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Jacobs R-755 / L-4



Jacobs R-755



The Jacobs R-755 (company designation L-4) was a seven-cylinder, air-cooled, radial engine for aircraft manufactured in the United States by the Jacobs Aircraft Engine Company.



The R-755 was first run in 1933 and was still in production in the 1970s. With a bore and stroke of 5.25 in × 5 in (133 mm × 127 mm) the displacement was 757 cu in (12.4 L), power ranged from 200 hp to 350 hp (150 kW - 260 kW). The engine features steel cylinders with aluminum-alloy cylinder heads. An R-755E variant was developed for use in helicopters.



The Jacobs  must be primed on every start; two or three strokes on warm days, and five to seven strokes on cold days. Then, with mixture "Full Rich," carb heat at Cold, and prop set at "Low rpm" (to keep from taking too much oil from the engine), master switch "On," throttle cracked, and prop clear, turn the ignition key to "Battery Start- and press the starter button. As soon as the engine fires, turn the ignition to "Both Run". Another shot or two of prime may be needed to keep the engine running at first.
During the take-off run, it needs full right rudder trim to handle the torque, and the 195 will leave the ground in a slightly tail-low attitude at about 60 mph, using about 900 feet of runway. You are allowed full throttle for one minute with this engine; but a good cruise-climb with 2,000 rpm at 23 inches manifold pressure at 115 mph will usually produce around 500-600 fpm.


Anahuac Tauro
Avro Cadet
Beechcraft Staggerwing
Boeing-Stearman Model 75
Cessna AT-17
Cessna 195
Funk F-23
Grumman Ag Cat
Kellett KD-1
Morane-Saulnier MS-505
Waco CG-15
Waco Custom Cabin series (YKC, YKC-S etc.)
Waco Standard Cabin series (YKS-6 etc.)


Type: 7-cylinder air-cooled radial piston engine
Bore: 5.25 in (133 mm)
Stroke: 5.0 in (127 mm)
Displacement: 757 cu in (12.4 L)
Length: 27.25 in (692 mm)
Diameter: 44 in (1,188 mm)
Dry weight: 505 lb (229 kg)
Valvetrain: 2 valves per cylinder, pushrod-actuated, Sodium cooled exhaust valves
Fuel system: Single Stromberg NA-R7A carburetor
Fuel type: 73 octane
Oil system: One pressure pump, two scavenge pumps
Cooling system: Air-cooled
Reduction gear: Direct drive, right hand tractor
Power output: 245 hp (183 kW) at 2,200 RPM at sea level
Specific power: 0.32 hp/cu-in (14.8 kW/L)
Power-to-weight ratio: 0.48 hp/lb (0.8 kW/kg)



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