Developed for Tommy Sopwith, the Sphinx proved that Armstrong Siddeley could build a sportscar
Armstrong Siddeleys were regarded by most of the sporting fraternity as pedestrian, even sluggish, but that was not entirely true. In 1933 there was the remarkable Siddeley Special with its 4.9-litre hiduminium engine, which was timed over a Brooklands quarter-mile at 87.38mph. The more normal four-cylinder 234, with ugly saloon body conforming to Cyril Siddeley’s requirement for a roof high enough to enable a top hat to be worn, was not exactly a sluggard either.
However, a racing Armstrong Siddeley was another ball-game, to mix metaphors. But Bill Smith reminded his audience at the W O Bentley Society lecture of The Sphinx, which the Parkside factory built for Tommy Sopwith to race under the Equipe Endeavour banner.
I saw it on its racing debut at Goodwood in March 1954. The chassis was that of an Allard JR (actually No 3405 only six other Js were built) with coil spring de Dion rear suspension, and the engine was a development of that used in the Armstrong Siddeley 346, retaining the 90mm bore and stroke dimensions. More power was obtained by the fitting of a high-lift camshaft, three double-choke Weber carburettors and a free-flowing six branch exhaust system. The crankshaft was specially designed to withstand the increased power output (227bhp at 5100rpm), the car being run on 100-octane petrol which could withstand the 9.5:1 compression ratio.
At first a 3.55 to 1 axle ratio was used but this would have been changed to suit different circuits in conjunction with 6.00 x 16 Dunlop racing tyres on knock-off wire wheels. It is interesting that there was sufficient faith in the electrically controlled pre-selector gearboxes to incorporate it in this sports-racing Sphinx. Front suspension was by the Allard-type split axle in conjunction with coil springs and telescopic dampers were fitted. The Alfin brake drums measured 12 x 2.25in, actuation being hydraulic, with two leading shoes.
The Equipe Endeavour, named after Sopwith’s Americas Cup racing yachts, ran the car at circuits such as Goodwood, OuIton Park and Snetterton. Tommy Wisdom, that all-round racing driver and motoring journalist, proved at OuIton that it could hold off the C-type Jaguars. It is said the Sopwith family persuaded their son to give up sports car racing in favour of saloon cars, which he did with a 346 Armstrong Siddeley Sapphire. WB