Wright R - 975 The Wright R - 975 Whirlwind was a series of nine - cylinder air - cooled radial aircraft engines built by the Wright Aeronautical division of Curtiss - Wright . These engines had a displacement of about 975 in³ ( 16 . 0 L ) and power ratings of 300 - 450 hp ( 225 - 335 kW ) . They were the largest members of the Wright Whirlwind engine family to be produced commercially , and they were also the most numerous . During World War II , Continental Motors built the R - 975 under license as a powerplant for Allied tanks and other armored vehicles . Tens of thousands of engines were built for this purpose , dwarfing the R - 975 ' s usage in aircraft . After the war , Continental continued to produce its own versions of the R - 975 into the 1950s ; some of these produced as much as 550 hp ( 410 kW ) . Design and development . Wright introduced the J - 6 Whirlwind family in 1928 to replace the nine - cylinder R - 790 series . The J - 6 family included varieties with five , seven , and nine cylinders . The nine - cylinder version was originally known as the J - 6 Whirlwind Nine , or J - 6 - 9 for short . The U . S . government designated it as the R - 975 ; Wright later adopted this and dropped the J - 6 nomenclature . Like all the members of the J - 6 Whirlwind family , the R - 975 had larger cylinders than the R - 790 . The piston stroke of 5 . 5 in ( 14 . 0 cm ) was unchanged , but the cylinder bore was expanded to 5 . 0 in ( 12 . 7 cm ) from the R - 790 ' s bore of 4 . 5 in ( 11 . 4 cm ) . While the R - 790 was naturally aspirated , the R - 975 , like the other J - 6 engines , had a gear - driven supercharger to boost its power output . Wright gradually developed the R - 975 , at first using suffix letters to indicate successive versions . The original R - 975 ( or J - 6 - 9 ) was rated for 300 hp ( 224 kW ) , while the R - 975E of 1931 could do 330 hp ( 246 kW ) thanks to an improved cylinder head design . Wright later added numeric suffixes to show different power levels . The R - 975E - 1 , introduced the same year as the R - 975E , was rated at 365 hp ( 272 kW ) thanks to higher - compression pistons and a slightly greater RPM limit . An even more powerful version , the R - 975E - 3 , was also introduced that year , with greater supercharging and a still higher RPM limit , and was progressively refined until the final model of 1935 could reach 450 hp ( 336 kW ) for takeoff . Operational history . As the most powerful Whirlwind engine to be commercially produced , the R - 975 also became the most popular . It was a powerplant for a variety of civil utility aircraft , such as the Beechcraft Staggerwing , and was also used for some early airliners , like the Ford Trimotor 4 - AT - E and the Lockheed Electra 10B . In addition , it powered several U . S . military training aircraft , the North American BT - 9 and Vultee BT - 15 Valiant for the Army and the Curtiss - Wright SNC - 1 Falcon for the Navy . It was even used in a fighter aircraft , the Curtiss F9C Sparrowhawk parasite fighter used on U . S . Navy airships . However , the R - 975 faced heavy competition from Pratt & Whitney ' s R - 985 Wasp Junior and from their larger R - 1340 Wasp . Pratt & Whitney sold many more Wasp Juniors for aircraft use than Wright sold R - 975s . Wright ' s production of the R - 975 continued until 1945 , with over 7000 engines being produced by the company . Production by Continental Motors . In 1939 the U . S . Army , which had already been using Continental R - 670 radial engines in its light tank s , chose Continental Motors to build the R - 975 under license as the engine for its M2 medium tank s . Subsequently , the same engine was selected for the M3 Lee medium tank , the M4 Sherman medium tank , the Canadian Ram tank , the M7 Priest self - propelled gun , the M18 Hellcat tank destroyer , and other Allied armored vehicles based on these . Continental versions of the R - 975 for armored vehicles included the R - 975E - C2 , the R - 975 - C1 , and the R - 975 - C4 . Continental built over 53 , 000 R - 975 engines for armored vehicles , far more than were ever built by Wright . When installed in a tank , the R - 975 did not have the benefit of being cooled by an air slipstream or propeller blast , so a cooling fan was attached to the power shaft and surrounded by a shroud to provide the same effect . After the war , Continental introduced its own R - 975 version for aircraft , the R9 - A . Though it was basically similar to other R - 975 engines , and its compression ratio and supercharger gear ratio were unchanged from the R - 975E - 3 , other improvements in the R9 - A allowed it to achieve 525 hp ( 391 kW ) for takeoff , surpassing any Wright version . A military version , the R - 975 - 46 , could reach 550 hp ( 410 kW ) , and was used in Piasecki ' s HUP Retriever and H - 25 Army Mule helicopters . Continental ' s production of R - 975 engines continued into the 1950s . Other license - built R - 975s . The engine was built in Spain as the Hispano - Suiza 9Q or Hispano - Wright 9Q without modification apart from the use of Hispano ' s patented nitriding finishing process and , on one version only , the 9Qdr , an epicyclic output speed reducer . The R - 975 was also produced under licence by Fábrica Nacional de Motores in Brazil . Engines on display . Some museums which have R - 975 engines on display : The following Federal Aviation Administration type certificate data sheets , all available from the FAA ' s Regulatory and Guidance Library :