BMW N55 Engine (2009)

Available from 2009, the BMW N55 was a 3.0-litre inline six-cylinder petrol engine which had a single twin-scroll turbocharger. With its 84.0 mm bore and 89.6 mm stroke, the N55 engine had a capacity of 2979 cc.

The N55 engine replaced BMW’s N54 engine which had two parallel turbochargers.

N55 Block

Like the N54, the N55 engine had an open deck design with a two-piece aluminium crankcase and bedplate. Due to its high torque outputs, the N55 crankcase was manufactured in a pressure die-casting process from AL 226. The N55 engine had grey cast iron cylinder liners and, in a new development, the webs between the cylinders had a groove so that coolant could flow from one side of the crankcase to the other.

The N55 engine had a lightweight, cast iron (GGG70) crankshaft with asymmetrical counterweights and was 3 kg lighter than the forged crankshaft in the N54 engine. Like the N54, however, the N55 engine had a maximum engine speed of 7000 rpm.

Cylinder head and valvetrain

The N55’s aluminium cylinder head housed double overhead camshafts (DOHC), variable intake and exhaust camshaft timing (dual VANOS) and four valves per cylinder. Compared to its N54 predecessor, the N55 engine had:

• New VANOS solenoid valves;
• Variable valve lift (BMW’s ‘Valvetronic III’);
• Larger intake valves (32 mm diameter compared to 31.4 mm diameter);
• Hollow, sodium-filled exhaust valves; and,
• Lightweight camshafts that were manufactured using an internal high pressure forming process.

For the N55 engine, the connections for the VANOS non-return valves were removed as they were integrated in the solenoid valves.

Borg Warner B03 turbocharger

The N55 engine had a journal bearing Borg Warner ‘B03’ turbocharger which had a custom version of the smallest Borg Warner EFR billet compressor wheel (62 mm) and custom inducer sizing. For the N55 engine, maximum boost pressure is understood to be around 0.61 bar (8.8 psi).

In vehicles fitted with a twin-scroll turbocharger, the exhaust manifold separates the channels for cylinders that can interfere with each other, so that the pulsating exhaust gases flow through separate spirals (or scrolls). For the N55 engine, cylinders 1-3 (bank 1) and cylinders 4-6 (bank 2) were paired together. As a result, the low pressure tail component of an exhaust pulse could be used to

• extract all the combustion products from the cylinder head, and,
• induct the intake charge during periods when both intake and exhaust valves are partially open/

Because of its turbocharging, the N55 engine had a pressure-limiting crankcase breather so that oil would not be drawn in through the breather into the intake manifold as a result of high vacuum pressures.

Injection and combustion

The N55 had direct petrol injection via Bosch solenoid valve fuel injectors (HDEV5.2) – with multi-hole nozzles – and could run on 91-98 RON unleaded petrol, though 95 RON was recommended. Furthermore, the N55 engine had Bosch MEVD 17.2 engine management, a Siemens SIMAF GT2 hot-film air mass meter and operated at a compression ratio of 10.2:1.

^*With BMW M Performance Power Kit