BMW N73 Engine (2003)

The N73 was a V12 petrol engine that was introduced in 2003 and replaced the M73B54 engine. For the BMW E65 760i and E66 760Li, the N73B60 engine had an 89.0 mm bore and an 80.0 mm stroke for a capacity of 5972 cc. Furthermore, the N73B60 engine weighed 280 kg.

For the F01 760i and F02 760Li, the N73 was replaced by the twin-turbocharged N74 engine.

N73 crankcase

The N73 crankcase was made entirely from an aluminium-silicon alloy (Alusil) and had a one-piece, ‘open deck’ design. The cylinder bores were finished by an etching procedure which involved etching out a thin aluminium layer from the cylinder walls to expose the high-strength silicon crystals. Within the combustion chamber, the iron-coated cast aluminium alloy pistons were ‘domed’ with valve reliefs in the crowns and recesses which directed the air-fuel mixture directly under the spark plug.

Furthermore, the N73 engine had a forged steel crankshaft which had 12 countereweights (two for each throw) and cracked forged steel connecting rods.

N73 cylinder head

The N73 cylinder heads were made from aluminium and manufactured using gravity die-casting. The N73 engine had double overhead camshafts (per cylinder bank) that were driven by roller type chains (one for each cylinder bank). The camshafts featured infinitely variable vane-type VANOS for variable intake and exhaust valve timing; the VANOS units were designed as integral components of the chain drive and secured to their respective camshafts with a central bolt. For the N73 engine, the intake and exhaust camshafts had adjustment ranges of 63 degrees and 60 degrees, respectively, relative to the crankshaft.

The N73 engine also featured BMW’s ‘Valvetronic’ system which varied intake valve opening lift between 0.3 mm and 9.85 mm, according to engine speed and load. Each cylinder head had a Valvetronic assembly – including a motor, control module and position sensor – which consisted of a bridge support with eccentric shaft, intermediate levers with retaining springs, drag lever and the intake camshaft.

With Valvetronic, air volume could be controlled according to engine requirements, rather than by the throttle valve. As an example of its operation, slower cylinder filling from the intake valves with partial lift could result in more turbulence in the combustion chamber for better air-fuel mixing and more efficient combustion. Furthermore, the variability of the Valvetronic system could enhance cylinder filling and scavenging throughout the engine’s entire operating range.

Injection and ignition

The N73 engine had direct petrol injection at pressures from 50 to 120 bar. Furthermore, the N73B60 was controlled by two MED 9.2.1 engine management units and had a compression ratio of 11.3:1. The firing order was 1-7-5-11-3-9-6-12-2-8-4-10.