After reading a thread about the supposed disadvantages of the N55 vis-a-vis the N54 twin turbo, I came across a N55 BMW Training Manual online that sheds further light on the subject.
Some of the advances incorporated into the N55 have been well publicized: Average 8% better fuel economy; lower CO2 and NOX emissions; peak torque earlier (1200 rpm vs 1400 rpm); and no turbo lag. But a review of the BMW Training Manual on the N55 shows many detailed refinements that taken together make a great engine (N54) even better. The following is a summary (the numbers in parenthesis refer to the page numbers). The training manual pdf can be viewed by Googling “pdf n55 engine”.
New coolant passages between cylinders to improve cylinder cooling (p.14 & 81).
New lightweight cast iron crankshaft (6.5 lbs lighter) (p.14).
New design con rod that spreads power-stroke forces more evenly on the wrist pin (p.19).
Oil supply in oil pan vacuum pump now lubricated by filtered oil vs. unfiltered oil (p.22).
New ecu-controlled electric oil pump reduces parasitic power losses and optimizes both volume of oil pumped and oil pressure (p.24).
New cylinder head with third generation Valvetronic reduces emissions and improves fuel economy, and now has cooling passages for the fuel injectors (p.32 & 80). New Vanos is lightweight aluminum and provides faster adjustment of valve timing (p. 44).
New cylinder head cover incorporates completely redesigned crankcase ventilation system to better separate oil from blow-by gases (p. 34 & 38). Improved crankcase ventilation extends oil service life (p.16).
The camshafts are now a lightweight hollow design (p.41).
New, simpler, less complicated Bosch solenoid fuel injectors specially adapted to the combination of Valvetronic III and turbocharging (p.49).
New vibration damper on the belt pulley reduces belt load for better belt life (p.53 & 54).
New twin scroll turbo provides enhanced throttle response and incorporates a new electric diverter valve that eliminates surges in boost pressure and reduces stress on the turbo bearings (p.65 & 91).
Less complicated exhaust system that provides one catalytic converter vs. two, as on the N54 (p. 67). Improved oxygen sensors that provide faster response (p.95 & 96).
A new engine-mounted DME module that considerably simplifies and shortens wiring (p.82).
Ignition coils redesigned and “sturdier” (per the manual) (p.94 & 100).
As to the sturdiness of the N55: Two N55-powered f30 335’s were raced by a South African team in 2012 (including being raced in Germany at the 24 Hrs of the Nurburgring). Other than a modified turbo, racing ecu, and aftermarket exhaust, the N55 engines were stock and produced 408 hp and 443 lb ft of torque. According to a representative of the team, in an email, the engines ran the entire 2012 season without any engine failure or need to be “refreshed”.