BMW brought the downsizing trend to its sporty M models in 2014 with the in-line six-cylinder bi-turbo S55 engine from the M division. It was a special milestone for the class because the S55 is the first turbocharged engine under the hood of a BMW M car.
In the previous generation of the M3, BMW still relied on an S65 naturally aspirated V8 engine. Regardless of the more modest cylinder numbers, the change from naturally aspirated motors to turbocharging came bearing good news for fans. Horsepower is expanded from 414 to 425 hp (309 to 317 kW), and torque went up from 295 to 406 lb/ft (400 to 550 Nm). Here’s everything you need to know about BMW’s S55 engine.
Article updated on 08/12/21. Originally published on 11/16/18
What Makes the S55 Engine Special
The S55 is characterized by a high constant torque typical of a turbo, which is available in a wide range from 1850 rpm to 5500 rpm. The optimal performance band comes in from 5500 rpm to 7300 rpm and the maximum speed can be achieved at 7600 rpm. The engine offers the best of two worlds: turbo and high speed.
Diehard M car fans who understand the history of these cars were always skeptical about turbocharged motors. After all, some of the best M cars were built NA, which was their biggest strength. Yet, when BMW announced a twin turbo I6 engine for the next generation of M-cars, the fans understood the potential positives of such a move.Unlike the N55, the BMW S55 features a closed deck engine block and crankcase design, making it resilient to the higher cylinder pressures associated with the ramped up output.
The bore X stroke are slightly undersquare, at 84mm X 89mm. The longer stroke helps the S55 achieve excellent low end torque, with peak grunt available from well under 2,000rpm. Peak horsepower continues flatly all the way past 7,000rpm.
History of the S55 Engine
The S55 uses the N55 engine from BMW as the basis. The original N55 engine was first used in 2009 on the BMW 5 Series Gran Turismo. Bore (84.0 mm), stroke (89.6 mm), and displacement (2979 cc) are identical in the N55 and S55. The compression ratio of 10.2: 1 is also the same for both engines.
With the newly developed in-line 6-cylinder engine, BMW M GmbH was working with both the traditional and the new system: The aim was to combine the advantages of a high-performance, naturally aspirated engine with the strengths of modern turbo technology.
The effects on the response characteristics of the M3 and M4 models are obvious: 431hp and a maximum of 7,600 rpm show the potential of the 3.0-liter engine. With a maximum torque of 550Nm – from 1,850 rpm. Fast sprints are performed faster than ever before, and acceleration from 0 to 100 km/h now takes just 4.1 seconds if the 7-speed M dual-clutch transmission takes over the shifting. The engine had quickly proven to be more efficient than its predecessors.
The Difference Between N55 and S55
The S55 is essentially a strengthened version of the N55. While the N55 from BMW is equipped with only one turbocharger with “TwinPower” technology (a turbo-charger that is driven twice by two separate exhaust manifolds), the S55 has two classic turbochargers. The structure of the engine block is also more complex in the M engine. As a result, it’s able to generate class leading throttle response, low CO2 emissions, and a stratospheric 141 horsepower per liter of displacement.
The engine comes with several innovations and differs from the base model thanks to a number of technical refinements. Nevertheless, genes from BMW are unmistakable when it comes to the crankcase or the very special type of liner coating.
One of the main differences is the closed-deck design of the crankcase. The advantage of this special design compared to the open-deck design is the higher rigidity of the cover plate, which has a positive effect on both the cylinders and the acoustics. In addition, higher cylinder pressures and overall better power output can be achieved.
It’s worth mentioning, however, that numerous BMW series engines, as well as engines from other manufacturers, have a crankcase in closed-deck design, it is not a unique feature of the S55.
Together with the variable valve control (Valvetronic), gasoline direct injection HPI, and the double Vanos, the throttle response is noticeably improved even at low engine speeds. The turbocharged engine reaches the 370 lb. Ft of torque, which is 30% more than what its predecessor had to offer. At the same time, the fuel consumption is down by 25%.
Pistons are Grafal-coated ‘slipper’ style aluminum alloy (AlSi12Cu4Ni2Mg) units manufactured by Mahle, made to ride in sleeveless cylinders. The interior of the S55 cylinder jugs are coated with what is called a 2-wire arc spray process (LDS), reducing weight. With this system, the surface is coated with a thin but extremely strong atomized spray of molten metal, and is commonly used for extreme applications such as turbine blades. I cannot find any specifics on what metal they use, but be sure it is some kind of super alloy.
The bottom of the crankcase has special ventilation holes bridging cylinders 1-2-3 and 4-5-6 for improved pressure equalization to reduce pressure oscillation. Oil squirters are utilized to spray the bottoms of the pistons for crown cooling.
Two single scroll Mitsubishi TF035 turbochargers provide around 18lbs of peak boost to the cylinders, sending power to a nitrocarburized forged steel crankshaft (42CrMoS4 Mod) featuring a symmetrical counterweight arrangement and 7 main bearings.
The twin turbos each have an electronically actuated waste gate, further enhancing throttle response while also deleting vacuum demand. The design of the waste gates help negate the need for a blow-off or recirculation valve. The physical design of the turbos are similar to the N54, with integrated cast alloy steel exhaust manifold for optimum flow.
In direct comparison with the N55 engine, the fundamentally revised cooling system of the S55 engine is one of the features that make it stand out. In addition to the main cooler, it comes with an additional cooler for the high and low-pressure temperature circuit. The turbochargers and the transmission both benefit from an electric cooling water pump, which enables a balanced temperature.
Last but not least, there is also a scavenger oil pump added to the main oil pump system. The secondary pump collects oil from the corners of the oil pan, thus making oil starvation a non-factor even under aggressive cornering.
Which Cars Have the S55 Engine
BMW has used many applications of the S55 engine on different M models over the years since 2014.
You can find the list below:
2020-present F87 M2 CS Racing (275 hp – 360 hp)
2019-2021 F87 M2 Competition (405 hp)
2020-2021 F87 M2 CS Racing (444 hp)
2014-2018 F80 M3 (425 hp)
2016-2018 F80 M3 Competition (444 hp)
2018 F80 M3 CS Racing (453 hp)
2014-2020 F82 – F83 M4 (425 hp)
2016-2020 F82 – F83 M4 Competition (444 hp)
2017-2020 F82 M4 CS Racing (453 hp)
2015-2016 F82 M4 GTS (493 hp with water injection system)
2017 F82 M4 DTM Champion Edition (493 hp with water injection system)
Which Parts Does Your S55 Engine Need?
Like all other engines made by BMW, the S55 works best with genuine or OEM quality parts. It’s a known truth that it can be hard to find the right parts according to your engine’s specifications if you’re not a professional mechanic.
We have an easy solution for your problem here! Please choose your car’s make and model from the drop-down menu. You will be presented with a list of parts specific to your car & engine.