BMW 4 Series Turbo

From the N47 to the B-series: when BMW switched to fixed-geometry turbos on the petrol Série 4

The Série 4 is built around two distinct engine families. On the diesel side, theN47D20C, B47D20Aunits cover the 418d and 420d withvariable-geometryturbos: the adjustable vanes allow the gas flow cross-section — and therefore boost — to be varied across a wide rev range, but they are prone to progressive fouling from carbon deposits, particularly with short urban journeys. On the petrol side, theB46B20B, B48B20, B48B20A, B48B20Bunits (420i, 428i xDrive, 430i, 430i xDrive) and theN55B30Ain the 435i runfixed-geometry turbos with a wastegate. Boost pressure is regulated by opening the wastegate rather than by movable vanes; thermal stresses are high, particularly on the 435i, whose N55B30A endures exhaust gas temperatures significantly higher than those of the four-cylinder units.

Two diesel generations on the 418d: N47 and B47, two different turbos

The 418d designation covers two engines with no direct mechanical relationship. TheN47D20Cis the older common-rail indirect-injection unit, whereas theB47D20Abelongs to BMW's modular engine generation. These two engines do not take the same turbo: the manufacturer references differ entirely. For the 418d in its 136 hp and 150 hp outputs, both engine codes coexist depending on the year of production; checking the code stamped on the block is the only way to avoid an ordering error. On the 420d, the same logic applies: N47D20C or B47D20A depending on the date of manufacture.

Causes and symptoms to address before replacing the turbo

On variable-geometry diesels, premature wear is often caused by fouling of the variable-geometry mechanism or by insufficient lubrication — itself the result of degraded oil or excessively long oil-change intervals. On the petrol B48 and N55 units, the most common causes are an ingested foreign object, an air leak on the intake circuit, or a lubrication failure following a harsh cold start. A characteristic whistling noise, smoke, or a faulty wastegate actuator are the usual warning signs. In every case, identifying and correcting the root cause — oil quality, intake circuit integrity, condition of the CHRA — is what determines the service life of the replacement turbo. All turbos offered in this collection arenew turbos sold as a straight sale, with no old unit to return.