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Evolution of the BMW Inline-6 Diesel
BMW is thought by many to have stronger diesel than petrol engines. This is in fact a new phenomenon. Today, I go over the evolution of 5 BMW inline-6 diesels and rank them into three tiers.
BMW is thought by many to have stronger diesel than petrol engines. This is in fact a new phenomenon. Today, I go over the evolution of 5 BMW inline-6 diesels and rank them into three tiers.
M21
- 1983-1991
- 2.4L, 80x81mm, 22:1
- SOHC, NA or turbo (no IC), Indirect injection (mechanically control -1987 / electronically control 1987+)
- Cast iron block, closed deck, cast crank
- 24d, 24td
The M21 was BMW’s first 6-cylinder diesel engine. Based largely on the M20 petrol, the M21 has a similar SOHC design with cast iron block and cast steel crankshaft. Its internals are reinforced to cope with higher compression and diesel combustion. The M21 also uses a timing belt. Its mechanical fuel pump was initially controlled mechanically, while later units had electronic control. The M21 was intended to be turbodiesel exclusively, but BMW introduced a naturally aspirated version in 1985 recognized by its “d” instead of “td” designation. There is no intercooler installed from factory.
The M21 is lethargic and gutless even in turbocharged form compared to the M20 petrol, much less the contemporary Mercedes OM603. Its cylinder head tends to crack, and timing belt failure is not uncommon. Part of the reason why BMW diesels are remembered fondly is because the M21 has been forgotten.
M51
- 1991-2000
- 2.5L, 80x82.8mm, 22:1
- SOHC, Turbo (IC only in tds), Indirect injection
- Cast iron block, closed deck, cast crank
- 25td, 25tds
The M51 was an evolution of the M21 with a similar cast iron block and cast steel crankshaft. Its small increase in displacement was achieved by having a slightly longer stroke. A main difference is the adoption of a timing chain as well as hydraulic tappets. In models designated “tds”, an intercooler is also fitted from factory.
The adoption of the intercooler significantly increased the M51’s power output, while the lesser “td” version had virtually no gain over the M21. It is considered more reliable than the M21, but the cooling and fuel systems are not faultless, and the cylinder head and block can still crack when overheated. While E36, E34, E39, and E38 are some of the most loved BMW chasses, the 25td and tds are not.
M57
- 1998-2013
- 2.5L / 2.9L / 3.0L, 16.5-18:1
- DOHC, Single or twin turbo, CDI
- Cast iron (-2005) / Aluminum block (cast iron liner, 2005+), closed deck, forged crank
- 25d, 30d, 35d
The M57 is likely where BMW earned its reputation in diesels. It was a completely new design with DOHC head, common rail direction injection, and considerably lower compression ratio. After TU2, the M57 also ditched cast iron in favor of aluminum for its crankcase construction. In the latter case, cast iron liners are used. BMW also added a forged crankshaft. Standard variants have a single Garrett turbocharger, while the more powerful TOP variants have two turbos in a series setup.
The M57, especially the TOP variants, has competitive performance. Even though more complex than the M51, the M57 is more reliable and more durable, thanks to lessons that BMW has learnt from earlier attempts to build diesel engines. Main issues include the swirl flaps in the plastic intake manifold that can be dislodged and ingested by the engine.
N57
- 2008-2019
- 3.0L, 84x90mm, 16-16.5:1
- DOHC, 1/2/3 turbo, CDI
- Aluminum block, cast iron liner, closed deck, forged crank
- 25d, 30d, 35d, 40d, 50d, M50d
The N57 was an evolution of the M57, bearing much resemblance to the M57TU2. A main difference is the revised timing chain setup at the rear of the engine. “U” and “O” variants have a single variable geometry turbocharger, while the “T” and “S” variants have two and three turbos respectively.
Performance-wise, the N57 was a small step forward in standards variants, while the “S” variants in the 50d and M50d models are able to pull V8-level performance. While the engine is considered reliable, the N57 suffers from timing chain problems and is generally less dependable than the M57. It is also said that the N57 seldom runs right after a rebuild, probably reflecting the engine’s tighter tolerances and resulting shorter service life.
B57
- 2015-
- 3.0L, 84x90mm, 16-16.5:1
- DOHC, 1/2/4 turbo, CDI
- Aluminum block, arc wire sprayed, closed deck, forged crank
- 30d, 40d, M40d, 50d, M50d
The B57 is part of BMW’s current B-series modular engine lineup, sharing basic design with the B37 and B47 diesels as well as the B38, B48, and B58 petrols. As such, the B57 has arc wire sprayed cylinder walls instead of cast iron liners. Still, BMW is wise enough to forgo VANOS and Valvetronic found in the B58, unlike the Mercedes OM656 that brought low compression and variable valve timing to diesel. BMW also had the good sense to retain closed-deck crankcase and an air-to-air intercooler setup. One main update with the B57 is the adoption of quad turbo setup in the “S” variant used in 50d and M50d models, which has sadly been discontinued in 2020.
The B57 develops immense torque across all variants, while the “T” and “S” variants also put out competitive power. Thanks to its relatively simple mechanical setup, especially compared to the B58 and OM656, it is also rather reliable. Notably, it has addressed the N57’s timing chain problem. Failures points of the B57 are mostly related to emissions systems. In my opinion, it is the best German diesel passenger car engine today.
Tier List
Tier 1: M57, B57
Tier 2: N57
Tier 3: M21, M51
Tier 2: N57
Tier 3: M21, M51
