The Sunbeam Manitou was an aero-engine produced by Sunbeam. Unsuccessful as an aero-engine, it is best known for having powered the Sunbeam 350HP racing car.
The Manitou was a further development of the V-12 Maori III. Work on it began by Louis Coatalen in 1917. It used aluminium blocks rather than cast iron, cast in blocks of three cylinders, a typical Sunbeam feature. Bore was increased to 110 mm, but stroke remained at 135 mm. The banks were at a 60° vee, with twin overhead camshafts on each bank operating four valves per cylinder. There were two Claudel-Hobsoncarburettors and two British Thomson-Houston (BTH) ignition magnetos. For aircraft use a reduction gear was fitted. The engine developed 300 hp at 2,000 rpm, later increased to 325 hp.[1]
Only one engine was produced before the end of the First World War, as the factory was busy with the vibration and other problems with the Arab engine, and it was very nearly cancelled. As was not unusual for a Sunbeam engine, it was only ever fitted to a single aircraft for trials, a Short 184 seaplane, and never went into production. After the war and Sunbeam's financial problems in the war-surplus glutted aero-engine market, they were offered to the less-critical powerboat market, again not an unusual move for Sunbeam. During the war 840 had been ordered, but only 13 were delivered before cancellation of the order.[2]
In typical Coatalen fashion, he redesigned the engine substantially even though this was just a one-off with no other likely sales. The twin-cam four-valve head was replaced by the single-cam three-valve head, possibly from the Arab. An increase in bore to 120 mm allowed space for these larger valves.
Another change affected the stroke, also a technique from the Arab, where it had been responsible for the engine's failure. A multi-cylinder V engine offers little length for connecting rods. Coatalen's usual solution was to use articulated connecting rods, where one rod runs on the crankshaft journal, but the other acts indirectly, through a journal on the other rod. The difficulty is that this gives a slightly different piston stroke for each bank, leading to unbalance and possible vibration problems, as with the Arab. Stroke was thus 135 mm on one bank and 142 mm on the other.[2]
