Firefly

It was in late 1930 that T.G. John proposed that a cheaper 4-cylinder car be produced as a successor to the 12/50 by redesigning the chassis and engine. Work was well in hand by May 1931 and deliveries could have commenced in September of that year, but in the event development of the six-cylinder Speed 20 took priority. It was not until May 1932 that the first experimental prototype was completed. Savings in the cost of production were achieved by using as many parts in common with the Speed 20 as possible. The chassis was essentially a shortened (9ft 10.5ins wheelbase) version of the 'double dropped' Speed 20 chassis, with the same suspension and cable braking system, but the radiator and hence bonnet line had to be taller since cooling was by thermo-syphon, with no water pump or fan. The engine was also similar to the Speed 20, with overhead valves, but of course with only four cylinders, with a bore of 69mm and stroke of 100mm giving it a capacity of 1,496cc, and a rating of 11.9 h.p.. Carburetion was in the form of a single down-draft S.U. Early cars have an S.U. electrical fuel pump, whilst later cars have an A.C. mechanical pump driven off the camshaft.

From the start, an ENV pre-selective gearbox was offered as an option for an additional £15, the standard fitting being a non-syncromesh gearbox manufactured by Alvis, unit mounted. Unlike some other manufacturers, who used fluid flywheels between the engine and gearbox, on the Firefly the pre-selective gearboxes were self-contained, with the gears being selected by clutch bands within the box. Well over 50% of all Fireflies produced were sold with pre-selective gearboxes.

Alvis sub-contracted the body-work to local companies, offering, initially, four catalogued body styles for the Firefly: a 4-light saloon; an open 4-seater; a drop-head coupé; and a 2-seater (of which, very few were sold). For 1934 a 'De Luxe' 6-light saloon was added to the catalogue but only 15 were produced. Prices ranged from £455 for a 2-seater to £520 for a 6-light saloon. A few bare chassis were sold at £395 to agents or customers who wanted to commission their own bespoke bodywork. The majority of bodies were contracted out to nearby Cross & Ellis, but some 27 saloons were built first by Carbodies, then 74 by Charlesworth, and, right at the end of production, 40 were built by Holbrook in Wolverhampton. Grose of Northampton also built a handful of saloons to their own, very elegant, design.

SA 11.9: Fireflies produced between October 1932 and November 1933 were designated by the factory as the SA 11.9, which was then superseded by the SB 11.9, but in brochures and advertisements they were always referred to simply as ‘Firefly Twelves’. The first 50 engines produced had the distributor mounted in front of the dynamo, but it was then repositioned to be closer to the timing chain. Minor modifications were made during the life of the SA 11.9: to the brake shaft and cables, to the oil pump body and spindle and to the camshaft profiles to provide more valve overlap.

The Firefly was redesignated as the SB 11.9 in November 1933. The main difference was in the choice of pre-selective gearbox, which was changed from the ENV Type 75 to the ENV Type 110, although some late production SA 11.9s had already been fitted with the Type 110. The vast majority of SB 11.9s were sold fitted with the pre-selective gearbox, and the option of an Alvis gearbox was withdrawn from May 1934. The only other known modification was to the wheel spinners and hub grease caps. In August 1934 the Firefly was superseded by the Firebird SA 13.22, but the last Firefly was not despatched from the factory until 19th February 1935.