The BMW N55 is a turbocharged straight-six petrol (gasoline) engine that began production in 2009. The N55 replaced the BMW N54 engine and was introduced in the F07 5 Series Gran Turismo.

The N55 was BMW's first straight-six engine to use a twin-scroll turbocharger. It also won three straight Ward's 10 Best Engines awards in 2011–2013.^[1]

Following the introduction of the BMW B58 engine in 2015, the N55 began to be phased out.^[2]

The BMW S55 engine is a high performance version of the N55 made by BMW M GmbH, which is used by the F80 M3, F82 M4 and F87 M2 Competition/CS.

The main differences between the N55 and its N54 predecessor are the use of a single turbocharger, the addition of Valvetronic and the type of fuel injectors. Whilst the N54 used a twin-turbo arrangement, the newer N55 uses only a single twin scroll turbocharger.^[3][4] Valvetronic (variable valve lift)^[5] is claimed to improve throttle response, low-rev torque, exhaust emissions and to reduce fuel consumption by 15%.^[6][5][7] The direct injection system uses solenoid-type injectors, instead of the piezo-type fuel injectors used by its N54 predecessor. The piezo injectors were more expensive and BMW decided they were not worthwhile outside of Europe, because the potential benefits of lean-burn operation could not be fully realised.^[5]

The exhaust manifold design, called Cylinder-bank Comprehensive Manifold (CCM) by BMW, aims to reduce the pressure fluctuations to reduce throttle lag and exhaust back-pressure.^[8] The twin-scroll turbocharger uses 2 sets of exhaust duct to turn 1 turbine wheel, with cylinders 1–3 and 4–6.^[9] The engine management system is Bosch MEVD 17.2, and compatible fuels are ROZ (RON) 91–98 octane (minimum RON 95 is recommended),^[10][11]

As per the N54, the compression ratio is 10.2:1, the bore is 84.0 mm (3.31 in), the stroke is 89.6 mm (3.53 in) and the displacement is 2,979 cc (181.8 cu in).

The S55 engine is the high performance version engine developed from the N55 engine by BMW M. It was introduced in the F80 M3, F82 M4 and later to the F87 M2 Competition/CS, replacing the BMW S65 naturally aspirated V8 engine used in the previous generation M3.

Differences compared with the N55 include a closed-deck engine block, lightweight crankshaft, different crankshaft bearings, strengthened pistons/rods, different springs/valve material, twin turbos, twin fuel pumps, active exhaust, revised cooling system and intercoolers.^[18][19]

• List of BMW engines