BMW M73

Compared with its M70 predecessor, the M73 features an updated roller-rocker valve-train and an increased displacement, due to a bore increase of 1 mm (0.04 in) ending with a cylinder bore of 85mm and a stroke increase of 4 mm (0.16 in) to have a final piston stroke of 79mm.[1] While most other engines in the BMW range had switched to dual overhead camshafts with four valves per cylinder, the M73 used a single overhead camshaft and with two valves per cylinder. However, BMW engineers did create a prototype four-valve per cylinder V12, called the M72, which developed 265 kW (360 PS; 355 hp) and 530 N⋅m (391 lb⋅ft) of torque. However, this 48-valve engine did not fulfill the comfort demands of the large sedan class in all respects and, as a result, did not go into production.[2]

Some versions of the engine have two separate Bosch Motronic engine control units,[3] while others use a single Siemens ECU.[4]

The hydrogen versions of the engine feature a ${\displaystyle \lambda \geq 3.0}$ lean burn concept,[5] which allows quality torque control similar to a Diesel engine (i. e. no engine throttling).[6] Due to the lower LHV of a hydrogen-air mixture, and the fact that the engine was designed as a Bi-Fuel (petrol and hydrogen) engine, the power output is reduced by 38 per cent compared to the petrol only version of the engine.[5]

• 1999 International Engine of the Year - Best Above 4.0 Litre category winner[2]

• List of BMW engines