One problem with scaling up any piston engine design is that eventually a point is reached where the crankshaft becomes a major engineering challenge. This was a problem that affected almost all engines of the 2,500 hp (1,900 kW) class, including BMW's own 18-cylinder BMW 802 project. For the 803 the engineers decided to avoid this problem by simply not using a common crankshaft, and driving a set of independent contra-rotating propellers. The front engine drove the front propeller directly, while the rear engine drove a number of smaller shafts that passed between the cylinders of the front engine before being geared back together to drive the rear prop. This layout resulted in a rather large gearbox on the front of the engine, and the front engine needing an extended shaft to "clear" the gearbox.
With no common crankshaft it became more practical for all of the accessories to be powered by one engine alone, in this case the rear engine. The supercharger itself used up several hundred horsepower, so the rear propeller delivered considerably less power than the front one.
The engine weighed 2,950 kg (6,490 lb) dry, and 4,130 kg (9,086 lb) fully loaded, displacing 83.5 litres. It delivered 3,900 PS (Pferdstärke) (2,868 kW). Although this made it the most powerful German engine design, its power-to-weight ratio was not impressive, at about 0.60 hp/lb, compared to other large designs like the Junkers Jumo 222 at 1.04 hp/lb. Specific power was likewise poor, at about 34.4 kW/l, compared to the 222's 40 kW/l, as was specific fuel consumption, at 380 g/kWh (0.63 lb/hp·h).
As with most coupled engines, the 803 was not a success on the test-bed, and did not enter production.
The engine was intended to be used only on the largest of designs, notably the Focke-Wulf Fw 238, the Focke-Wulf Ta 400 six-engined Amerika Bomber design competition competitor, and other large bombers. The May 1942-approved Amerika Bomber contract was for a trans-Atlantic range strategic bomber designed to attack New York City from European bases. A single example remains in the Deutsches Museum.
Type: 28-cylinder 7-bank liquid-cooled coupled inline radial engine
Bore: 156 mm (6.14 in)
Stroke: 156 mm (6.14 in)
Displacement: 83.5 L (5,095 in³)
Diameter: 160 cm (63 inches)
Dry weight: 4 130 kg (9,100 lbs)
Valvetrain: One intake and one sodium-cooled exhaust valve per cylinder
Fuel system: Fuel injection
Cooling system: Liquid-cooled
Power output: 2 940 kW (3,950 hp)
Specific power: 34.3 kW/L (0.75 hp/in³)
Compression ratio: 6.5:1
Specific fuel consumption: 380 g/kWh (0.63 lb/hp·h)
Power-to-weight ratio: 970 W/kg (0.59 hp/lb)