BMW’s new N55 engine family’s 306 horsepower and 400Nm power and torque output figures reminds us of the original moment when BMW first announced it was bringing back forced induction into the BMW engine line-up. The N54 engine was factory rated to put out that exact same amount, and it did that via the use of gasoline direct injection and two turbochargers running together, fed gas from three cylinders each.
Three years later, we still have 306 horsepower and 400Nm of torque, but BMW has achieved it using only one turbocharger instead of two. The new N55B30 engine also churns out peak boost and torque faster than the N54, with 400Nm hit at the 1,200rpm point instead of the N54B30’s 1,3000rpm mark.
The single turbocharger uses twin scroll technology. A twin scroll turbine housing uses dual side by side exhaust gas inlets into the housing, fed by two sources of exhaust pulses. A single path is also not able to take advantage of all the exhaust gas pulses effectively. Since there are two passages, each has a smaller overall volume than a single scroll turbo path, thus the exhaust velocity of each pulse can be maintained.
The N55B30 is not the first BMW engine to use a twin scroll turbocharger, as the BMW Prince engine in the MINI Cooper S also uses one. In the Cooper S, the turbocharger’s twin-scroll principle is fed by two sets of two cylinders each, while in this N55 inline-6 engine it is two groups of 3 cylinders each.
The N54 did not feature Valvetronic because in a turbocharged engine, throttle losses can be countered by the positive pressure of the turbocharged air being blown into the engine. But the main reason there was no Valvetronic is because the complexity of designing a cylinder head that could fit Valvetronic mechanisms as well as direct injection. It is because of this reason that the N53 normally aspirated direct injection engines also do not feature Valvetronic.
BMW engineers probably finally managed to cram all that technology into the engine and make it possible for them to work all together. The new N55B30 has both a new generation Valvetronic variable valve management system and High Pressure Injection (direct injection at up to 200 bar). Valvetronic replaces the throttle butterfly functions with a control unit infinitely varying the valve stroke on the intake valves. This means the intake is not obstructed by a partially closed throttle during low load operations. The new N55 also features a map-controlled oil pump which takes up less energy, which BMW says is a first in a turbocharged engine.
One of the variations of the Volkswagen EA888 engine also features variable valve management combined with direct injection but the EA888’s valvelift system is only fitted on the exhaust valves of the engine and works in two stages – a high cam and a low cam, unlike Valvetronic which is continuously variable.
Fitted in the newly unveiled BMW 535i GT, the new N55B30 engine takes the machine from 0 to 100km/h in just 6.3 seconds, up to a top speed of 250km/h.