BMW N74 Engine (2009)

The BMW N74 engine was a V12 petrol engine that had twin turbochargers. The N74 engine replaced the naturally aspirated N73 engine and shared many features with BMW’s N63 engine. The N74 engine had a 60-degree ‘V’ and the N74B6U0 engine for the F01 760i and F02 760Li had 89.0 mm bores and an 80.0 mm stroke for a capacity of 5972 cc.

N74 crankcase

The N74 engine had a closed deck design and an Aluminium-silicon alloy (Alusil) cylinder crankcase with lowered side walls (deep skirt), double main bearing bolting and additional side wall connections in the form of threaded support sleeves and bolts. Within the crankcase, separate channels were used the oil return from the cylinder heads and for crankcase ventilation to reduce the amount of oil in the blow-by gases.

The cylinder walls were hardened through exposure honing – a ‘soft stripping’ process that removed the aluminium from the cylinder surface such that hard silicon particles remained. Coolant passages were also used to cool the area between the cylinders.

The N74 engine had a forged crankshaft with hardened running surfaces, cracked forged connecting rods with trapezoidal wrist pin bosses and iron-coated pistons. Maximum engine speed for the N74 was 6500rpm.

N74 cylinder head

The N74 had an aluminium alloy cylinder head which used ‘diagonal cooling’ whereby coolant flowed diagonally across the cylinder head from the outer side of the engine towards the ‘V’ chamber.

The N74 engine had double overhead camshafts per cylinder with variable intake and exhaust valve timing, BMW’s double VANOS. Both the intake and exhaust VANOS units had 50 degree crank angle adjustment. The camshafts were thermally jointed and had forged cams, a steel flange for the VANOS units and a sintered camshaft sensor wheel; the intake camshafts also had a three-way cam to drive the high pressure pumps. The camshafts were driven by a tooth-roller type chain and used roller cam followers to actuate the valves.

The N74 had four valves per cylinder and was not fitted with BMW’s ‘Valvetronic’ variable intake valve lift system. As such,

• The intake valves had 8.8 mm of lift; and,
• The sodium-filled exhaust valves had 8.6 mm of lift.

The N74 engine had a die-cast aluminium cylinder head cover.

Turbocharging

Each cylinder bank of the N74 engine had single-scroll Garrett GT2260S turbochargers. At full load, the turbochargers provided peak boost pressure of 0.7 bar in the intake manifold. Charging pressure was regulated by the Digital Motor Electronics (DME) via wastegate valves that were operated by vacuum units. Once the desired level of boost pressure was attained, the wastegate valve would open to direct some of the exhaust flow past the turbine wheel. To reduce unwanted peaks in the charge air pressure – which could occur when the throttle valve was closed quickly – the blow-off valves were incorporated into the turbochargers.

The N74 engine had separate cooling units for the engine and turbochargers; for the latter, two air-to-coolant intercoolers were mounted on top of the engine.

Injection and ignition

The N74 engine had High-Precision Injection (HPI) via outward-opening piezo injectors. The piezo injectors were integrated into the cylinder head with the spark plug in the centre of the combustion chamber (between the intake and exhaust valves). At all times, the direct injection system provided homogeneous operation.

The N74 engine had primary and secondary MSD87-12 engine control units (ECUs), while the air intake duct contained hot film air mass meters (digital HFM) and integrated electric throttle valves. The N74 engine had a compression ratio of 10.0:1 and the following firing order: 1-7-5-11-3-9-6-12-2-8-4-10.