Sunday, 16 January 2011

AC Cobra

The electronic time trap credited me with 183mph. I was at the wheel of the Le Mans AC Cobra 39PH. It was 1963 and it felt quick. Alas The Motor’s technicians pooh-poohed electronics, their slide-rules calculating that the change in axle ratio, described in the feature published July 17, rendered it more likely to be about 170mph.
Click to enlarge

Peter Bolton and Ninian Sanderson had just driven 39PH to seventh place at Le Mans. The following week I met Sanderson by chance, outside Harrods. I knew him through my association with Ecurie Ecosse and he suggested AC might lend the car for test. My colleague on the road test staff Roger Bell, later editor of The Motor and an accomplished saloon car racer, joined me at the Motor Industry Research Association (MIRA) test track. We did just short of 140mph, about 22.5mph per 1000rpm, through an electronic trap on the banked circuit, so at 6,500rpm it might have managed 146-147mph. On the Le Mans axle it was doing 160mph on Mulsanne at 5,500rpm, or around 29mph per 1000rpm, so had it been able to pull 6,500rpm with its rather blunt aerodynamics, that would be 189mph.

On the fresh axle ratio, even at 7,000rpm, 165mph at MIRA was more likely than 180, so discretion suggested that the figure be excluded from the feature. MIRA had a second electronic time trap on the road course, inside the banked outer circuit, on which you could go faster before braking hard for the next corner. Photographer Maurice Rowe took a fine picture of Roger lifting the Cobra’s inside wheel on one. Perhaps the slide-rulers at The Motor (they were usually precise) were not perverse to rob me of my 183mph but that assumes, of course, that all the other calculations were right.

When AC stopped making cars in 1939, they were using an engine already 20 years old. John Weller’s aluminium 1,991cc single overhead camshaft wet liner 6-cylinder was first shown at the London Motor Show in 1919. Production resumed in 1945 with the same engine in a saloon not long for the automotive mainstream. There was little to distinguish it from 1930s counterparts, except that the headlamps had sunk into the wings and the grille curled over. So long as cars remained in short supply it held its own. Traditionally a sports car manufacturer, AC wanted to make 2-seaters so engaged John Tojeiro whose sports cars were doing well in British racing. He had been hired by Charles and John Cooper to plan the front-engined Cooper-MG. Tojeiro’s formula was straightforward, his twin-tube frame accommodated the Weller engine much the same as it had obliged the 4-cylinder MG.

The shape of the AC Ace was cribbed, without much alteration and certainly no acknowledgement, from a contemporary Ferrari Barchetta. It used Weller’s now 34 year old engine and went on sale in 1953 at a premium price. The chassis was simple, a frame of two 3in diameter tubes and independent suspension both ends. The frame was stiff and the handling exemplary; still good in the 1960s after nearly 700 had been made. A coupe, the Aceca (320 made) became a collector’s piece and through steady evolution an excellent, intuitive design improved, although the power was insufficient to exploit the excellent road holding. In 1956 as an alternative to Weller’s 102 bhp, AC offered the Bristol (neĆ© BMW) 2 litre with 125 bhp, providing over 115 mph.

The Ace was a classic, the Ace-Bristol spectacular but in 1961 Bristol stopped making the engine. A modified Ford Zephyr pushrod, of 170 bhp, scant refinement and great weight was unsatisfactory.

Above: AC Aceca In the nick of time the United States Cavalry arrived, led by colourful Texan Carroll Shelby. The first Cobra prototype of 1962 was basically an Ace chassis altered to take a Ford V-8, with wider tyres and body modifications to cope with more than twice the horse power of the Zephyr. For sheer bravura, nothing could match it. There were 4.2 or 4.7 litre V8s, then from 1965 a 7 litre giving up to 345bhp in road trim and a top speed around 145mph. The standing quarter-mile took under 13sec.

The V8 made immense demands on the chassis, and changes were wrought, starting with rack and pinion steering. Like many carry-over designs of the 1930s the Ace continued using drop-arms and drag links, until the tendency of rack and pinion to lock-up at inconvenient moments was curbed. The Cobra's suspension was changed, coil spring and damper layouts with wishbones replacing transverse leaf springs.

Cobras went under a lot of names. Sometimes AC was dropped altogether; it was known as a Shelby Cobra, A Shelby American, and sometimes a Ford Cobra. AC provided it with a Frua body and called it simply the 428, a stylish but unsuccessful model that formed the sole AC exhibit at the London Motor Show long after production effectively stopped. The Cobra was probably the most copied, most replica-ed sports car ever. And when I see 39PH I bask briefly, just a little, in some of its glory.

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