R-4360 Development History

Pratt & Whitney Aircraft
R-4360 Wasp Major Development History
Compiled by Kimble D. McCutcheon
Published 18 Apr 2025; Revised 4 Jun 2025

Starting in 1942, additional engine model designations were assigned for new aircraft projects or production models. The production models were identified as "B Series" engines.

As 1943 ended a total of six engines had been shipped for Army project and two for Navy projects. A total of 7,200 hrs of test stand operation had been accumulated and $10,000,000 expended on engineering. During January and February 1944, a total of eight "B Series" engines was shipped, including four R-4360-4s, one R-4360-7, one R-4360-8, one R-4360-11, and one R-4360-13.

During the remainder of 1944, a total of 19 more "B Series" engines were shipped, including six R-4360-5s, one R-4360-13, four R-4360-10s, four R-4360-4s, one R-4360-9, and two R-4360-8s. At 1944's end, a total of 17 Army and 13 Navy "B Series" engines were shipped. Test stand operation had reached 12,000 and engineering cost was $15,300,000.

During Summer 1945, it became necessary to provide additional takeoff power for the Boeing B-50. To accomplish this, the B3 Series engine was developed. This engine reduced the compression ratio from 7.0:1 to 6.7:1, cylinder baffles were changed to protect the rocker boxes and ignition systems from exhaust heat (hooded baffles) and power section reduction gearing was strengthened.

Late in 1945 it was realized that to have an engine that could be rated in excess of 3,500 hp it would be necessary to redesign the "B Series" engine to provide additional strength throughout. To accomplish this, the "C Series" engine was laid out. It differed from the "B Series" in all major parts. The crankshaft was changed to permit crankcase strengthening. The crankcase was made heavier and stronger, the mounting system was changed to permit the use of stronger main case attachment, the cylinder was changed to provide better cooling, and the rear case was changed to accommodate direct fuel injection, and the reduction gear was strengthened to transmit higher power. During 1945, engine shipments totaled 30 "B Series" engines and 81 production engines, for at total of 62 Army and 79 Navy engines. Development test time reached 17,000 hrs and engineering costs reached $19,700,000.

During 1946 a total of 323 Air Force and 221 Navy engines were shipped. Test time was increased to 24,000 hrs and engineering costs reached $25,700.00. During "C Series" engine development, studies of various compounding means showed that by removing the engine-driven supercharger and using exhaust energy in a turbosupercharger to do all the engine supercharging, a 500 hp takeoff rating increase could be realized along with an 8% cruising fuel consumption reduction. "C-series" development work was carried out to permit its use with such arrangements.

During 1947, 790 engines were shipped, bringing the total to 1,025 Air Forces engines and 289 Navy engines. Engineering costs were at $33,200,000.

During 1948, 1,005 engines were shipped, bringing the total to 2,009 Air Forces engines and 310 Navy engines. Engineering costs were at $40,600,000.

Through Jun 1949, 719 engines were shipped, bringing the total to 2,644 Air Forces engines and 394 Navy engines. Engineering costs were at $42,900,000.

In order to understand why so many engine models were designed, it is necessary to review the different airplane types for which they were intended. The following is a list of R-4360-powered airplanes, with some notes regarding each

Vultee Model 85 – A Pratt & Whitney experimental test ship sponsored by the Army Air Corps for testing single-stage R-4360 engines.
Vought V-326 high-altitude test ship for testing multi-stage or turbo R-4360 applications; two of these airplanes were built
Republic XP-72 used the R-4360-19 Its predominant feature was a remote-mounted, engine driven auxiliary stage supercharger. In this particular airplane the pilot was positioned between the engine and the auxiliary stage. Both dual rotation and single rotation propellers were tested on this airplane
Vultee XA-41 – R-4360-9. This was primarily a low-level attack ship carrying either a torpedo or bomb load and using a two-speed single-stage supercharged engine. This airplane is currently being used by Pratt & Whitney Aircraft for experimental flight test activities.
Goodyear XF2G-1 using the R-4360-3. A single-stage variable-speed rear engine. The first airplane of this type was a converted F4UM constructed by the Pratt & Whitney Aircraft Installation Department on a Navy-loaned Vought F4U airplane. The final F2G sported several improvements, such as a bubble canopy, greatly improved stability characteristics, particularly upon landing, and a trick tab arrangement for counteracting propeller torque under carrier wave-off conditions. During the testing of this airplane it was never felt that a dual rotation reduction gear was necessary to help torque characteristics with the possible exception of carrier wave-off, a problem that was cleverly overcome by Goodyear Aircraft. This airplane had a terrifically high rate and high angle of climb and for this reason this airplane was used to develop the engine scavenge system, more details of which will be discussed later.
Martin AM-1 (Mauler) used the R-4360-4 engine.
Boeing XF8B-1 fighter used the R-4360-10 two-stage engine and also a dual rotation reduction gear.
Curtiss-Wright XBTC-2 using the R-4360-14.
Douglas XTB2D-1 used the R-4310-8.
Boeing B-50, C-97 and 377 prototype, the XB-44. The R-4260-33 two-stage engines were used in the XB-44 and the XB-35, which is a single-stage turbo type engine, is intended to be used on the B-50 and 377. The B-55 was loaned to Pratt & Whitney by the Army and the four R-4360's installed under Army contract through joint cooperation between the Army, Boeing and Pratt & Whitney Aircraft personnel, As a result of tests on this airplane, it was decided before the end of the war with Japan to get this airplane into mass production just as fast as possible. At the present time, a large proportion of our post-war business is centered around the B-50, the C-97 cargo chip, and the 377 Boeing commercial Stratocruiser.
Douglas C-74 used R-4360-27 engines similar to the Navy R-4360-4 with the exception of the reduction gear. A medium number of these airplanes were built.
Republic AF-12 Rainbow, which used the R-4360-35.
Northrop XB-35 used the R-4360-17 and -21 engines, their only difference being the extension shaft length. This is perhaps the most unusual engine model Pratt & Whimsy Aircraft has ever built and has required a more extensive testing facility outlay. Originally, this engine had bean scheduled to use a two-speed dual-rotation reduction gear. However, this was abandoned because of its extreme complexities and weight. This engine had a turbo-type rear and a two-speed fan drive, also equipped with a fan brake.
Martin Mars used the R-4360-4.
Hughes XF-11, One of two airplanes was lost on its first flight. The plane used R-4360-31 engines, featuring dual rotation reduction gear.
The giant Consolidated RB-36 was equipped with six R-4360-25 pusher engines. These engines, like the XB-35 engines, were equipped with a fan drive and power take-off. One hundred of these airplanes were scheduled to be built.
Hughes H-4 eight-engine flying boat. This project used R-4360-4A engines and originally started as the brain child of Henry Kaiser, who is no longer associated with the protect.
Lockheed XR-60, more commonly referred to as the Lockheed Constitution, used the R-4360-18 engine. Two of these airplanes were being built for the Navy, one of which was Aero Sudest (French) four-engine airliner. This was to use a modal similar to the R-4360-20W. The original four "Power Pack" nacelles were engineered by Pratt & Whitney Installation Engineering,
Martin XP4M-1 used two R-4360-4 engines plus two General Electric jet engines.
Fairchild C-119A, C119B, C-119C and XC-120 used two R-4360-20, -20A, or -20W engines.

In developing a new engine, the ratings at which initial development is started are usually at the current brake mean effective pressure (bmep) rating of the most recent production model of the next largest size engine. In the R-4360's case, initial development centered around ratings similar to that of the R-2800 "B Series". Usually engine ratings rapidly exceed those of previous engine models; the R-2800 exceeded that of the R-1830, etc. This is generally caused by the inability to stay within dimensional limitations for installation or service replacement requirements and it became necessary to introduce an entirely new engine. This point is only now being reached in the R-4360 engine where P&WA was now going from an R-4360 "B Series" to the R-4360 "C Series". The "A Series" R-4360 never materialized much beyond the tinkering stage as it was found in early development that the engine was capable of 3,000 hp instead of the original 2,800 hp "A Series" rating. It does appear at this time that the current 3,500 hp takeoff rating for the "B Series" engine was about as high as P&WA could go, although war emergency ratings up to 4,400 hp or higher were probably possible.

During development, an R-4360 was drawn from production and operated in excess of 4,400 hp for three days and accumulated approximately 22 hours of operation in this range. This same engine was also calibrated and run for about an hour as high as 4,850 hp and subsequently for 100 hours at 3,000 hp and approximately 50 hours at 3,500 hp before disassembly. This engine was still in serviceable condition.

In the course of R-4360 development the problems common with other engine models and currently being investigated by the other P&WA engine groups were very often sidetracked and the efforts of the R-4360 Group concentrated entirely on features peculiar only to the R-4360. As an example, R-2800 and R-4360 piston and ring assemblies at elevated powers were subject to ring sticking, a constant source of annoyance. The R-4360 Group suffered along by overcooling or babying the piston assemblies through endurance testing and the R-2800 Group carried on with development of a piston and ring assembly that would be free of sticking. At the conclusion of their investigation, the piston and ring assembly adopted for the R-2800 was used in the R-4360. This was possible to do in most cases but often cases arose where items used satisfactorily in other engines could not be used on the R-4360.

Hose clamps and connections on R-2800 rocker drain systems proved quite satisfactory, but R-4360 rocker drain system oil temperatures were considerably higher than in the R-2800, which resulted in severe and rapid rocker drain hose deterioration. It was therefore necessary to develop a rocker drain system using a more heat-resistant material. This was very satisfactorily done by the use of the new silicone rubbers. Silicone, however, had an unusual property of flowing readily and therefore had to be completely confined. This rubber's flow characteristics were addressed by a spring-loaded compressive installation that eliminated the need to service these parts during engine operation.

The following items are some of the more interesting and complex problems encountered during R-4360 development and for the most part are peculiar to that engine type. At some time or other, almost every engine part required some improvement or design change and parts that were initially trouble-free became troublesome as the engine was further developed and accrued more and more time at elevated powers. Probably the most interesting part of engine development was the constant possibility that anything from a cotter pin to a crankshaft could be the next item to be troublesome as the power conditions were elevated.

The crankshaft actually lost weight during its initial development. In some experimental overspeed testing conditions (not conditions encountered for any appreciable time in service) it was possible, after 50 hours of operation at this condition, to develop cracks at the crank cheek. This condition was traced to stiffness in this crankshaft, which, because of the flexibility of other shaft portions, resulted in stress concentration under the resonant vibration characteristics experienced at this overspeed point. By removing material from the crankshaft and providing greater radii at the edges, this cracking was eliminated even though the actual crankshaft twisting was probably increased. Also, during engine development, it was noticed that the front intermediate main bearing was subject to frequent unexplained test stand failures. It was further found that this failure could be induced either by reducing main oil pressure below 75 psi or introducing air into the oil pump inlet, a condition prevalent in a large number of aircraft not having a good oil system and means of eliminating air entrained in the oil leading to the tank from the engine. This bearing burning problem was solved in two ways. Engineers instrumented the oil passage through the crankshaft and noted that when the main oil pressure was at 75 psi, only 10 to 15 psi reached the front intermediate main bearing. There were two main oil supplies to the engine nose, one through the crankshaft and the second through oil passages at the crankcase top. However, due to the crankshaft's oil flow requirements, oil entering the rear crankshaft section only reached the front intermediate main bearing; the front crankshaft section was fed from the line traversing the crankcase upper half. The second cure for this bearing burning was the installation of air separators in the main journals where small diameter holes were drilled on the exact crankshaft center line or ones that led from this center line. The crankshaft's whirling motion tended to separate the oil from the air, concentrating air at the crankshaft center. Since it is also necessary to provide sludge traps in the crankshaft, the oil feeds from one crankshaft section to the other were necessarily taken as close to the center line as possible, which resulted in feeding air rather than oil. These small bleed holes offered little resistance to the air but sufficiently resisted oil flow so that no detrimental pressure losses were encountered.

According to G.E. Armbruster, "…the initial R-4360 wouldn't scavenge nohow." This problem was rectified by a scheme that presented with no breathing difficulties or excessive power loss up to 40,000 ft. In order to obtain this scavenging efficiency, a large numbers of changes were made over a period of 2.5 years, changes that involved increasing the scavenge pumps' capacity, adding scavenge pump stages or locations, revising scavenge pump inlet conditions, and installing screens in certain compartments to act as air separators that reduced the oil volume and increased its density so it could be efficiently handled by the scavenge pumps. The fact that this was a dry sump engine meant that oil had be collected by the pump "on the run." Several scavenge rigs were made and engines were tested both on the test stand and in flight. A converted Vought F4U was used for flight testing because of its high climb angle and high rate of climb, which provided the most severe scavenge conditions that could be encountered.

Although additional scavenge pumps with increased capacity contributed greatly, effective scavenging was not achieved until screens were used. These screens worked on the theory that oil in the engine was whipped to a froth similar to soap suds in the kitchen sink. When draining the sink, the solid water under the soap suds flowed out of the drain, but if this water contained enough soap, removing the drain plug after beating into a froth had no effect on the draining the soap suds. A similar condition existed is the engine and the scavenge pumps could not remove the oil suds. Since the oil foam was traveling at high velocity and had more inertia than the air in it, when the foam impinged upon a screen the large bubbles could not pass through small hole, but the oil, by virtue of its own inertia, seperated out, passed through the screens and emerged with a greatly reduced air content, thus approaching a more nearly liquid condition prior to entering the scavenge pumps.

An engine sometimes threw oil out the breathers instead of scavenging, which was evidenced by a high temperature rise and horsepower loss. Initially, the R-4360 with a four-coupling variable-speed supercharger drive developed around 200 less horsepower less for a given manifold pressure than the same engine with a two-gear fixed supercharger drive in the same compartment; the couplings were whirling the oil around at such a rate that it could not be scavenged from the compartment. The R-4360-4 blower drive compartment had a 120 lb/min oil flow, yet the compartment's oil capacity was less than 15 lb. It was in the blower drivev compartment that screens contributed their most valuable results by reducing horsepower loss from over 200 to less than 35.

Fortunately, no great amount of trouble was experienced with the use of the fore-and-aft forged head cylinder. There were a few weaknesses that showed up such as exhaust valve guide erosion, intake valve seat warpage, exhaust rocker ear valve gear coking, and rocker cap gasket oil leakage. Exhaust valve guide wear was corrected by the use of steel-tipped guides that scraped lead deposits off the valve stems. As the performance numbers of fuels increased, the increased lead content icaused combustion chamber and exhaust passage lead deposits to become more and more troublesome. Exhaust rocker ear coking was primarily caused by too few cooling fins on that ear and very little cooling air circulation, a condition non-existent in other Pratt & Whitney engines up to that time. The revised baffle system on the R-4360-35 engine greatly helped to alleviate this condition. This, combined with an oil cooling jet in the ear and the use of silicone rocker cap gaskets alleviated most of the troubles. The intake valve seat warpage was simply eliminated by changing from a bronze to a steel seat plus a change in valve seat width and valve spring action.

The initial R-4360 baffle system was such that ignition harness, rocker ear, intake pipes, etc., cooling was dependent upon the care the aircraft manufacturer took in providing proper exits for the hot air from the engine. Experience indicated that the engine experience of people employed by the aircraft manufacturers with such details was necessarily limited and that it was far better for Pratt & Whitney to completely enclose the engine so as to control these operating conditions. Further, the use of the R-4360-35 type of hooded baffle greatly reduced temperatures in these parts by enclosing them in the cooling system's cold-air side, resulting in considerably longer parts life.

R-4360 intake pipes had much larger radii on the engine than other models. The cylinders were very flexible and under high power operation weaved about considerably, imposing considerable stresses on the intake pipes. It became necessary to impose very wide gaps between the pipes on adjacent cylinders in order to allow sufficient flexibility without pipe strain. Whereas this was successful in reducing pipe failures, serious hose connection failure epidemics resulted. A successful solution to hose and coupling bursting caused by backfires was accompanied by installing a metal shield supported by a bulge in the coupling. However, these rubber hoses were still subject to deterioration and development continued to find a satisfactory all-metal coupling. After considerable experimentation a metal coupling was developed.

Another problem with the large-radius intake pipes was the matter of fuel and oil draining from the lower intake pipes. Since the intake pipe was considerably lower than the intake valve ports, pulling the engine through did not insure against hydraulic locks, as a very large amount of gasoline and/or oil could accumulate in the intake pipe lower part. The engine could run at low speeds with this fuel/oil collection in the pipe, but when the throttle was advanced and the air velocity within the intake pipe increased, a sudden wave of this liquid could develop and be forced into the front cylinder bank, wrecking the engine. To eliminate this, automatic drains were installed in the lower intake pipes; these drains would close whenever a differential air pressure in either direction was imposed. Since relatively small force was available to operate these valves, they had to be light; consequently, they were not very durable, particularly in conditions that promoted backfires and could stick very easily by gum or dust. Since, for normal shutdown periods oil generally did not leak past pistons and into cylinders in quantities that would cause hydraulic locks, and since most hydraulic locks were the result of improper mixture control during starting, engineers decided that if the fuel could be trapped in the blower before it got to the intake pipe, the danger of hydraulic locks could be eliminated.

Engines left in storage for any length of time had plugs in the lower pipes for manual drainage, but for day-to-day operation an annulus around the intake pipe as it left the blower leading to a chamber with a float valve was quite satisfactory in removing liquid fuel tending to flow to the lover pipes from the blower. Operating instructions specified that an engine should be started on the prime and that the mixture control should not be moved out of idle cut-off until the engine started to fire. With the new R-4360 priming system that injected fuel into the blower rim , this was a very easy and simple way to start, However, there remained a large number of people schooled in the theory that the only way to start an engine was to jockey the mixture control.

If during R-4360 starting, 15 psi boost pressure was supplied and the mixture control and throttle were moved to their full open positions, which had been known to happen among some of the very careless personnel, it required a 1.125" diameter hole in the intake pipe to drain enough fuel to prevent hydraulic lock. This not only resulted in a prohibitive hole size but might result in serious fire hazard should exhaust system torching set off this fuel, which would be pouring to the ground or the deck of an aircraft carrier. In the space allowed with the float type drain valve, P&WA was unable to get a valve of sufficient size to take care of more than 75% of this requirement. However, this extreme example was so unusual and rare that the probability of it happening was very remote.

Early engines used seven pressurized Bendix Scintilla D4RN-2 high-tension shielded magnetos with integral distributors operating at 1/2 crankshaft speed. Each magneto provided dual ignition for the bank of four cylinders directly behind it. Pratt & Whitney chose to use seven magnetos for simplicity, durability, and continued engine reliability should enemy action damage one or two of the magnetos. These parts produced relatively little trouble. Considerable controversy, however, arose over the merits of a single-piece molded and sealed ignition harness versus ones with detachable and leads. Later ignition harnesses featured detachable leads.

Later engines used four Bendix Scintilla S14RN-15 low-tension magnetos with integral distributors operating at 1/2 crankshaft speed. Magnetos on the engine's left side were designated L1 and L2; L1, the top-left magneto, fired the intake plugs on cylinder rows B and D; L2, the bottom-left magneto, fired the intake plugs on cylinder rows A and C. Magnetos on the engine's right side were designated R1 and R2; R1, the top-right magneto, fired the exhaust plugs on cylinder rows B and D; R2 the bottom-right magneto fired the exhaust plugs on cylinder rows A and C.

Pratt & Whitney Aircraft produced R-4360s through 1955 for total production of 18,769 units.

**Airlines Scheduled to use Wasp Major Engines**
Airlines	Orders
Pan American World Airlines	20 Boeing 377 Stratocruisers
Northwest Airlines	10 Stratocruisers
American Overseas Airlines	8 Stratocruisers
United Airlines	7 Stratocruisers
British Overseas Airways	6 Stratocruisers
A.B.A. (Swedish)	2 Stratocruisers
D.D.L. (Danish)	1 Stratocruiser
D.N.L. (Norwegian)	1 Stratocruiser
Air France	Aero-Sudest

Armbruster, G.E. "History of the R-4360" Pratt & Whitney Service School Handbook
History of 4360 Type Engine. Pratt & Whitney publication.
Parts Catalog for Models R-4360-20, -20A, -20B, -20C, -20T, -20AT, -20W, -20WA, -20WB, -20WC, -22W, -35, -35A, -35B, -35C, -49, and -49A Aircraft Engines. (NAVY) AN 02A-10HE-4; (USAF) T.O. No. 2R-R4360-14. 1 Dec 1955.
Overhaul Manual Wasp Major (R-4360) B6 and CB2 Engines. Pratt & Whitney Aircraft Part No. 201529. Dec 1953.

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