The 18WI, introduced in 1926, had a different construction than former Framan water-cooled W-18s. It was also adaptable to inverted installation. Its bore was 110 mm (4.331″), its stroke 125 mm (4.921″), its displacement 21.382 l (1,304.8 in³) and its compression 5.7:1. It was rated 550 hp at 2,600 rpm but could produce 730 hp at 3,400 rpm.
Its bare weight was 701 lb, but with a 2.46:1 propeller reduction gear, starter and propeller hub it weighed 930 lb. It was 67.69″ long without supercharger, 80.29″ long with supercharger, 30.7″ wide and 42.67″ high. Each six-cylinder row, including cylinder heads and valve ports, was cast en-bloc of Alpax, a ductile silicon-aluminum alloy. Steel liners were pressed into these castings from inside. The three blocks were mounted on an aluminum crankcase at a 40° included angle. The valves seated upon inserts and were operated via rocker arms and overhead camshafts enclosed in aluminum housings that were bolted to the cylinder blocks. The crankcase was divided horizontally along the crankshaft centerline and was closed at the front by the reduction gear casing. The six-throw, seven-main-bearing crankshaft used articulated rods with H-section link rod shanks. Light alloy cast aluminum pistons were fitted with full-floating pins. Spur gear trains at the engine front drove the camshafts. Intermediate gears in the outer block camshaft gear trains drove two 18-cylinder ignition distributors and a water pump on the center block. The dry-sump lubrication system furnished pressurized oil to all bearings. Two Zenith carburetors were suspended below the cylinders; one fed the aft nine cylinders and the other the nine forward cylinders. When fitted with a Rateau supercharger, the engine could maintain 600 hp up to 5,684 m (18,000 ft).
All Aero 