# Allison V-3420

The Allison V-3420 was a large experimental piston aircraft engine, designed in 1937 by the American Allison Engine Company.

## Design and development

In 1937, at the behest of the United States Army Air Corps, the Allison Engine Company agreed to design and build a large-displacement high-power aircraft engine. The resulting V-3420 was essentially a pair of 12-cylinder Allison V-1710 engines mated to a common crankcase with a 30° angle between the inner cylinder banks. The crankshafts of the two V-1710 engines were geared together to drive a common propeller shaft. Most V-3420 parts were interchangeable with those for V-1710-E and -F engines.
The V-3420 had a power-to-weight ratio of 1.6 kW/kg or 1 hp/lb, excellent for its time. It was envisioned as a powerful yet compact engine for several advanced USAAF projects of the day, including the Douglas XB-19, the Boeing XB-39 Superfortress, the Lockheed XP-58 Chain Lightning, and the Fisher P-75 Eagle. As none of these designs saw more than limited production, only about 150 V-3420s were built.

## Variants
### V-3420-A16L (-13)

    left-hand rotation of propeller, single-stage supercharger with single-stage turbocharger and intercooler

### V-3420-A24

    Supercharger ratio 7.2:1

### V-3420-B (-23)

    Similar to the -A but with mechanically driven supercharger in two variable-speed stages

## Applications

* Boeing XB-39 Superfortress
* Douglas XB-19
* Fisher P-75 Eagle
* Lockheed XP-58 Chain Lightning

## Engines on display

* A V-3420-23 (B10) is on public display at the Aerospace Museum of California in North Highlands, California.
* A V-3420 is on public display at the Glenn H. Curtiss Museum in Hammondsport, New York.
* A V-3420-23 engine is on public display at the National Museum of the United States Air Force in Dayton, Ohio alongside a Fisher P-75 Eagle

## Specifications (V-3420-A18R (-17))

Data from Aircraft Engines of the World 1946

### General characteristics

* Type: 24-cylinder turbosupercharged double-"Vee" liquid-cooled piston engine
* Bore: 5.5 in (139.7 mm)
* Stroke: 6 in (152.4 mm)
* Displacement: 3,420 in$^3$ (56 L)
* Length: 97.7 in (2,482 mm)
* Width: 60.0 in (1,421 mm)
* Height: 38.7 in (863 mm)
* Dry weight: 2,655 lb (1,204 kg)

### Components

* Valvetrain: Single overhead camshaft per 6-cylinder block, two intake and two exhaust valves per cylinder, sodium-cooled exhaust valves.
* Supercharger: Single-speed one-stage gear-driven supercharger
* Turbocharger: General Electric turbocharger with intercooler
* Fuel system: Bendix-Stromberg PR-58B3 three-barrel injection-type downdraught with automatic mixture control.
* Fuel type: 100/130 Octane
* Oil system: Pressure fed at 60-70 psi (4.2 - 4.9 kg/m$^2$)
* Cooling system: Liquid-cooled with a mixture of 70% water and 30% ethylene glycol, pressurized.

### Performance

* Power output:

#### Take-off

    2,600 hp (1,940 kW) at 3,000 rpm at 46.0 inHg / +8.0 Lb boost for 15 minutes

#### Military

    2,600 hp (1,940 kW) at 3,000 rpm at 25,000 ft (7,625 m) for 15 minutes

#### Normal

    2,100 hp (1,565 kW) at 2,600 rpm at 25,000 ft (7,625 m) maximum continuous power

#### Max. Cruising

    1,575 hp (1,175 kW) at 2,300 rpm at 25,000 ft (7,625 m)
* Specific power: 0.76 hp/cu in (34.6 kW/L)
* Compression ratio: 6.65:1
* Specific fuel consumption: 0.41 lb/hp/hr (249.4 g/kW/hr)
* Oil consumption: 0.025 lb/hp/hr (15.2 g/kW/hr)
* Power-to-weight ratio: 1.0 hp/lb (1.64 kW/kg)