BMW S55 I6 Engine

First introduced in 2014 for the F80 M3 and F82 M4 Coupe, the BMW S55 was a 3.0-litre inline six-cylinder petrol engine with direct injection and parallel single-scroll turbochargers. Like the N55 engine on which it was based, the S55 engine had an 84.0 mm bore and 89.6 mm stroke for a capacity of 2979 cc.

The S55 replaced BMW’s S65 engine.

S55B30 block

The S55 engine had a die-cast aluminium alloy (AlSi7Cu0.5Mg) block, a closed-deck crankcase and liner-less cylinders due to the use of a twin-wire arc spraying process that deposited molten iron on the cylinder walls (also known as LDS or Lichtbogendrahtsprizen coating). The S55 crankcase also had longitudinal ventilation holes bored between the lower chambers of the cylinders to improve the pressure equalization of the oscillating air columns created by the motion of the pistons.

Unlike the N55, the S55 engine also featured:

• A lighter, forged steel alloy (42CrMoS4 Mod) crankshaft which was then ‘nitrocarburised’ (a hardening process); and,
• Full slipper skirt pistons that were manufactured by Mahle from an aluminium alloy (AlSi12Cu4Ni2Mg). The piston skirts also had a ‘Grafal’ coating to reduce frction.

The S55 engine operated at a compression ratio of 10.2:1 and maximum engine speed was 7600 rpm.

S55 cylinder head and valvetrain

The S55’s aluminium cylinder head housed double overhead camshafts (DOHC), variable intake and exhaust camshaft timing (dual VANOS), variable intake valve timing (BMW’s ‘Valvetronic III’) and four valves per cylinder (with sodium-filled exhaust valves).

For the S55, the cylinder head was modified so that it no longer had a built-in accumulator for the vacuum system and the VANOS system was optimised for faster adjustment.

Mitsubishi TF035 turbochargers

The S55 was fitted with two Mitsubishi TF035 turbochargers which operated in parallel to provide maximum boost pressure of 1.3 bar (18.85 psi). Unlike the N55‘s pneumatic wastegate valves, the S55 engine had electrical wastegate valves – activated by the engine management system – which provided faster adjustment, more precise boost control and greater closing force.

Fuel supply and injection

The S55 engine had two high pressure fuel pumps: one which operated continuously and a second which only activated beyond 3000rpm. Furthermore, the S55 engine had direct petrol injection via Bosch solenoid valve fuel injectors (HDEV5.2) with multi-hole nozzles, Bosch MEVD 17.2.G engine management and a Siemens SIMAF GT2 hot-film air mass meter.