Jacobs L-6

R-915
Type	Radial engine
Manufacturer	Jacobs Aircraft Engine Company
Developed from	Jacobs R-755

The Jacobs R-915 or L-6 is a seven-cylinder, air-cooled, radial engine for aircraft manufactured in the United States, production started in 1936.

The R-915 was effectively an enlargement of the R-755 with strengthened stressed parts. With a bore and stroke of 5.5 in × 5.5 in (140 mm × 140 mm) the displacement was 914 cu in (15 L), takeoff power was around 330 hp (246 kW). The engine features steel cylinders with aluminum-alloy cylinder heads. The R-915A4 variant features a drive shaft for autogyro use.

Data from Jane's

...
Wikipedia