Get this, due to a shortage of high horsepower engines in the United States in the late 1930's and early parts of WW II, American Tanks used Wright Cyclone 9 cylinder radial engines as the best available engine. M-2, M-3, and M-4 tanks and M-10 & M-18 tank destroyers were all users of the radial engine and some used the hydramatic transmission while the others used manual transmissions. Simultaneously to the early production of these tanks, there were other engines developed for tank use, such as Ford and Cadillac flathead V-8s and the Chrysler engine made up of five flathead 6 cylinder engines radially arranged around a common crankcase (30 cylinders!). These were all used as tank power plants later in the war. There were other engines used as well.
Correction: The Ford V-8 was an 1100 cu-in OHC engine - not a flathead - my BAD! For more info: http://www.fordgaaengine.com/
The Jalopy Journal
