The Wright R-790 Whirlwind was a series of nine-cylinder air-cooled radial aircraft engines built by Wright Aeronautical Corporation, with a total displacement of about 790 cu.in (12.9 L) and around 200 hp (150 kW). These engines were the earliest members of the Wright Whirlwind engine family.
The R-790 Whirlwind began as the Lawrance J-1, a nine-cylinder air-cooled radial developed in 1921 by the Lawrance Aero Engine Company for the U.S. Navy. The Navy was very enthusiastic about air-cooled engines, which it felt were better suited for naval use than liquid-cooled ones. But Lawrance was a small company, and the Navy doubted it could produce enough engines for their needs.
Despite urgings from the Navy, the major U.S. aircraft engine makers, Wright and Curtiss, were satisfied with their liquid-cooled engines and showed no interest in building air-cooled ones. Since the Navy was already a major purchaser of Wright engines, it decided to force the issue by suggesting that Wright purchase Lawrance and build the J-1 itself, while informing the company that the Navy would not buy any more of its existing engines or spare parts. In order to keep the Navy’s business, Wright was thus compelled to buy Lawrance in 1923, and the Lawrance J-1 became the Wright J-1.
By the time Lawrance merged with Wright, it had already developed the J-2, a more powerful version of the J-1 with slightly enlarged bore and displacement. However, Lawrance decided the J-1 was large enough, and the J-2 never went into production; only two examples were built.
Over the next two years, Wright gradually refined the J-1 engine, introducing the J-3, J-4, J-4A, and J-4B. The changes improved the engine’s reliability, cooling, and fuel consumption, but the basic design, dimensions, and performance were unaltered.
The J-4 was the first engine to bear the Whirlwind name; previous engines had no name, only a designation.
The J-5 Whirlwind, introduced in 1925, was a complete redesign of the engine which greatly improved its cooling and breathing, further increasing its reliability and reducing its fuel consumption. Among the more visible changes were a much wider separation between the valves, for better cooling airflow, and completely enclosed pushrods and rocker arms, rather than exposed ones as on the earlier engines.
The U.S. government later designated the J-5 Whirlwind as the R-790, but it did not apply this designation to the older engines.
All these engines had a bore of 4.5 in (11.4 cm), a stroke of 5.5 in (14.0 cm), and a displacement of 788 cu.in (12.91 L).
The J-5 was the last of the original nine-cylinder Whirlwinds. In 1928, it was replaced by the seven-cylinder version of the Whirlwind J-6.
Many Whirlwind engines were used in U.S. Navy aircraft, mostly in trainers but also in some ship-based observation and fighter aircraft. As the engines were refined and their reputation for reliability grew, their usage expanded to U.S. Army trainers and a wide range of U.S. civil aircraft, including the earliest versions of the Fokker Trimotor and Ford Trimotor airliners.
The reliability of J-5 Whirlwind engines also led aviators to use them for a number of record-setting distance and endurance flights. The most famous of these is Charles Lindbergh’s solo transatlantic flight from New York City to Paris on May 20–21, 1927, in the Spirit of St. Louis, powered by a single Whirlwind J-5C. During Lindbergh’s flight, the engine ran continuously for 33.5 hours. Lindbergh’s achievement greatly boosted the Whirlwind’s already good reputation.
Some other historic long-duration flights made in aircraft powered by the J-5 Whirlwind:

• Clarence Chamberlin and Bert Acosta made a record endurance flight of 51 hours, 11 minutes, 25 seconds in a single-engined Wright-Bellanca WB-2 over New York City in April 1927.
• Chamberlin and Charles Levine flew non-stop from New York City to Eisleben, Germany, in the same Wright-Bellanca on June 4–6, 1927, in a flight lasting 42.5 hours (3,920 mi).
• The first successful flight from the continental U.S. to Hawaii was made by Albert Hegenberger and Lester Maitland in the Fokker C-2 Bird of Paradise from Oakland, California, to Honolulu, Hawaii, on June 28–29, 1927, lasting 25 hours 50 minutes (2,400 mi).
• The first flight across the Pacific was made by Sir Charles Kingsford Smith in the Fokker trimotor Southern Cross from Oakland to Brisbane, Australia, with stops in Hawaii and Fiji, from May 31 to June 9, 1928. The leg from Hawaii to Fiji lasted 34.5 hours over 3,100 mi (5,000 km) of open ocean.
• A record endurance flight of 150 hours, 40 minutes, and 14 seconds was made by U.S. Army fliers in the Fokker C-2A Question Mark over southern California on January 1–7, 1929. Achieved with the help of aerial refueling, this flight only ended when a pushrod failure stopped one of the engines.
  Charles L. Lawrance, who developed the original Whirlwind series and became president of Wright, won the 1927 Collier Trophy for his work on air-cooled radial aircraft engines.
  The Whirlwind J-5 was also produced under license in Poland by several makers. Among these were Polskie Zakłady Skody, the Polish branch of Škoda Works, which built about 350 to 400 engines from 1929 to 1931, and the Polish firm Avia, which manufactured a further 300 engines from 1935 to 1938. Polish-built J-5s were used in numerous Polish aircraft, mostly in military training, observation, and liaison aircraft.

Variants
J-1: Lawrance J-1 as built by Wright Aeronautical in 1923.
J-3: Wright’s first refined version, 1923.
J-4: Improved version, 1924. First to be named “Whirlwind”.
J-4A, J-4B: Further refinements of J-4.
J-5 (R-790): Complete redesign with improved reliability and performance, 1925.

Applications:
U.S. aircraft-
Boeing NB-1
Buhl-Verville CA-3 Airster (J-4)
Buhl-Verville CA-3A Airster (J-5)
Consolidated NY-1 and NY-2
Consolidated O-17
Consolidated PT-3
Curtiss AT-5 and AT-5A Hawk
Curtiss N2C-1 Fledgling
Fairchild FC-2
Fokker F.VIIA-3m and F.VIIB-3m
Fokker Universal (Model 4)
Ford Trimotor 4-AT-A and -B
Keystone Pronto
Lockheed Vega 1
Naval Aircraft Factory N3N-1 Canary
New Standard D-25
Pitcairn PA-5 Mailwing and PA-6 Super Mailwing
Ryan B-1 Brougham
Spirit of St. Louis
Stearman NS-1
Stearman C2B and C3B
Stinson Detroiter SB-1 and SM-1
Stinson Junior SM-2AB
Texas-Temple Aero C-4
Texas-Temple Commercial Wing
Travel Air 4000
Vought FU
Vought UO
Waco 10-W (ASO) and 10-T (ATO)

Polish aircraft, using Polish-built engines-
Bartel BM-5d
Lublin R-XIII
PWS-18
PWS-26
PZL Ł.2

Specifications:
R-790 Whirlwind J-5
Type: 9-cylinder naturally aspirated air-cooled radial piston engine
Bore: 4.5 in (114 mm)
Stroke: 5.5 in (140 mm)
Displacement: 788 cu in (12.91 L)
Length: 34 to 40 in (86 to 102 cm)
Diameter: 45 in (114 cm)
Dry weight: 520 lb (236 kg)
Valvetrain: 2 valves per cylinder, pushrod-actuated
Fuel type: 50 octane
Power output: 220 hp (164 kW) @ 2000 RPM
Specific power: 0.279 hp/cu-in (12.7 kW/L)
Compression ratio: 5.1:1
Power-to-weight ratio: 0.423 hp/lb (0.696 kW/kg)