The last FIAT racing engine described is the mysterious type 406 1 1/2-litre with two six-cylinder blocks in line and geared-together crankshafts. Destined for the model 806 car, development on this engine was began in 1927. With it FIAT Won the last G.P. race in which they ever appeared, on the car’s sole appearance. The Occasion Was the 1927 Milan Grand Prix—Bordino won his heat at 92.88 m.p.h., the race itself at 94.57 m.p.h., beating Campari’s P2 Alfa-Romeo and lapping at 96.59 m.p.h. on a wet Course. So the details Motor ltalia provides and the Antique Automobile translates are of exceptional interest. The bore and stroke were 50 by 63 mm. and the cylinder structure as before, with a single crankcase for each row of three blocks of two. The inclined o.h. valves were of 30 mm. overall, 27 mm, face dia and had a lift of 7 mm. Each had three guided springs. Valve operation was by three o.h. camshafts, the centre one actuating the inlet valves of each bank, the outer ones the exhaust valves, via fingers in each instance as in the other FIAT engines. The camshafts, running in plain bearings, were driven from the rear of the crankshaft, via Oldham couplings, by one very large and five small spur gears on anti-friction bearings. A Roots supercharger was mounted centrally at the front of the engine and driven from the r.h. crankshaft by a pinion meshing with one of the actual rotor gears via a multi-plate clutch. Three pinions united the two crankshafts, which were of built-up type by Hirth, running each in four plain bearings. Normal plain big-ends were used. The main and big-end journals were 40 mm. in dia., the former 30 mm. long except for the front bearing, which was 32 mm, long, the latter 41 mm. The connecting rods were 5.118 in. long, or over 2 times stroke.
The pistons were again supported by their rings, of which there were two per groove, in three grooves per piston; 18 mm. sparking plugs extended well into the hemispherical combustion chambers. The oil pump, at the back of the engine, ran at less than engine speed.
Vaglienti had a hand in the design of this remarkable engine. It weighed 381 lb. and the model 806 car in which it was installed turned the scales at just over 13 cwt. Maximum speed is quoted as 149 m.p.h. The engine developed 173 b.h.p. at 7,500 r.p.m. and 187 b.h.p. at 8,500 r.p.m., figures that are truly remarkable. A test in July, 1927, shows 160 b.h.p, at 8,000 r. p.m. at a manifold pressure of 12.52 psi., the temperature of the compressed mixture being 162° F. (72° C.). This was an experimental engine and it is confidently stated that the aforementioned 187 b.h.p., equal to a power/weight ratio of over 1 h.p. per kg., was subsequently attained. Compared with the present-day 1 1/2-litre engine with two-stage supercharging, which gives 300 – 400 b.h.p. on special fuels, the output of this 1927 twelve-cylinder FIAT unit still ranks as exceedingly noteworthy.—W.B.