Soon after the 801 entered testing, BMW engineers turned to building much larger versions. One idea was to "simply" bolt two 801's back to back. Although seemingly simple, the resulting BMW 803 was in fact fantastically complicated, even giving up on air cooling to be liquid-cooled instead, and generally proved that piston engines of this size were unworkable. Another idea was to simply add more cylinders to the 801 design, and since radials need to have an odd number of cylinders per row, the next "size up" was a two-row 9-cylinder design, the 802, which emerged having an almost identical displacement to the Wright R-3350 Duplex Cyclone American 18-cylinder twin-row air-cooled radial aviation engine.
One problem with the 801 was its poor altitude performance, due almost entirely to the simple single-stage two-speed mechanical supercharger it used. Since the 802 was not a necessity given the success and emerging flexibility of the 801's basic design, the engineers decided to take the time needed to address this problem by including an improved three-speed supercharger. The lowest-speed setting would not "rob" as much power at low altitudes, allowing the engine to produce 2,600 PS (1,912 kW) for takeoff, and still produce 1,600 PS (1,176 kW) at 39,000 ft (12,000 m). This was a dramatic improvement on the 801A's 1,600 PS (1,176 kW) for takeoff and 1,380 PS (1,015 kW) maximum at 15,100 ft (4,600 m), especially notable considering the engine was less than 30% larger in displacement.
Development was still underway in late 1943 when BMW decided the project simply wasn't worthwhile. With their BMW 003 axial-flow turbojet engine finally maturing and considerably larger models of turbojet and even turboprop powerplants entering the prototype phase from both BMW and their competitors, it appeared that large piston engines simply weren't worth building.
A further improvement led to P.8011, which replaced the supercharger with two smaller turbochargers, driving contra-rotating propellers. This raised the takeoff power to about 2,800 PS (2,059 kW), (some report 2,900 PS (2,133 kW)) and dramatically improved altitude performance. As with most German turbocharger projects, the lack of quality high-temperature alloys meant the project was never able to enter production.
Type: 18-cylinder supercharged two-row radial engine
Bore: 156 mm (6.142 in)
Stroke: 156 mm (6.142 in)
Displacement: 53.671 L (3,275.2 in³)
Valvetrain: One intake and one sodium-cooled exhaust valve per cylinder
Supercharger: Gear-driven single-stage three-speed
Fuel system: Fuel injection
Cooling system: Air-cooled
1,912 kW (2,563 hp) for takeoff
1,176 kW (1,575 hp) at 12,000 m (39,000 ft)
Specific power: 35.6 kW/L (0.78 hp/in³)
Compression ratio: 6.5:1