed limit on UK Motorways which, up until that year, had no speed restrictions. However, government officials have cited the increasing accident death rate in the early 1960s as the principal motivation, with the exploits of the AC Cars team merely highlighting the point.

The AC Cobra was a financial failure that led Ford and Carroll Shelby to discontinue importing cars from England in 1967. AC Cars kept producing the coil spring AC Roadster with narrow fenders, a small block Ford 289 and called the car the AC 289. It was built and sold in Europe until late 1969. AC also produced the AC 428 until 1973. The AC Frua was built on a stretched Cobra 427 MK III coil spring chassis using a very angular steel body designed and built by Pietro Frua. With the demise of the 428 and succeeding 3000ME, AC shut their doors in 1984 and sold the AC name to a Scottish company. The company's tooling, and eventually the right to use the name, were licensed by Autokraft, a Cobra parts reseller and replica car manufacturer owned by Brian A. Angliss.