by F. W. Stiles
late Managing Director of Alfa-Romeo
British Sales, Ltd.
PART V —
The Supercharged 2nd Series 1¾-litre
Before we leave the 1,750-c.c. models I must describe the second-series cars, the first of which was produced early in 1930, as a development of the 1929 Tipo 6C 1½ and 1¾-litre supercharged models. It was three of these second-series cars which I entered for the 1930 T.T. and which, as mentioned in the previous instalment, finished in the first three places, taking the Team Prize. The three cars actually finished within 16 seconds of each other.
The main differences between the second-series cars and the earlier series were:
(a) A larger Roots supercharger, mounted horizontally, and driven at engine speed, the rotor-gears being driven by an internally-toothed gear in place of the former straight-toothed gear, resulting in quieter running.
(b) A shorter finned induction pipe to suit the new supercharger, and twin-type Memini carburetter on the off side of the supercharger, which gave the engine a cleaner appearance.
(c) A larger clutch, first used for the 1980 Targa Florio race.
(d) Improved bonnet fasteners, later-type Bosch ignition switchbox, fuse boxes mounted on either side of the 1¾-gallon reserve fuel tank on the pressed steel dash, and quick-action filler cap for the rear fuel tank.
The wheelbase remained at 9 ft. in., although a few 9 ft. 6 in. wheelbase chassis were produced. The bodies fitted were usually two and four-seaters, but occasionally closed bodies were supplied. The bigger supercharger gave an increased boost at maximum engine speed and these second-series 1,750-c.c. cars had outstanding acceleration and could reach 100 m.p.h. in road trim. The compression ratio was 5.75 to 1. A typical two-seater weighed 16 cwt. 8 qr. 24 lb. and axle-ratios of 4.08, 8.8, 8.7 and 3.5 to 1 were provided (12 by 49, 18 by 51, 18 by 40 and 13 by 46, respectively). The constant-mesh gears of the four-speed gearbox were 17 by 27.
The rear bearings and gears of the new supercharger were lubricated by a duct formed in the cylinder block, which took oil from the supply to the valve gear. The supercharger front bearings were lubricated through a Tecalemit nipple on the detachable front cover. The cover should be removed occasionally and the bearings packed with petroleum jelly. The five-bearing counter-balanced crankshaft with 42 mm. journals was retained. The new clutch had five male and five female plates as hitherto, but these were approximately a third larger, which gave smoother engagement and longer life, obviating, too, the tendency for the female plates to buckle under maltreatment. The need to occasionally dismantle the clutch and clean thoroughly the splines in both drum and boss remains, however, if maximum efficiency is to be achieved. The servicing notes detailed in Parts II and III apply to the second-series 1,750-c.c. cars. For racing, use 75 per cent. petrol, 25 per cent. benzole, with 2 per cent. of light mineral oil. Petrol should be of .760 density. The oil pipe to the camshaft bearings must be slightly proud of the block, to keep lubricant out of the water jackets. We used Champion R.3 plugs for racing, R.1 for touring, with plug gap set .020 in., or wider with a heavy-duty or “Oilcoil” coil. Valve timing is: Inlet opens seven degrees before t.d.c., closes 59 degrees after b.d.c.; exhaust opens 51 degrees before t.d.c., closes 15 degrees after b.d.c. T.d.c. is indicated by an arrow on the flywheel. Tappet settings are 0.012 in. inlet, 0.014 in. exhaust, engine cold; set these before re-timing. Oil pressure should be 2½ kg. (40 lb./sq. in.) at maximum revs., and ½ k.g. when idling, using Prices C, Castrol XXL or BB. Distributor points should have 0.0012 in. to 0.0016 in. gap, carburetter settings are as given in Part IV. Run 5.25 by 18 tyres at 28 lb./sq. in. When overhauling the engine completely, remove the copper plugs from out all sludge, using methylated spirits as a solvent if necessary. Wash out afterwards; the plugs may prove somewhat tricky to refit.
With English coachwork the pear-shaped rear fuel tank was sometimes lowered slightly, but normally it sat on brackets above the chassis, the spare wheel being carried on these brackets.
Early in 1932 a few detuned second-series chassis came to England fitted with gearboxes having synchro-mesh on third gear. These chassis were usually supplied with Italian two-seater coupé bodies and about the same time a 10 ft. 2 in. wheelbase “Gran Turismo Supercharged” chassis was made, also with the synchromesh third speed and with the springs mounted outside the side-members.