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The Pratt & Whitney R-4360 "Wasp Major" Engine

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A cutaway of the R-4360

4360 engine running on the test stand. The exhaust collectors have been removed.

The Pratt & Whitney Aircraft Division of the United Aircraft Corporation in East Hartford, Connecticut, concentrated during World War II on the manufacture of high-powered air-cooled radial engines. Military requirements for Pratt & Whitney engines during the war necessitated the opening of six company-owned manufacturing facilities and licensed production of Pratt & Whitney designs by four automotive companies and two other aircraft companies.

The remarkable R-4360 Wasp Major was developed by Pratt & Whitney beginning in 1942, but culminated twenty years of piston engine development at the company.

For all its mass, this 28-cylinder, 4,360 cubic inch powerhouse was the lightest engine for its horsepower of any aircraft powerplant of its time. It weighed 3,405 pounds but produced 3,650 horsepower, a ratio of .93 pounds per horsepower. It was fuel-injected, gear-supercharged, and featured one crankshaft with four throws, each throw connecting seven cylinders to the crankshaft. One of each throw's pistons was a "master" piston, having the connecting rod form the crank bearing, maintaining alignment of the bearing with the crankcase. The other six piston connecting rods were connected to the master rod's bearing with pinned hinge connections. Each cylinder had two spark plugs and was offset to maximize cooling as air moved across the engine.

The supercharger revolved six times for each revolution of the crankshaft and the propeller shaft turned once for every two revolutions of the crankshaft. The revolutions of the propeller had to be slowed to avoid exceeding the speed of sound at the tips, which would have drastically reduced propeller efficiency.

The R-4360 was a very reliable engine, but each flight usually required that aircraft mechanics re-tighten all engine mounting bolts! The Wasp Major was intended for use on the Boeing B-29D Superfortress, which ultimately evolved into the B-50 bomber project. The engine was subsequently used on several variants of the B-50, the Boeing C-97 Stratofreighter transport (which was based on the basic B-29 design), and the Douglas C-124 Globemaster II. Later versions of the engine were used on the Fairchild C-119 Flying Boxcar and Convair B-36 Peacemaker. The Engine Repair Shops at Hill Field repaired and overhauled variants of the Pratt & Whitney R-4360 Wasp Major engine during World War II and for years afterward.

The R-4360 engine was used to power various post-WW-ll USAF bombers, cargo-transport, and aerial tankers, including B-50, KB-50, B-36, C-97, KC-97, C-119, and C-124 aircraft. It represents the most technically advanced and complex reciprocating aircraft engine produced in large numbers in the United States. The passing of the KC-97 and C-97 series aircraft from Air Force inventory in the late 1970s marked the closing of the era of both the large piston and turbo-supercharger within the USAF

The Pratt & Whitney R-4360 Wasp Major was a large radial piston aircraft engine designed and built during World War II. It was the last of the Pratt & Whitney Wasp family and the culmination of its maker's piston engine technology, but the war was over before it could power airplanes into combat. It did, however, power the last generation of large piston-engine aircraft before the turbojet and turboprop took over.


Design and Development


The R-4360 was a 28-cylinder four-row radial engine. Each row of pistons was slightly offset from the previous, forming a semi-helical arrangement to facilitate cooling of the successive rows of cylinders. A mechanical supercharger geared at 6.374:1 ratio to engine speed provided forced induction, while the propeller was geared at 0.375:1 so that the tips did not reach inefficient supersonic speeds.

Although reliable in flight, the Wasp Major was maintenance intensive. Improper starting technique could foul all 56 spark plugs, which would require hours to clean or replace. As with most piston aircraft engines of the era, the time between overhauls of the Wasp Major was about 600 hours when used in commercial service.

Engine displacement was 4,362.50 in (71.5 L), hence the model designation. Initial models developed 3,000 hp (2240 kW), but the final models delivered 4,300 hp (3200 kW) using two large turbochargers in addition to the supercharger. Engines weighed 3,482 to 3,870 lb (1,579 to 1,755 kg), heavy but giving a power to weight ratio of 1.11 hp/lb (1.83 kW/kg), matched by very few engines.

Wasp Majors were produced between 1944 and 1955; 18,697 were built.




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Pratt & Whitney R-4360 Wasp Major

A sectioned Engine


Specifications (R-4360-51VDT)


General characteristics





Pratt & Whitney R-4360 Wasp Major
Model: R-4360 CB2
Type: 28 cylinders, 4 row radial, air cooled, geared drive, turbo-supercharged, 4 cycle. T.C.247.
Construction: 5-piece forged aluminum alloy crankcase. Cylinders with steel barrels and shrunk on forged aluminum alloy cooling muffs, and forged aluminum alloy heads. Cylinders arranged helically around crankcase to facilitate cooling. 1 inlet valve and 1 exhaust valve (Inconel "M") per cylinder actuated by push rods. 4-throw 1-piece counterbalanced crankshaft supported in 5 plain bearings. Planetary spur reduction gear, ratio 0.375:1. Single propeller shaft, with provisions for Hydromatic propeller. Torque-meter.Hydro-Aire control actuators.
Supercharger: Gear-driven 1-speed supercharger, ratio 6.374:1. Provision for General Electric CH9 1-sage turbo-supercharger. Aftercooler.
Fuel system: 1 Bendix-Stromberg PR-100B3 4-barrel downdraft injection type carburetor with mixture control. Optional equipment: 1 Chandler Evans 100CPB9 downdraft injection type carburetor. Provision for water injection, with Bendix Products K2 or M1 water regulator.
Ignition: 4 Bendix-Scintilla S14RN-15 low-tension dual magnetos. 2 18-mm long reach spark plugs, with 1 high-tension booster coil per spark plug, per cylinder. Shielded ignition system.
Lubrication: Pressure feed, 80-85lb./sq. in. (5,6-6,0 Dry sump.
Starter: Optional. Eclipse 36E07-2 electric starter can be used.

The above ratings are at 500 ft. (150 m) and 700 ft. (200 m), respectively.



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R-4360 ready to be installed.

R-4360 installed.

R-4360 On Display

R-4360-20: Military engine similar to R-4360-CB2, but without water injection. Same dry ratings.

Military engine similar to R-4360-CB2, but with direct fuel injection and other modifications. 3,800 h.p./2,800 r.p.m./take-off, wet. 115/145 grade gasoline.

Military engines similar to R-4360-53

Note: Ford Motor Co., Engine Division manufactured the R-4360-53,-59,-63 engines.

Cylinders... The cylinders which are similar to those of the R-2800 Series, are arranged helically around the crankcase and pressure baffles are provided for individual cylinders and for each bank of four. All cylinders are completely interchangeable.

Pistons... Generally the same as for the Double Wasp, with reinforced internal bosses.

Connecting Rods... Rod assembly for each row of seven cylinders consist of master rod with detachable cap, two piece lead silver bearing, and six I-section articulated or link rods. Each link rod has a bronze bushing at the piston end and rides on a silvered knuckled pin.

Crankshaft... One piece crankshaft of forged steel has four throws and is supported in the crankcase by five steel-backed lead-silver main bearings. Weights of the reciprocating parts connected to the crankpin are counterbalanced by two fixed and two bifilar counterweights.

Airscrew Shaft... Supported at the crankshaft end by a plain lead-bronze bearing and at the air-screw end by a roller-bearing to carry radial loads and a deep-groove ball-bearing witch absorbs engine thrust.

Crankcase... The power section case made up of five sections, all except the front and rear sections being substantially alike. The parts for the power section are machined from aluminum forgings and are held together with through bolts. All other crankcase sections are magnesium castings. Attached to front of power section by studs is the magneto section, whichmounts four interchangeable magnetos and contains the torquemeter. The nose section houses the planetary reduction gearing and has provision for full-feathering, reversible airscrew governor. Attached to rear of power section by studs in the supercharger housing, enclosing the supercharger drives and impeller. Behind the supercharger housing, also attached with studs, is the accessory section. On this is mounted a down-draft pressure type carburator. This section houses the accessory-drive mechanism and provides mounting pads for the radial mounting of all accessories and automatic power controls to permit greater accessibility for servicing. Normally nothing is mounted to the rear of the engine but co-axial accessory drive maybe provided as optional equipment.


Valve gear... Rocker boxes are part of the cylinder head, extending four and aft. Rocker arms with plain bearings are actuated by enclosed push-rods. Double-track self-mounted cams inside the crankcase between the rows of cylinders operate the exhaust valves of the forward row of cylinders and the intake valves of the aft row. Cams are driven by reduction-gears from the crankshaft at one-sixth crankshaft speed.

Induction System... Stromberg four-barreled pressure-type carburetor, with automatic mixture control, idle cut-off, primer tubing and distributor. Metered fuel is carried through internal passages and is thrown centrifugally through small holes between the impeller blades to mix with the combustion air. The fuel-air mixture, after passing through the diffuser to the blower rim, is carried to the cylinders through seven intake pipes, one for each bank of four cylinders.

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R-4360 Final Assembly

Supercharger... There are two basic types in unrestricted engines: a single-stage single-speed supercharger suitable for use with an exhaust driven turbo-supercharger, and a single-stage hydraulically-driven variable-speed supercharger. The two-piece impeller assembly consists of a machined impeller the straight blades of which are blended with the curved entrance blades of an inducer.

Ignition System... Four Scintilla magnetos, each with an internal distributor, operate at one-half crankshaft speed. Dual ignition.

Lubrication... Forced feed lubrication provided by a gear-type oil pump to all parts of the engine.

Reduction Gear... The planetary reduction gears are of Pratt & Whitney design and are spur gears with ratio of .375:1.

Accessory Drives... All accessories mounted radially on the periphery of the rear section. All accessory drive-gears driven by a bevel-gear revolving at crankshaft speed.

Bore: 5.75 in. 146 mm
Stroke: 6.00 152 mm
Displacement: 4.363 71.5 lit
Compression ratio: 6.7:1 6.7:1
Length: (engine only) 96.5 in. 2 451 mm
Diameter: 55.0 1 397 mm
Frontal area: 16.5 sq.ft. 1.35 m
Weight: (engine only) 3,670 lb. 1 665 kg
Weight/Horsepower: 1.05 lb./h.p. 0.48 kg/hp
Fuel Consumption: (cr.) 0.43 lb./h.p./hr. 195 g/hp/hr
Oil consumption: (cr.) 0.015 lb.h.p./hr. 11 g/hp/hr
Gasoline Grade: 115/145 grade 115/145
Oil Viscosity: 100 S.U. secs. 20.5 cs
Output/Displacement: 0.80 h.p./ 49.0 hp/lit
Output/Piston Area: 4.81 h.p./ 0.75 hp/cm
Piston Speed: (max) 2,700 ft./min. 13.t m/sec.
B.m.e.p.: (max) 235 lb./ 16.5 kg/cm
Rating: (take off, wet)..3.500 h.p./2,700 r.p.m./60.o in. (1 524 mm)+15.0 lb.
Rating: (take off dry)...3250 h.p./2,700 r.p.m./60.0 in. (1 524 mm)+15.0 lb.
Rating: (normal)...2,650 h.p./2,550 r.p.m. /5,500 ft. (1 680m)
Rating: (max. cruising)...2,800 h.p./2550 r.p.m./3,500 ft. (1 070 m)




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