Inline-6s are everywhere. We can see them in ancient airplanes, trucks, industrial motors, motorbikes, and premium cars because of their inherent balance and robustness. Many automobile manufacturers produce good inline-6s, but no one is more famous for that than BMW. Looking back into history, the first engine ever produced by BMW, or BFW back then, was the inline-6 BMW IIIa aircraft engine used on Fokker D.VIII. Even BMW Motorrad applies the transverse 1.6L I6 on K1600 Series today. If we take another glance at the current automobile lineup of BMW, we can see that the inline-6 is available in almost all series. The inline-6 is indeed an icon of BMW just as the flat-6 of Porsche and the V12 of Ferrari. This article talks about the popular modern inline-6 of BMW starting from the Neue Klasse era.
The Neue Klasse models marked the new nomenclature of engine codes with the introduction of the M10/M20/M30. “M” intuitively stands for “Motor”: M10 stands for I4s; M20 stands for “small sixes” with displacement up to 2.7L; M30 stands for “big sixes” with displacement from 2.5L and above. M30 powered the early E3 and E9 lineups and stayed around with the 5/6/7 Series, competing with high-end sports and luxury cars. Two turbocharged versions were made exclusively for the E23 745i —the 3.2L M102 and the 3.4L M106, and this was considered a bold move in this segment as competitors were offering large V8s at the time.
The small sixes, on the other hand, expanded the access to 6-cylinder engines down to the compact class. A 4.3-meter sedan with I6 under the bonnet would be crazy today, but BMW did so on 3-Series back in the day. Mercedes-Benz did pack the M103 2.6L I6 engine into 190E, but M103 was not designed to be a compact-car engine. M20 served as the basis for the diesel M21 which is first introduced on E28 524td. This initiated the strategy of offering diesel variants alongside petrol siblings for later generations. Both M20 and M30 were rather long-lived, powering many BMWs from the 70s to 90s.
The high-performance variant M88 derived from M30 powered the very first M car — BMW M1 — and the early M5/M6 models. The E12 M535i was the first car ever to be badged with the letter “M”, but it had an enhanced M30 variant (M30B35LE) instead of M88. The E24 M635CSi did have the M88/3 engine but was only called “M6” in selected markets. The E28 M5 got its naming settled from the premiere and was equipped with the same M88/3 as M635CSi, proving that a sedan can be just as fast as a Porsche. The S38 on E34 M5 that replaced the M88/3 was the ultimate evolution of the M30, with S38B38 putting out 340 PS without forced air intake.
In the 90s, the SOHC M10/M20/M30 were replaced by the DOHC M40/M50/M60. While the M40 and M50 were still I4s and small sixes, M60 marked the revival of BMW V8 engines, dropping the long-lived big sixes. The I6 engine lines have been merged into a single stream ever since, and the BMW I6s we see today are legitimate decedents of small sixes. The small sixes got the codes M52 and M54 to denote the improvement throughout the years. The components of M52 and M54 are highly interchangeable. One can even replace the aluminum engine block of the M54 with the cast-iron block of the M52 for better tuning durability. The displacement of the mainstream small sixes reached 3 liters for the first time when the M54B30 was introduced. M54B30 is regarded as the most all-rounded and durable engine ever.
The corresponding diesel versions for this generation were the 2.5L M51 and later the 2.5L/3.0L M57. M51 was introduced together with M50; M57 came with M54 but was kept on the market with the later N52/N53/N54 petrol variants. M57 was the first common-rail injection 6-cylinder diesel of BMW, and the output increase over the M51 was substantial. The mightiest variant of the M57 lineup was the 3.0L M57 with one low-pressure and one high-pressure turbo. It put out 286 PS and 580 Nm on a variety of 35d/3.0sd models. M57 earned the name “diesel M” at the time and paved the way for future M-Performance diesel models.
The M-variants of this generation were the summits of naturally aspirated I6. The EU-spec S50B30 on E36 M3 matched the old M88 in horsepower, and the later S50B32 introduced later pushed the output over the 300 PS threshold. The US-spec E36 M3, however, had different S50 and S52 options because of the regulations. The story did not end here as the E46 M3 shocked the world with S54B32 yet again. A maximum of 360 PS was available on the E46 M3 CSL, which is one of the most-wanted old M cars on the market today.
Most competitors switched to the V6 engine in the 2000s because V6 was easier to pack into a small car and can thus be shared between large and compact models. BMW persisted in refining the I6 to the limit, nevertheless, and developed a revolutionary N-generation engine. The completely reengineered N52 features a lightweight magnesium alloy structure and Valvetronic, and the twin-turbocharged N54 reminded the world that BMW was once good at turbos.
N52 and N54 were inherently different, however, as the N54 carried the aluminum engine block design from the old M54 for better durability under turbocharging. N54 thus had no Valvetronic and shared the same 2979 cc displacement with M54, while N52 had a slightly larger 2996 cc. The performance of the turbo I6 was so incredible that the F01/F02 740Li could outperform the V8 twin-turbo 750Li before 80 kph. BMW later increased the output of 750Li from 407 PS to 450 PS to avoid such an awkward comparison. A modified variant of N52 coded N53 was later introduced to the EU market exclusively. N53 shared the same displacement with N52, but ditched the Valvetronic and replaced the port injection with high-pressure direct injection. The changes made N53 susceptible to high sulfur fuel, so N52 kept its offering in other regions where the fuel quality could not match.
N52 and N53 are, from hindsight, the last naturally aspirated BMW I6 and will stay a legend. The E87 130i will remain a unicorn forever as a “N/A I6 3-liter manual 5-door hatchback”. No corresponding M-variant was introduced for this generation as BMW provided an even larger V8 and V10 for the M family at the time.
The introduction of N55 officially declared the retirement of N/A I6s and the pursuit of simplification. The only configuration in the generation was a 3.0L twin-scroll single-turbo I6 with Valvetronic and direct injection. N55 shared the same displacement with N54 whose structure can be traced back to M54. N55 was the first BMW I6 to be electrified. The I6 hybrid system was applied to ActiveHybrid models at the time.
I6 was back to the M3/M4 but with turbochargers this time. The S55 put out significantly more torques than the N/A V8 previously, and such big torque made the RWD M3/M4 hard to launch from the still. The extra torque brought by the turbo suggested that AWD would be a better choice for these torque monsters. As the subsequent M cars have gone more civilized with the aid of torque converters and AWD, the F8X M3/M4 were indeed the last untamed beasts in M history.
N57, the successor of M57, was introduced at about the same time as the N55. N57 only came as a 3.0L diesel, but the various turbocharger configurations gave it different output numbers. The single turbo 30d variants provided at most 258 PS; the twin-turbo 40d variant offered at most 313 PS; the mind-blowing tri-turbo 50d variant delivered 381 PS on 750d/Ld and the first-ever M-Performance diesel models including M550d, X5 M50d, and X6 M50d. The M50d models kicked off the M-Performance family, which serves as the second most powerful variant of the range even today. This nomenclature somehow paid tribute to the original M-models — M535i and M635CSi.
The current generation B58 is the peak of the BMW I6. B58 is part of the modular engine lineup including the I3 B38 and I4 B48. The development of new modular engines coincided with the Z4/Supra project, so Toyota took place in the testing of the I4 B48 and the I6 B58. It is believed that B48/B58 is robust enough to meet the quality standard Toyota. B58 appears in all series, from 1 to 8, Z to X, 2- to 7- seaters, and hybrid to sports cars. B58 features a closed deck cooling design similar to the previous M-version S55. The previous mainstream version N55, in contrast, featured open deck cooling and was thus limited in the maximum power output. Such a design unlocks the potential of B58. We can see tuners trying to squeeze out a thousand horsepower from the B58-powered Toyota Supra. B58 showed its flexibility again on PHEV models, but this time it is badged as “45e” on 5-Series, 7-Series, and X5.
The concept of modularization was applied to diesel engines as well. The modular diesel engines are the I3 B37, I4 B47, and I6 B57 now. BMW implemented similar configuration tactics on B57 by providing one displacement with different turbocharger layouts. The 3.0L single-turbo 30d and the twin-turbo 40d were just the same as their predecessors, but BMW astonished everyone with the 50d variant. The quad-turbocharged B57 delivered 400 PS, making it the most powerful 6-cylinder diesel engine and the only two quad-turbo engines on the market (the other one is the Bugatti). Tuners could easily squeeze 500 PS out of this engine without a significant hardware upgrade. The party of crazy diesel ended recently, however, as BMW ceased the production of 50d as a result of low demand and the difficulty of quad-turbo assembly. The configurations of B57 were reduced to 30d and 40d, both of which have come with two turbochargers.
The M-version S58 further proved the potential of this generation. The street-legal M3/M4/X3 M/X4 M Competiton versions have 510 PS available. The M4 CSL, which revived the CSL name since the E46, has a S58 producing 550 PS. B58 is the first 6-cylinder M engine to be paired with the AWD and the torque converter automatic gearbox (ZF 8HP). Such a layout makes the M cars easy to handle and launch. The racing version P58 on M4 GT3 has almost 600 PS at its disposal on the track. This is the first 6-cylinder GT3 racecar ever produced by BMW.
The I6 is gaining back its popularity in the premium segment thanks to the downsizing trend. Mercedes-Benz and Jaguar have already revived their I6 engines although only available in full-size vehicles. The future of the internal combustion engine is likely to feature modularized 3.0L I6 and 2.0L I4. The once prevalent V6 is declining as manufacturers are not putting 6-cylinder in compact cars now. Throughout the years, the only one who never bothers to make a V6 is, without a doubt, BMW.
