Bücker Bü 181
Type: 4-cylinder air-cooled inverted inline
Bore: 105 mm (4.134 in.)
Stroke: 115 mm (4.528 in.)
Displacement: 3983 cm3 (243.1 cuin)
Length: 978 mm
Width: 420 mm
Height: 665 mm
Dry weight: 91.50 kg (core)
Valvetrain: 1 inlet and 1 exhaust valve per cylinder
Fuel type: A3 (80 octane)
Cooling system: air
Power output: 105 hp at 2500 RPM
Compression ratio: 6:1
Fuel consumption: 210 -230 gr /PSh
Oil consumption: 3g /PSh
Hirth HM 500 Wikipedia
The Hirth HM 500 was a German four-cylinder air-cooled inverted inline engine developed from the Hirth HM 504 in 1938. Although developing the same output of the HM 504 (105 hp) and keeping the same capacity and bore, the HM 500 was a very different engine; the new HM 500 had a one-piece "closed" crankcase for simplified manufacture. The new cooling system reduced cylinder temperatures considerably (15 °C less), and the fuel consumption was also reduced. The HM 500 carried a twin-magneto instead of the two separate magnetos as on the old HM 504. All these changes made that the HM 500 resulted a 12% lighter than the HM 504 and the front surface a 37% less than the previous HM 504.
Otherwise the HM 500 continued the typical Hirth use of built-up crankshafts and roller-bearings for crankshaft & connecting rods (Hirth patents) as well as magnesium-alloy crankcases which made Hirth engines so popular in the 1930s.
Due to low fuel consumption and excellent reliability, the HM 500 was chosen as the powerplant for the Bücker Bü 181.