Fiat AS.6

The Italian 1931 Schneider Trophy entrant, the Macchi M.C.72, required an engine of 2,300 horsepower (1,700 kW) with the capability of producing up to 2,800 horsepower (2,090 kW) while having a weight of not more than 840 kilograms (1,850 lb). The contract was awarded to Fiat, but their most powerful V engine to that date was the 1,000-horsepower (750 kW) 12-cylinder Fiat AS.5 that had also been used for Schneider Trophy racing.

The solution was to couple two AS.5 engines in tandem to produce a V-24 with each engine independently driving one of a pair of contra-rotating propellers through co-axial shafts. The rear engine drove the front propeller through a reduction gearbox, located between the two engines, up to a shaft that passed between the cylinder banks of the reverse-mounted forward engine. The latter's crankshaft drive faced rearwards and drove the rearmost propeller in a likewise manner, but instead via a hollow shaft through which the rear engine driveshaft passed. A large rear-mounted supercharger was used to boost the engine, with the fuel/air mixture being delivered to the cylinders through a manifold that was centrally mounted between the cylinder banks of both engines.
 
The apparent advantages were that this configuration maintained a small frontal area, and that Fiat already had previous experience with the AS.5, while having a combined displacement of 50 litres (3,066 cu in) it was thought that the power requirement would be easily met.
 
In practice the engine was plagued with technical problems that led to the Italian team being unable to compete in the 1931 Schneider Trophy event. On 23 October 1934 however, after backfiring problems had been solved with the assistance of the British engineer "Rod" Banks, the AS.6 was used by Francesco Agello to set a new airspeed record for piston-powered seaplanes at 709.2 kilometres per hour (440.6 mph), a record that still stands today.

Applications
Macchi M.C.72

Specifications:

AS.6
Type: 24-cylinder V-engine
Bore: 138 mm (5.4 in)
Stroke: 140 mm (5.5 in)
Displacement: 50.25 L (3,066 cu in)
Dry weight: 930 kg (2,050 lb)
Valvetrain: Two intake and two exhaust valves per cylinder
Supercharger: Single-stage, centrifugal running at up to 19,000 rpm. Boost pressure 1.82 bar.
Fuel system: Carburettor
Cooling system: Liquid-cooled
Power output: 3,100 hp (2,300 kW) at 3,300 rpm
Compression ratio: 7:1
Power-to-weight ratio: 2.47 kW/kg (1.5 hp/lb)