Model history: Along with works driver and development engineer Ken Miles, Carroll Shelby looked at ways to further improve the performance of the Cobra racing car ahead of the 1965 season. In good American tradition, although Miles was actually British, the two found that shoe-horning an even bigger engine into the AC-supplied chassis to be best solution. This was, however, such a substantial change that for homologation purposes 100 examples had to be produced. Shelby had had no problems selling his Cobras before, so he believed this to be a relatively minor issue.
Miles and Shelby had set their sights on a very special '390' Ford V8 that was being developed for NASCAR. Cast from aluminium, it was exceptionally light but considerably more powerful than the small-block V8 previously used. One was used in an experimental Cobra developed and raced very successfully by Miles. Unfortunately, these high performance engines were in short supply and Ford's NASCAR teams objected to sharing them with the Cobra program. This forced Shelby and Miles to compromise and settle for the more readily available and far less exotic '427' big-block V8.
Displacing just under seven litres, it was heavier and bigger than the existing Cobra engines. The weight was kept down by using thin-wall casting but nevertheless substantial changes to the chassis were required to improve the handling characteristics. The single biggest improvement was the uprated suspension, which now used coil springs on all four corners instead of the transverse semi-elliptic leaf springs of the original design. With the help of Ford's sophisticated computers, the chassis design was revised, growing both in width and length to cope with the size and power of the engine.
The revised designs and an order for 100 chassis were sent to AC in England where the chassis were constructed. Unknown to Shelby, the design was further compromised when AC got a great deal on the original length tubing. So the short wheelbase was retained while the width was increased. Wider wheels and tyres were also fitted to cope with the additional power. These required a restyled body with sizeable, flared fenders. Fitted with the body, which was finished in primer, the rolling chassis were airlifted from England to Shelby's facility in Venice, California for completion.
By the time the FIA inspectors arrived at Shelby American, early in 1965, only 51 examples had been produced and homologation for the GT class was refused. At that time 16 had already been shipped to customers in full racing trim but the rest were instantly made unsaleable. In June, the FIA announced a new 'Competition GT' class for which the production minimum was conveniently set at 50 examples. Although this seemed perfect for Shelby, it wasn't as eligible for the same class was the Ford GT40. This program had priority, and to prevent any embarrassing situations, Shelby promised Ford, he would not campaign the new 427 Cobra.
Even though the big-block had been homologated, only two additional racing cars were sold as many of Shelby's customers switched from GT to sports car racing. Only raced by privateers, the Cobra 427 was nevertheless very successful. One of the best results was a rare outright win against prototypes for an open Cobra, at Brands Hatch in 1966. The 30-odd competition cars that had already been completed lingered on Shelby's lot until it was decided to sell them for use on the street. Equipped with only the bare necessities, these were known as the Semi Competition or S/C and marketed as the fastest road car in the world.
In addition to the Competition and S/C variants, Shelby eventually also constructed a run of over 200, slightly more docile Cobra 427 road cars. So his early intuition had proven correct but he had just had too little time to get the cars ready in time for homologation. Production ceased in 1967, and the 427 remains as the final iteration of the original Cobra. Thanks to its heavy engine and short chassis the big-block Cobra's handing does not have the best of reputation, although its slightly lazier engine does reputedly make it easier to drive than the high strung 289 Cobra. We can only imagine what a phenomenal machine it could have been had the 390 V8 been available and the proper length tubing been used.
Invoiced by AC on October 26, 1967 this is the very last Shelby Cobra constructed. Built as a road car, it is powered by a big-block engine that is rated at around 425 bhp. Like the final 60 cars built, it does not have an additional oil cooler opening under the radiator. Having covered only around 6,000 miles from new, this very important Cobra has survived in remarkably original condition. Chassis CSX3360 is seen here during the 2012 Pebble Beach Concours d'Elegance, where the 50th anniversary of the Cobra was celebrated.