History and developmentLike many British specialist manufacturers, AC Cars had been using the smooth, refined 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 cid (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 260ci engines. In January 1962 mechanics at AC Cars in Thames Ditton, Surrey fitted the prototype chassis CSX0001 with a 260ci Ford V8 the 221ci 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.
AC's system was as follows: The Ace chassis numbers read AEX... "A" being the car series and "X" being for export with left hand drive. Later with the introduction of the Bristol engine the chassis numbers ran "BEX..." When switching to the Ford 2.6 engine the Ace chassis numbers ran "RS...". The fist Cobra chassis was left hand drive and given the next letter in the alphabet,ie C. Therefor the numbers ran CSX2... for all the export Cobras. Otherwise, the prefix was "CS2..." The four following numbers ran from 2000 sequentially. When the MKIII Cobra was built the identification ran from CSX3000 (for left hand drive export). There were other numbers used such as "COB...." and "COX...."
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 litre 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 engine (4.2 L). The remaining 51 Mark I model were fitted with a larger version of the Windsor Ford engine, the 289 in³ (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).
By 1963 the leaf-spring Cobra was losing its supremacy in racing. Shelby tried fitting a larger Ford FE engine of 390 cubic inches (6.4 L). Ken Miles drove and raced the FE-powered Mark II and pronounced the car was virtually undrivable, naming it "The Turd". A new chassis was developed and designated Mark III.
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 the 1st of 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 70s. 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.
AC Cobras had an extensive racing career. Shelby wanted it to be a "Corvette-Beater" and at nearly 500 lb (227 kg) less than the Chevrolet Corvette, the lightweight car did just that. It was February 2, 1963 at Riverside International Raceway that driver Dave MacDonald beat an impressive field of Corvettes, Jaguars, Porsches, and Maseratis to give the Cobra its 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.
The Cobra was perhaps too successful as a performance car and reputedly contributed to the implementation of national speed limits in the United Kingdom. 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. 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 risk.
Although successful in racing, the AC Cobra was a financial failure, which 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 handsome 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.
Super SnakeShelby Motors built twenty two 427 competition roadsters. In 1965, one was selected and converted into a special model called the 427 "Cobra to End All Cobras." The first one of these (number CSX3015) was originally part of a European promotional tour before its conversion. This conversion called for making the original racing model street legal with mufflers, a windshield and bumpers amongst other modifications. But some things were not modified, including the racing rear end, brakes and headers. The most notable modification is the addition of Twin Paxton Superchargers. This gave the car a claimed 800 brake horsepower (600 kW) and 800 lb·ft (1,085 N·m) of torque at 3000 rpm. Officially 0–60 mph (0–97 km/h) was achieved in 4.5 seconds.
Another non-competition 427 roadster, CSX3303, was converted and given to Shelby's close friend, Bill Cosby. Cosby attempted to drive the super-fast Cobra, but had issues with keeping it under control. This was humorously documented in Cosby's album titled Bill Cosby, 200 M.P.H.. Cosby gave the car back to Shelby, who then shipped it out to one of their dealers in San Francisco, S&C Ford on Van Ness Avenue. S&C Ford then sold it to customer Tony Maxey. Maxey, suffering the same issues as Cosby did with the car, lost control and drove it off a cliff, landing in the Pacific Ocean waters.
Shelby's original model, CSX3015, was kept by Carroll Shelby himself over the years as a personal car, sometimes entering it into local races like the Turismos Visitadores Cannonball-Run race in Nevada, where he was "waking [up] whole towns, blowing out windows, throwing belts and catching fire a couple of times, but finishing." The CSX3015 was auctioned on 22 January 2007, at the Barrett-Jackson Collector Car Event in Scottsdale, Arizona, for $5 million plus commission (£2.8 million).