 Despite the low demand for luxury motorcycles during the Great Depression, Indian not only continued production of the Four, but continued to develop the motorcycle.One of the less popular versions of the Four was the "upside down" engine on the 1936-1937 models.While earlier (and later) Fours had inlet-over-exhaust (IOE) cylinder heads with overhead inlet valves and side exhaust valves, the 1936-1937 Indian Four had a unique EOI cylinder head, with the positions reversed.