The Franklin O-425 (company designation 6AG/6V6) was an American air-cooled aircraft engine that first ran in the mid-1940s. The engine was of six-cylinder, horizontally-opposed layout and displaced 425 cu in (7 L). The power output was between 240 hp (179 kW) and 285 hp (213 kW) depending on variant. The O-405-13 (6V6-300-D16FT) of 1955 was a vertically mounted, turbocharged and fan cooled version for helicopters.

Variants

6AG6-245

Geared propeller drive at 0.623:1, 245 hp (183 kW) at 3,300 rpm, (XO-425-5).

6AGS6-245

Supercharged and geared, 250 hp (186 kW) at 3,200 rpm, (XO-425-3).

6V6-245-B16F

Vertically mounted, fan-cooled helicopter version, 245 hp (183 kW) at 3,275 rpm, (XO-425-1).

6V6-300-D16FT

Vertically mounted, turbocharged, fan-cooled helicopter version, 285 hp (213 kW) at 3,275 rpm, (O-425-13).

O-425-1

O-425-2

O-425-3

O-425-5

O-425-9

O-425-13

Applications

• Convair XL-13
• Republic YOA-15 (intended)
• Sikorsky S-52
• Sikorsky YH-18
• Stinson L-13

Specifications (O-425-5)

Data from Aircraft engines of the World 1950.^[1]

General characteristics

• Type: 6-cylinder air-cooled horizontally-opposed aircraft piston engine
• Bore: 4.75 in (120.7 mm)
• Stroke: 4 in (101.6 mm)
• Displacement: 425.3 cu in (6.97 L)
• Length: 47 in (120 cm)
• Width: 33.3 in (84.6 cm)
• Height: 30 in (76 cm)
• Dry weight: 405 lb (184 kg)

Components

• Valvetrain: 1 x exhaust and inlet OHV driven by pushrods and rockers
• Fuel system: 1 x Bendix-Stromberg PS-780 up-draught carburetor with automatic mixture control
• Fuel type: 80 Octane / US Army 2-103 all-purpose motor fuel
• Oil system: Pressure system at 55 psi (380 kPa)
• Cooling system: Air-cooled

Performance

• Power output:
  

• Take-off: 245 hp (183 kW) at 3,300 rpm
• Normal: 240 hp (180 kW) at 3,200 rpm at sea level
• Cruise: 168 hp (125 kW) at 2,840 rpm at sea level

• Specific power: 0.58 hp/cu in (26 kW/L)
• Compression ratio: 7.4:1
• Specific fuel consumption: 0.51 lb/hp/h (0.086 kg/kW/ks)
• Oil consumption: 0.015 lb/hp/h (0.0025 kg/kW/ks)
• Power-to-weight ratio: 0.606 hp/lb (0.996 kW/kg)
• Reduction gear: Planetary reduction gearing at 0.623:1