by F. W. Stiles
late Managing Director of Alfa-Romeo British Sales, Ltd.
Part III — The Supercharged and Non-Supercharged Twin-o.h.c. 1 1/2-litre Models
The non-supercharged twin-o.h.c. version of the 1 1/2-litre Alfa-Romeo was first produced early in 1928, and was marketed in this country with bodywork by Young, Vanden Plas and Hoyal, a limited number of cars being imported with Italian coachwork.
Known as the Tipo 6C, this car was enhanced by more lively acceleration and a higher maximum speed than its predecessor, lower axle and gearbox ratios being used, the constant-mesh ratio being 16:28, and the axle ratio 10:54, although other ratios were available if required. The chassis, however, with a wheelbase of 9 ft. 6 in. and a track of 4 ft. 7 in., was practically the same as that of the 1 1/2-litre single o.h.c. car, although the overall radiator dimensions were smaller and, as the engine showed no signs of overheating, the friction-driven fan was dispensed with.
The twin camshafts were driven by gears from the vertical shaft at the rear of the engine and the valves, in hemispherical combustion spaces in the detachable cast-iron head, were actuated direct. Three valve springs per valve were used, inner, outer and an anti-bounce spring. Each camshaft ran in four plain bearings, the crankshaft in five bearings, the former being bronze-lined, with cast-iron caps. The camshafts were lubricated by an oil supply tapped from the main supply and brought via a tube in the cylinder block casting to steel pipes secured to the head. Surplus oil returned to the sump via the camshaft gears. Otherwise drives, tappets, etc., were similar to those of the single-camshaft engine.
The alloy pistons had two compression, one bevelled and one oil-control ring each and were of Italian manufacture. These were often replaced by Specialloid and Wellworthy pistons, the standard compression-ratio being 5.75 to 1. Carburation was by a double Zenith, with twin 18-mm. choke tubes, a 75 main jet and 100 compensator jet, although an 80 main and 95 compensator were sometimes substituted. In a few cases a special cast induction pipe and two S.U.s were used, which gave excellent results and 25 m.p.g.
The rear-springs were firmly attached to the axle by a steel anchor pin mounted in two bronze bushes and a lug cast on the axle banjo, the rear of the spring being shackled. On later models, in an attempt to improve roadholding, the springs were shortened and mounted on brackets about 2 in. from the end of the chassis frame. The nine-gallon fuel tank was now at the rear, feeding to an auxiliary dash tank, a 3/4-gallon reserve supply being provided. Italian Excelsior shock-absorbers were fitted, between the front dumb-irons on early cars, outside the dumb-irons on later models.
Early models had s.s wheels, later cars 5.25 by 18 tyres, either Pirelli Superflex or l.p. Dunlops, running at 28 lb./sq. in. Electrics are usually 12volt, single-pole Bosch, except when, in a few instances, Manelli equipment was substituted. Servicing data: Champion R3 plugs were recommended. The oil filter is at the end of the base chamber under the oil pump and removable by undoing two 10-mm. nuts and the small steel cover-plate. Oil pressure (hot) is 5 lb./sq. in. idling and 20-25 lb./sq. in. normally. Use Castrol XXL, Price’s C de Luxe, etc. The oil-pressure release valve on the base chamber can be adjusted by removing the cap and hexagon-headed nut in or out. The camshafts must be a first-class running fit in their bearings, otherwise oil will bypass them and run down the valve guides, resulting in oiling-up. Ignition timing should be 40 degrees advance before t.d.c. with hand control retarded and points just breaking with No. 1 piston at t.d.c. If the clutch refuses to free properly the fault is likely to be buckled female plates or powder from the composite male plates filling the splines of either the clutch boss, clutch drum or both. Dismantling and thorough cleaning and replacement of buckled plates is the solution.
The first supercharged 1 1/2-litre car arrived also in this country in 1928. It was practically identical to the unblown model, except that the engine was 15 in. further back in the chassis and a Roots-type supercharger was mounted at the front, its rotors driven at one-and-one-eighth engine speed by a straight-tooth pinion pressed on to the crankshaft and secured by a taper-pin.
The alloy inlet pipe was in two parts, feeding to the opposite side of the engine to that on which lived the six-branch exhaust manifold. The carburetter was a twin Memini, and the blower supercharged at 5 lb./sq. in. Flat-top pistons were used, reducing the compression-ratio to 5.25 to 1. Two spring-loaded blow-off valves were incorporated in the induction tract and slightly larger engine valves of special material and slightly altered cam contours were used. The gearbox had higher-ratio constant-mesh gears and the axle ratio was 12:54. Whereas the unblown car did around 75 m.p.h., the blown cars not very impressive in appearance, would do 90 m.p.h. on the road when carrying a light two-seater body, having detachable alloy panels on an angle-iron frame, a pear-shaped fuel tank above the chassis at the rear and carrying the spare-wheel bracket, and a 48-amp.-hr. battery in a metal container slung from the chassis. The acceleration was remarkably clean, as the fully-equipped two-seater weighed approximately 15 1/2 cwt. The fine all-round performance and splendid roadholding of these twin-cam cars rendered them very desirable, the controls accurate, the steering light and high-geared, the springing perhaps a trifle harsh by modern standards.
Major Coe drove a sports two-seater at Shelsley Walsh in 1928 and I later entered two cars, a supercharged model for Guilio Ramponi and an unsupercharged model for Carlo Bruno to drive, in the 1928 Essex M.C. Six-Hour Race. Ramponi won, covering 417.44 miles at 69.51 m.p.h. Bruno averaged 64.84 m.p.h. This encouraged me to enter for further races. Ramponi won the 1929 “Double Twelve” after an astonishingly close finish from Davis’ Bentley, the Alfa-Romeo covering 1,824 miles at 76 m.p.h. Then, at Phoenix Park in 1930, Eyston was second on handicap in the 1 1/2-litre race to Gillow’s Riley, averaging 74.83 m.p.h., Don fourth and Ivanowski sixth, dominating the 1 1/2-litre class, and in that year’s T.T. Kaye Don’s car set the 1 1/2-litre lap record to nearly 73 m.p.h. before crashing and catching fire; the 1,750-c.c. Alfa-Romeos finishing 1, 2 and 3 — but that is another story.