Fiat AS.6

The Fiat AS.6 was a liquid-cooled, 60-degree V-24 aircraft engine developed by Fiat Aviazione in the late 1920s as Italy's most powerful gasoline-fueled racing powerplant, featuring a displacement of 3,067 cubic inches (50.256 liters) and capable of producing up to 3,100 horsepower at 3,300 rpm with supercharging.^[1]^[2] Designed by engineer Tranquillo Zerbi under a commission from Italy's Ministry of the Air Force, it combined two Fiat AS.5 V-12 engines in tandem with independent crankshafts to drive coaxial contra-rotating propellers, addressing the need for extreme performance in seaplane competitions like the Schneider Trophy.^[3]^[1] Initial development faced challenges, including backfiring issues during early 1931 tests that contributed to crashes of prototype Macchi M.C.72 seaplanes, but these were resolved by 1932 through modifications like a ram air intake system devised by engineer Rod Banks, enabling reliable operation.^[1]^[2] The engine's key specifications included a bore of 5.4 inches (138 mm) and stroke of 5.5 inches (140 mm) per cylinder, a compression ratio of 7:1, dual Marelli magnetos for ignition, and a supercharger spinning at up to 19,000 rpm to deliver boost pressures of 11.5 psi (0.79 bar) in sprint mode; it weighed 2,050 pounds (930 kg) and measured 132.5 inches (3.365 m) in length.^[1]^[2] Primarily powering the Macchi-Castoldi M.C.72 floatplane, the AS.6 enabled historic achievements, including a world speed record of 423.83 mph (682.08 km/h) set on April 10, 1933, by Warrant Officer Francesco Agello, followed by Agello's absolute record of 440.682 mph (709.209 km/h) over a 3-kilometer course on October 23, 1934, at Lake Garda, Italy—a mark that stood until 1939.^[1]^[2]^[4] Only 12 units were produced, with three surviving today: one mounted on the record-setting M.C.72 MM.181 at the Museo Storico dell’Aeronautica Militare in Vigna di Valle, another restored exhibit at the same museum using parts from engines 7 through 11, and a third at Fiat's Centro Storico in Turin.^[3] In a notable recent event, a restored AS.6 roared to life on October 22, 2023, at Idroscalo di Desenzano after over 5,500 hours of work by the "Il Magnete" Association, highlighting ongoing efforts to preserve Italy's aviation heritage.^[3]

Development

Origins

In the late 1920s, the Italian Ministry of the Air Force commissioned Fiat to develop a high-performance aircraft engine specifically for competing in the 1931 Schneider Trophy seaplane races, aiming to secure victory against international rivals, particularly Britain.^[1]^[3] The initial requirements stipulated a minimum power output of 2,300 hp (1,700 kW), with scalability up to 2,800 hp (2,090 kW), while keeping the total weight under 930 kg (2,050 lb) to ensure compatibility with racing seaplanes.^[1] This engine, designated the AS.6, was intended primarily for the Macchi M.C.72 racing seaplane and represented an evolution from Fiat's earlier AS.5 V12 engines, with its technical configuration derived briefly from coupling two AS.5 units.^[1] Development progressed rapidly, culminating in the first bench run around 1930, which initiated extensive testing to meet the demanding performance criteria.^[1]

Design and Engineering

The Fiat AS.6 employed a distinctive V-24 configuration, created by coupling two Fiat AS.5 12-cylinder liquid-cooled V-12 engines back-to-back into a shared magnesium crankcase, resulting in a 60° dual overhead camshaft (DOHC) layout.^[2] This innovative arrangement featured two mechanically independent crankshafts—one hollow shaft nested within the other—driving contra-rotating propellers through a central spur-gear reduction system, which helped mitigate torque effects at extreme speeds.^[2] Each of the 24 cylinders measured 138 mm bore by 140 mm stroke and included four valves—two intake and two exhaust—for optimized airflow and combustion efficiency.^[1] The engine's total displacement reached 50.256 liters (3,067 cubic inches), enabling substantial power output from its liquid-cooled design.^[2] A key component was its single-stage centrifugal supercharger, driven by the rear engine section and operating at around 19,000 rpm to deliver up to 11.5 psi (0.79 bar) of boost pressure, though it consumed approximately 200-250 horsepower in the process.^[1] The supercharger fed a common induction system with an eight-jet carburetor, ensuring balanced fuel distribution across the coupled units despite the complexity of the shared manifold.^[2] Early development faced a significant engineering hurdle: severe backfiring during high-speed ground and flight tests, triggered by the ram air effect from airflow exceeding 400 mph (644 km/h), which leaned out the fuel mixture and caused predetonation in the intake tract.^[2] Fiat engineers addressed this by 1932 through redesigned carburetion for richer mixtures under ram conditions, modified intake geometry to counter the pressure imbalance, and adjusted ignition timing; these fixes were informed by wind tunnel simulations replicating race speeds and consultations with British specialist Francis Rodwell Banks.^[1] While the AS.6's architecture paved the way for subsequent Fiat designs, such as the AS.8 V-16 engine that adapted similar cylinder and valvetrain features for broader aviation use, the AS.6 itself stayed optimized for racing, prioritizing peak power over reliability or production scalability.

Operational History

Aircraft Applications

The Fiat AS.6 engine's primary and sole major application was in the Macchi M.C.72 floatplane racer, an experimental seaplane developed by Aeronautica Macchi for high-speed competitions over water.^[1]^[5] This Italian design, intended for events like the Schneider Trophy, relied on the AS.6 to achieve exceptional velocities in racing contexts, with no documented use in other aircraft models.^[6] In the M.C.72, the AS.6—a 24-cylinder V-configured unit formed by coupling two AS.5 V-12 engines in tandem—was centrally mounted within the fuselage, driving a pair of coaxial contra-rotating propellers via independent drive shafts from each engine section.^[1]^[5] This configuration minimized torque effects and enhanced propulsive efficiency for the floatplane's streamlined hull and wing-mounted surface radiators, optimizing it for low-drag operations on and above water surfaces.^[6] The setup shared a common induction system and supercharger, ensuring synchronized performance during sustained high-speed runs. Production of the AS.6 was extremely limited, with only five M.C.72 prototypes (serial numbers MM 177 to MM 181) equipped with the engine, built exclusively for racing purposes and without adaptations for military combat or civilian transport roles.^[1]^[5] Initial flight testing commenced in the summer of 1931, but persistent reliability challenges—including backfiring from lean fuel mixtures due to ram air effects, ignition failures, and supercharger issues—postponed full operational deployment until refinements were completed in 1933.^[1]^[6] By 1933–1934, the engine had achieved sufficient stability for demanding applications.^[5] The AS.6's peak power output enabled the M.C.72's participation in international speed record pursuits during this period.^[1]

Speed Records

The Fiat AS.6 engine achieved its first major success on October 23, 1934, when Italian pilot Francesco Agello set a world absolute speed record of 709.209 km/h (440.682 mph) over a 3 km course, flying the Macchi M.C.72 seaplane powered by the engine.^[7]^[8] This record, certified by the Fédération Aéronautique Internationale (FAI), remains unbeaten as of 2025 for piston-engined seaplanes.^[9] Prior to this, Agello had established an earlier benchmark on April 10, 1933, achieving 682.078 km/h (423.824 mph) over the same 3 km course with the M.C.72, marking the engine's initial world speed record.^[1]^[10] Agello secured additional seaplane class records in 1934, further validating the AS.6's performance capabilities.^[11] All these achievements took place at Lake Garda, Italy, where the engine's reliability was demonstrated following modifications to address high-speed backfiring issues caused by lean fuel mixtures and predetonation.^[1]^[12] These records, powered by the AS.6's V-24 configuration delivering over 3,000 horsepower, solidified Italy's dominance in 1930s air racing following the Schneider Trophy competitions.^[1]^[8]

Preservation

Surviving Examples

At least three examples of the Fiat AS.6 engine are known to survive, representing rare remnants of the limited production run of 12 units built in the early 1930s.^[3] One is installed in the Macchi-Castoldi M.C.72 seaplane MM.181, which set the 1933 world absolute speed record, and is preserved at the Museo Storico dell'Aeronautica Militare (MUSAM) in Vigna di Valle, Italy, where it has been on display since 1935.^[1]^[3] This engine, with its internal condition unknown, includes original components from the record-setting aircraft and serves as a static display alongside the historic seaplane.^[3] A second, complete example—assembled from parts of multiple original AS.6 engines—is also preserved at MUSAM in Vigna di Valle, having been placed on static display there since 1977.^[3] Another example, though incomplete and likewise assembled from various engines without a known serial number, is held at the Centro Storico Fiat in Turin, Italy, as part of the company's aviation heritage collection.^[3] These static displays highlight the engine's pivotal role in powering the M.C.72 to Schneider Trophy and speed records, underscoring their value as key artifacts of Italian interwar engineering innovation.^[1]^[3]

Recent Restorations

In 2023, a significant restoration project brought one of the rare surviving Fiat AS.6 engines back to operational condition after more than 80 years of inactivity. Led by the Italian aviation heritage organization "Il Magnete" under Leonardo Sordi, the two-year effort involved approximately 5,500 hours of work, including disassembly, extensive historical research, reconstruction of components, and reassembly. The project assembled parts from multiple engines (numbered 7-8-9-10 and 11), incorporating AS.5 cylinders to complete the V-24 configuration, while addressing challenges such as sourcing compatible elements for the liquid-cooling system and fabricating custom ignition magnetos and spark plugs.^[3] The restored engine was successfully started for the first time since the 1930s on October 22, 2023, producing its distinctive roar during a commemorative event that highlighted the engine's role in powering the Macchi-Castoldi M.C.72 to a world speed record of 440.682 mph (709.209 km/h) in 1934. This achievement overcame issues related to long-term corrosion, deteriorated fuel systems, and obsolete ignition components, marking a rare revival of 1930s Italian aero-engineering technology. Contributors including Andrea Rossetto, Giuseppe Barbetta, Davide Olivati, and Gregory Alegi supported the technical and documentation aspects, ensuring fidelity to original specifications.^[3] This restoration underscores the enduring fascination with the Fiat AS.6 among aviation preservationists, demonstrating the feasibility of reactivating such complex, high-performance engines from the interwar era. As the most powerful gasoline-fueled racing engine produced in Italy, its successful firing highlights potential for future heritage demonstrations, though no additional runs have been reported as of 2025. The project not only preserves technical knowledge but also connects modern enthusiasts to the legacy of Schneider Trophy-era innovations.^[3]

Specifications (AS.6)

General Characteristics

The Fiat AS.6 is a 24-cylinder, 60° V, liquid-cooled, supercharged piston aircraft engine manufactured by Fiat Aviazione.^[1] It was produced in limited numbers during the early 1930s specifically for high-performance racing seaplanes, such as the Macchi M.C.72.^[1] The engine's configuration consists of two mechanically independent Fiat AS.5 V-12 units coupled through a central gear case with coaxial output shafts driving contra-rotating propellers.^[1]^[13] Key physical dimensions include a bore of 138 mm (5.4 in) and a stroke of 140 mm (5.5 in), yielding a total displacement of 50.256 L (3,067 cu in).^[1]^[13] Length: 3.365 m (132.5 in); width: 0.702 m (27.6 in); height: 0.976 m (38.4 in). The dry weight is 930 kg (2,050 lb), reflecting its robust construction for extreme power outputs.^[1]^[13]

Components

The Fiat AS.6 features a V-24 layout formed by two coupled V-12 engine halves sharing a central gear case.^[13] Its valvetrain incorporates four valves per cylinder—two for intake and two for exhaust—actuated by dual overhead camshafts mounted in the cylinder heads.^[12]^[13] The supercharger is a single-stage centrifugal unit mounted on the rear engine half, geared to achieve rotational speeds of up to 21,000 rpm and deliver a boost pressure of 0.79 bar (11.5 psi).^[1]^[13] The fuel system employs two inverted four-barrel carburetors positioned ahead of the supercharger to supply a homogenized mixture to all cylinders via a shared induction tract, supplemented by special fuels containing anti-detonation additives such as TCP for high-performance operation.^[1]^[14]^[13] Cooling is provided by a liquid system using water as the primary fluid, circulated by four dedicated pumps (one serving each row of six cylinders) to surface radiators designed with streamlined profiles to minimize aerodynamic drag in racing configurations.^[13] The propeller drive system supports contra-rotating propellers through a central reduction gear housing that transmits power via two coaxial hollow shafts, with the forward V-12 driving the rear propeller and the rear V-12 driving the forward one to counteract torque.^[13]^[1]

Performance

The Fiat AS.6 engine delivered a takeoff power output of 2,300 hp (1,715 kW) for one hour duration in its early configuration.^[1] With modifications including increased supercharger boost to 11.5 psi (0.79 bar), the sprint version achieved up to 3,100 hp (2,312 kW) at 3,300 rpm.^[1] The engine featured a compression ratio of 7:1, which was optimized for the high-octane fuels available in the 1930s to support its high-performance operation.^[1] Operational limits included a maximum engine speed of 3,300 rpm, with reliable output at 3,000 hp (2,237 kW) attainable at 3,200 rpm under boosted conditions.^[12]