A TECHNICAL DESCRIPTION OF THE ALVIS ” FIREFLY “
THE appearance of the ” Speed 20″ early in 1932 was an event of the first importance for fast motorists. The low centre of gravity made possible by the double drop frame, the excellent brakes and unit construction of engine and gearbox were all highly successful developments. All these features and various others which will be enumerated were used in bringing the four-cylinder ” Firefly” into a pasition as advanced as that of the six cylinder car. The cyliader dimensions of the” Firefly” are the usual 69 and 100 mm., giving a capacity of 1,496 c.c., with an R.A.C. rating of 11.9. The cylinder head is detachable, and the cast-iron cylinder block is bolted to an aluminium crankcase. The overhead valves are operated by push-rods and the rockers and ball-ends are positively lubricated. A semi-downdraught S.U. carburettor, supplied by a petrol pump, is bolted to a V shaped induction pipe, which is in contact with a flange on the centre exhaust branch to form a hot-spot. The exhaust gases are led away from the front end of the engine, avoiding any possibility of overheating in the driving compartment. Coil, distributor and plugs are all accessible on the off-side of the engine. The distributor embodies an automatic advance
and retard with an additional hand control on the steering column. The aluminium alloy pistons carry 2 compression rings and 1 scraper. The bigend bearing surfaces, which are of white metal, are cast integral with the steel connecting rods and the three bearings for the balanced crankshaft are carried in webs extending the whole width of the crankcase. The camshaft is driven by chain from the rear end of the crankshaft, a lay-out which theory approves, but which finds a place on all too few cars. Its practical value is shown by the fact that on engines fitted with this rear dual location timing chains last at least four
times as long as when the drive is taken from the forward end. The torque damper which was fitted to the 12-60 engine has been found unnecessary on the “Firefly ” and is therefore not fitted.
The engine is cooled on the thermosyphon system, and the heat is dissipated by a large honeycomb radiator. The cooling water passes from the block to the cylinder-head through an aluminium casing at the rear of the engine, and there are no passages which depend for their tightness on the cylinder-head gasket. The flow of all the cooling water along the whole length of the engine and back, and the generous water spaces round the hot parts of cylinder and head should give complete freedom from local overheating, a most important consideration in the case of a small high-efficiency engine.
The oil pump is gear-driven from the camshaft and forces oil through a Tecalemit filter to the various bearings and there is another filter on the suction side. A gallery along the side of the engine supplies the main bearings with oil, whence it travels to the big-ends through the drilled crankshaft. Filter and oil filler are accessibly placed on the near side of the engine.
Engine, clutch pit, and gearbox are bolted together to form one unit of immense strength and the whole is flexibly mounted at three points, one in front of the engine, and the other two brackets at the rear end of the clutch housing. A single dry-plate clutch is used, and the four-speed gearbox has a silent third gear and a centre ball change brought back so that the lever is under the driver’s left hand. The speedometer is driven from the gearbox.
The propellor shaft is of the open type, tubular, and of large diameter. It is fitted with Hardy Spicer joints front and rear. The final drive is by spiral bevel, and the fully floating rear axle, a feature not often found nowadays, should ensure a long life for this much-stressed part of the transmission. The chassis frame is double-dropped following the well-known lines of the Speed Twenty, but the dimensions are naturally
somewhat less, the Firefly’s wheelbase and track being respectively 9ft. 14ins. and 4ft. 4ins. against 10ft. 3ins. and 4ft. Sins. on the larger car. The double-dropped frame allows of a low floor line without restricting head room, brings down the centre of gravity very considerably and allows plenty of movement for the axles without striking the side members The latter are braced by six cross members, most noticable of these being the pressing which unites the side members at the front mounting of the rear springs. Half elliptic springs are fitted all round, with Hartfords back and front, those at the rear being carried parallel with the axle. A Marles-Weller steering box is used, with an unusually long drop arm. The brakes are identical with those fitted to the “Speed 20” and operate on 14 inch drums. The shoes are expanded by a floating lever which carries two pins. When the lever is moved by the control cable, one pin bears against the end of each of the two shoes. The floating layout ensures that each shoe exerts the same pressure, and the brakes have a self-energising action. Another good feature is that there is no joint or lever exposed to dirt and wet. A knurled knob projecting through the floorboards at the
driver’s side operates a master-adjustment, while there is an adjustable cable stop on each brake-cable to allow the effect of all four brakes to be equalised.
Wire wheels with knock-on single centre nuts carry 30in. by Sin. Dunloptyres.
The electrical equipment is of the normal type, the dynamo being driven from the timing chain. Rotax head lamps. are used and the two six-volt batteries are arranged one on either side of the propellor shaft. The lights are controlled by a lever in the centre of the steering wheel, and the switching mechanism is accessibly mounted on top of the steeringbox.
Though the chassis is shorter than that of the “Speed 20,” the 4 cylinder engine of the Firefly actually allows a greater amount of space to be devoted to passenger accommodation than on the larger car. A particularly handsome saloon with a sunshine roof, and also a drop-head coupe are offered at £495, while the open four-seater costs £20 less. A road-test of this latter model appears elsewhere in this issue, and shows that the traditional high performance and comfortable travel which one expects in an Alvis car has been more than maintained.
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