The Alvis Firefly is based on the 9'10½" wheelbase chassis and running gear, the frame being double-dropped as on the SA Speed 20. Beam axles were used, with the standard Alvis 14" drum brakes, cable operated. Coachwork consisted of saloons by Cross & Ellis and Holbrook, plus tourers and dropheads also by Cross & Ellis. The Firefly engine was of 69x100 mm, 1496 cc, essentially the Speed 20 less the front two cylinders with the bore reduced to bring the capacity under 1½ litres and give the familiar 11.9 hp RAC rating. The timing chain was still at the rear of the engine, and a single SU carburetter sufficed. A unit mounting gearbox externally identical to the SA Speed 20 but with different ratios was used on the earlier Fireflies (SA 11.9), whilst for 1933 and 1934 (SB 11.9) ENV preselector units were listed.
The Firefly continued the high quality four-cylinder tradition and has long been derided in some books for being overweight. The writer suspects that some of these opinions are carried on from one author to the next and that those repeating them have never actually driven the cars. In fact they go very well indeed by comparison with others of the era, and at least one Firefly is currently campaigned with great success at the tough, competitive end of historic rallying. The Firefly was a particularly good-looking, small (by Alvis standards) car, and deservedly enjoys an enthusiastic following. The lowered chassis and relatively wide track confer excellent roadholding qualities and the willing little engine is easy to work on.
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