BMW S55 Engine Guide – Specs, Reliability, & Modifications

Prior to 2013, even thinking about a turbocharged BMW M3 was considered sacrilegious. But, then again, BMW always brushed controversy aside when it came to their star child. Before the F80 was the S65 V8-powered E9X M3 which was Judas 1.0 to the M3’s holy inline-6 recipe that began with the E36. Now it has become one of the community’s favorites. With a healthy dollop of deja vu, the same exact story repeated a generation later with the F80 M3 and the BMW S55 engine that powered it.

The BMW S55 3.0L turbocharged inline-6 marked a massive turning point for BMW’s M-division and the company at large. While BMW had dabbled in high-performance turbocharged M-variants of other engines prior to the S55, like the S63 initially used in the 2010 E70 X5M, the S55 proved that reliable turbocharged performance engines were not only possible to make but also the definite way forward. That is further evidenced by the fact that BMW had enough confidence to guinea pig the S55 in the company’s most famous model. 

In this guide, we’ll provide a comprehensive overview of the 3.0L twin-turbocharged BMW S55 inline-6, outlining its specs, upgrades compared to the BMW N55 off which it is based, its overall reliability, and some of the most popular S55 performance modifications. 

BMW S55 Engine Specs

Engine: BMW S55 Engine

Configuration: Inline-6 Cylinder

Displacement: 3.0L (2,979 cc)

Aspiration: Twin Mono Scroll Turbocharged

Valvetrain: DOHC, Dual-VANOS, Valvetronic, 4-valves per cylinder 

Block/Head: Aluminum

Bore x Stroke: 84.0 mm x 89.6 mm

Compression Ratio: 11.5:1

Engine Weight: 452lbs

Horsepower: 405-493 hp 

Torque (lb-ft): 295-406 lb-ft 

The BMW S55 is a 3.0L inline-6 engine that utilizes two mono-scroll turbochargers for each bank of three cylinders. Like the N55 engine that it is based on, the S55’s block and cylinder head are constructed from aluminum. The S55 engine makes use of a closed-deck engine block for added rigidity and strength. The BMW S55 is also equipped with both VANOS variable valve timing and Valvetronic variable valve lift technology. Many of the S55’s internal and auxiliary components, like the crankshaft, main bearings, and oil pan, were redesigned to save weight and improve strength. 

Overall, the S55’s use of turbo technology and improved engine design as a whole gives it a 30% increase in torque and a 28% improvement in fuel efficiency over the previous S65 V8 found in the E9X M3. 

BMW N55 vs S55

It only makes sense that we compare the BMW N55 and S55, as the S55 adopted 75 percent of its components from the non-M 3.0L turbocharged inline-6. The BMW N55 was introduced four years prior to the S55, in 2009, as a replacement for the beloved, if not sometimes frustrating, BMW N54 engine. The N55 introduced a number of important advances to BMW’s turbocharged inline-6 formula including twin-scroll turbo and Valvetronic variable valve lift technology. These advances would carry over into the BMW S55 and subsequent turbocharged engines like the BMW B58. 

The primary differences between the S55 and earlier N55 engines can be seen in their block design, cooling system design, turbocharger arrangement, and internal components. 

Open Deck vs Closed Deck Crankcase

The biggest difference between the engines is their crankcase design. While the N55 made use of an open deck design, meaning that it featured coolant channels surrounding the cylinders, the S55 utilizes a closed deck crankcase. While an open deck, like the one used on the N55, is great for cooling efficiency and reducing combustion temperatures, it also provides the least amount of structural support. 

The S55’s closed deck design is much more structurally rigid than the N55’s crankcase, meaning that it can withstand higher cylinder pressures and more power as a result. That is also one reason for the S55’s increased 7,600 RPM redline. However, with the closed deck block came the need for a revised cooling system to compensate.

The N55 and S55 also differ in terms of their cylinder liners. The N55 previously made use of cast iron cylinder liners which helped to provide the cylinder walls with some added strength and rigidity. The S55 got rid of these cast iron liners in favor of LDS-coated cylinder liners through a process called electric arc wire spraying. Not only do the S55’s LDS-coated liners provide just as much strength, but they also reduced the engine block’s weight by 4.85 lbs. 

Cooling System Design Differences

Another one of the biggest differences between the N55 engine and the S55 engine is their cooling system designs. As we mentioned before, the S55’s closed-deck crankcase, twin-turbo arrangement, and higher overall performance ceiling, mean that it needs a much beefier cooling system than the N55. 

The N55 already features a pretty efficient cooling system. Equipped with a main radiator, auxiliary radiator, hot climate oil cooler, and an oil-to-coolant heat exchanger, in combination with the open deck crankcase, the N55’s cooling system does a pretty good job of keeping the engine cool.

With that being said, the S55’s additional turbo and closed deck crankcase necessitate a more comprehensive system. The S55’s cooling system is split into two circuits. One is an engine cooling circuit and the other is a charge air cooling system. It also includes oil cooling for the engine oil and transmission. 

The S55 uses two auxiliary upstream radiators, one for the engine and one for the charge air circuit in addition to a large main radiator for both circuits. The charge air circuit also uses an additional indirect charge air cooler. The turbos are also fed coolant by an auxiliary electric coolant pump.

As an additional note, the S55 uses a mechanically driven (belt-driven) water pump for the engine cooling circuit instead of the somewhat troublesome electronically driven water pump found on the N54 and N55 engines. 

Turbocharger Setup

The S55 moved away from the single twin-scroll turbocharger used on the N55 in favor of a more traditional twin parallel Mitsubishi TF035 mono-scroll turbocharger arrangement. The S55 is more like the N54 in that regard, as the N54 was also known for its true twin-turbo setup. There are benefits and shortcomings to both a single twin-scroll turbo and a true twin-turbo design. 

Twin scroll turbos, like the one found on the N55, are the more efficiency-focused option of the two setups. BMW’s twin-scroll turbo BMW utilizes what’s called a divided inlet turbine. A twin-scroll turbo is divided into two “scrolls,” utilizing an exhaust manifold split into two headers based on firing order. 

Ultimately, the N55’s single twin-scroll turbo was designed to decrease boost lag, create a more even torque curve, and increase overall engine efficiency. The N55’s twin-power turbo is unquestionably good at what it is supposed to do but it doesn’t leave nearly as much room for aftermarket antics as the S55’s true twin-turbo arrangement.

By using two Mitsubishi TF035 mono-scroll turbochargers, the S55 bridges a fantastic middle ground between performance and efficiency. Ultimately, the S55’s turbo system doesn’t sacrifice much, if any, turbo response to the N55’s twin-scroll turbo while also providing better overall performance. Each turbo is responsible for a bank of three cylinders and the turbos work in parallel. Despite the use of two turbochargers instead of one, the S55’s turbo system is equally as light as the N55’s.

N55 vs S55 Engine Internals/Auxiliary Components

 While the BMW S55 adopted roughly 75% of its components from the N55, there are some very notable differences between the internals of both engines. All of the most important internal components that the S55 borrowed were reworked with strength and weight savings in mind.

We’ll start with the rotating assembly. The S55 is known for having a well-engineered forged steel crankshaft that is not only stronger than the one found in the N55, but also 4lbs lighter. The S55’s connecting rods were revised, using a different kind of aluminum and a differently shaped bore to help with load distribution. The S55’s pistons are another big topic of conversation, as they were also redesigned. The S55’s factory Mahle pistons are constructed from aluminum alloy for maximum lightness and Grafal coated to prevent friction.

The S55’s oiling system was also upgraded to allow for better track performance without the risk of oil starvation. In order to provide an uninterrupted flow of oil around the engine, BMW installed a second oil pump, also called a suction pump, which supports the return flow of oil from the turbochargers and the front areas of the oil pan back to the rear of the oil pan. As a result, the S55 is able to handle more strenuous acceleration, deceleration, and cornering forces.

What Cars Use the BMW S55 Engine?

As with most other BMW engines, the S55 was offered in different trim levels during its build cycle. For example, the Competition and CS variants of the M3, M4, and M2 received higher-output versions of the S55. This difference in power can be attributed to differences in the factory tunes. The only S55 variant that is physically different is the one found in the M4 GTS which features a water injection system. Here are the BMWs that make use of the S55 engine with their corresponding power output:

• 2020–present F87 M2 CS Racing (275-359 horsepower)
• 2019–2021 F87 M2 Competition (405 horsepower)
• 2014–2018 F80 M3 (425 horsepower)
• 2014–2020 F82/F83 M4 (425 horsepower)
• 2016–2018 F80 M3 Competition (444 horsepower)
• 2016–2020 F82/F83 M4 Competition (444 horsepower)
• 2020–2021 F87 M2 CS (444 horsepower)
• 2018 F80 M3 CS (453 horsepower)
• 2017–2020 F82 M4 CS (453 horsepower)
• 2015–2016 F82 M4 GTS (493 horsepower)
• 2017 F82 M4 DTM Champion Edition (493 horsepower)

Stock BMW S55 Engine Performance

Before we jump into how the S55 could potentially perform with the right performance modifications, let’s talk about the engine in stock form first. Both the minor and major upgrades made to the N55 platform to bring us the S55 were made in pursuit of performance. With the differences in design from the previous engine iteration, the BMW S55 also displays a number of improved performance characteristics. 

One of the most notable elements that gives the S55 better stock performance than most BMW turbocharged 6-cylinder engines before it is its twin-turbo setup. Combined with dual-VANOS and Valvetronic III variable valve lift, the two small mono-scroll turbos allow for almost immediate throttle response and instant torque which can be seen on the dyno chart below. 

The torque curve of a stock S55 typically levels out and stays consistent between 3,000 and 6,000 RPM, which is also backed up by dyno results. Overall, those characteristics are pretty similar to what you’d expect to see from an N55, just at a higher performance threshold. Power delivery is also extremely linear and predictable, which is something that you’d expect from a naturally aspirated BMW 6-cylinder.

There’s no doubt that the S55 is one of the best turbocharged inline-6 engine that BMW has ever made in terms of out-of-the-box performance. In fact, it is commonly reported that BMW vastly underrated the S55’s peak power output, often being much higher than the factory-reported figures. The fact that torque is so immediate and stays steady through the majority of the rev range means that you’ll always have usable grunt under your right foot no matter what.

BMW S55 Reliability

In the beginning, the reliability of BMW’s turbocharged engines was a serious point of contention amongst potential buyers. The N55 was truly one of the first turbo BMW engines that could truly be considered reliable. Luckily, that carried through to the S55 as well.

Since the S55 was released 10 years ago, it has proven to be an extremely reliable engine with very few common problems. Read our article on BMW S55 common engine problems for more on the below issues and reliability.

BMW S55 Crank Hub Issues

This S55 “common problem” is a very heated one in the M3/M4 community and one that has caused a lot of drama and argumentation in the forums as a result. Essentially, this problem stems from early reports that the M3 and M4, equipped with the S55 engine, were experiencing frequent crank hub failure. 

While there are some reports of this happening, the overall percentage of S55 owners that have experienced this issue is very low. Most of the coverage of this problem has stemmed from aftermarket communities trying to overreport the problem to sell replacement parts that have proven to be even more damaging than the factory crank hub. 

The exact cause of the issue is open to debate, but it is thought that S55 crank hub failure is the result of rpms changing rapidly during upshifting and downshifting, causing a nearly immediate change in crank speed to match the input shaft. The inertia caused by this abrupt change in crank speed can loosen the sprocket. Most of the time, this can cause the sprocket to slip a couple of degrees, throwing off the cam timing. 

The issue is reported more frequently in DCT-equipped cars and cars with upgraded transmissions due to the nearly immediate shift times. However, it is possible that the reports are skewed due to the fact that there are more DCT cars on the road than manuals. There is also some evidence to suggest that it is more common in cars with higher-than-stock power levels. 

Regardless, the number of S55-powered BMWs that this happens to is very low. It is commonly concluded, though, that swapping to an aftermarket S55 crank hub is a terrible decision with potentially disastrous consequences, as they are known to split entirely.

BMW S55 Charge Pipe Failure

This is another problem that isn’t very common in the traditional sense of the word. With that being said, there have been reports of S55 plastic charge pipes cracking or fatiguing over time. This is an issue that the S55 shares with the N55, which also had issues with its plastic charge pipe failing. This failure is simply the result of the material that BMW chose for the charge pipes themselves. With the constant heat cycling of the engine and the high engine temperatures that the S55 can reach, the plastic can warp or crack over time.

It is also common for the rubber couplers that connect the charge pipes to the valve-cover-mounted heat exchanger to come loose, allowing the charge pipes to detach from the heat exchanger itself. In this case, you’ll likely experience extreme power loss and limp mode. Some BMW dealers have solved this issue by degreasing the coupler and creating a rough mating area on the charge pipe with sandpaper to give the coupler a bit more bite. 

Upgrading to a set of aftermarket aluminum charge pipes is a very common solution to the problem, as there is no chance of them warping or cracking due to heat cycling. We also have a full guide on BMW S55 charge pipe upgrades.

Valve Cover and Oil Filter Housing Gasket Leaks

As with almost any engine, the BMW S55 is prone to oil leaks from the valve cover gasket and oil filter housing gasket as time goes on. As with the other problems on our list, these oil leaks are not a truly common problem for the BMW S55, affecting a large number of engines. However, knowing what we know about the oil leaks on high-mileage N55 engines and the similarities between the S55 and N55 valve cover and oil filter housing designs, it is safe to assume that high-mileage S55s will suffer the same issues. 

Ultimately, S55 valve cover gaskets are prone to degrading over time, exacerbated in part due to the S55’s high operating temperatures and constant heat cycling. The same thing can happen to the S55 valve cover itself, as it is made of plastic that is susceptible to warping and cracking as well. It is generally recommended to replace them at the same time. 

As with the valve cover gasket, the S55 oil filter housing gasket is also prone to degrading over time due to continuous heat cycling and age. This is another component that the S55 shares with the N55, with the N55 being notorious for oil filter housing gasket leaks. 

Best BMW S55 Modifications

So far, we’ve covered that the BMW S55 is an exceptionally strong, capable, and reliable engine in factory form. In most cases, that is the perfect recipe for an engine that has a lot more to offer when it is introduced to aftermarket modifications. That is certainly true for the BMW S55. Like the BMW N54 and N55 that came before it, the S55 has become a modder’s dream, with countless modifications, tuning options, and performance parts to count; and the S55 can handle them all. 

With the S55’s closed deck crankcase, twin-turbo arrangement, hefty cooling system, and upgraded oiling, a stock S55 is capable of handling 700-800 horsepower without a hiccup. In fact, it is relatively easy to get to pretty outrageous power figures with some simple bolt-on modifications. We’ll cover some of those in this section. If you are interested in learning more about BMW S55 tuning and modifications, take a look at our dedicated Best BMW S55 Engine Mods Guide.

BMW S55 Tune

Without a doubt, the easiest way to extract the most power out of an S55 engine for the least amount of money is a quality tune. For between $250 and $600, you can easily breach the 500 horsepower threshold with no other required modifications. The turbo inline-6 BMW tuning game is pretty much dominated by three names that have extensive credibility and customer praise to back them up: Burger Motorsport, MHD, and BM3. 

The Burger Motorsport JB4 is a piggyback tune, meaning that it retains the factory ECU tune and simply tricks the DME by adjusting sensor inputs. Piggyback tunes like the JB4 has the added benefit of being untraceable when uninstalled due to the fact that it doesn’t change any fueling or timing tables. The JB4 also supports on-the-fly map changes. The only downside to an S55 piggyback tune is that it is limited to the capabilities of the stock DME and not as customizable or precise as our next option.

The other route is an S55 flash tune. Unlike an S55 piggyback tune, flash tunes, like the ones available from MHD or bootmod3, do change stock DME parameters. This has a few notable benefits. S55 flash tunes allow for the highest levels of customization, as nearly every engine parameter can be changed and adjusted to your needs. Most flash tunes, including MHD and bootmod3 support custom tuning, so the sky is truly the limit. 

To read more about BMW S55 tuning options, take a look at our ECU vs Flash Tunes guide where we cover this subject in greater detail.

Upgraded BMW S55 Downpipes

As with most turbocharged engines, catless downpipes significantly affect performance, sound, and turbo response.  The OEM S55 downpipes are the source of perhaps the biggest restriction in the engine’s exhaust system. Due to the fact that both OEM downpipes are equipped with catalytic converters for emissions purposes, they create excess backpressure after the turbo. This negatively impacts turbo spool time.

Removing these catalytic converters with aftermarket BMW S55 catless downpipes is a fantastic way of reducing turbo backpressure, freeing up tangled power, improving turbo spool time, and giving your S55-powered BMW a bit more of a pronounced sound. 

The gains from upgraded S55 downpipes depend on the cat setup that you choose and also scale with other performance modifications. One major downside to catless S55 downpipes is that they’ll cause you to fail state emissions testing. High-flow catted S55 downpipes are another option that will boost performance marginally, albeit less than catless DPs, while still being able to pass emissions. 

It is also important to note the performance benefits of catless/high-flow S55 downpipes scale with other modifications, especially a tune. The harder you ask your S55’s turbos to work, the more they benefit from reduced backpressure. As a result, catless downpipes are often a recommended part when moving on to a stage 2 S55 tune. 

For more information about the difference between BMW S55 catless downpipes and high-flow S55 downpipes along with their benefits and shortcomings, take a look at our dedicated BMW S55 Upgraded Downpipe Guide. 

BMW S55 Fueling Mods

Most people believe that fueling modifications are reserved for ridiculously high horsepower S55 builds when in reality, S55 fueling mods can improve performance, reliability, and longevity for even modest S55 builds. 

E85 is one option in terms of an effective ethanol blend for the BMW S55. If you have access to E85, we highly recommend blending a minimum of 20-30% E85 with appropriate tuning. Not only does E85 burn at lower temperatures, but it also minimizes the risk of knock or pre-detonation.

The stock S55 fueling system can handle 100% E85 effectively until approximately 600whp. However, it is crucial to monitor air-fuel ratios (AFRs) closely. A proper tune is necessary to accommodate the increased fuel flow demands. With just a JB4 alone, you can safely utilize up to 30% E85. The JB4 MHD BEF maps offer a range of options from 0% to 100% E85, while MHD flash maps also provide different choices for E85 usage.

The horsepower gains achieved through E85 largely depend on the tuning and specific E85 mixtures that you choose to use. A full bolt-on (FBO) S55 tuned for 100% E85 will typically see horsepower improvements of around 30-50whp. On the other hand, a tune-only S55 using a 25% E85 blend may only see power gains of approximately 10whp. However, it’s essential to note that horsepower gains are just one aspect of the benefits offered by E85. Most notably, E85 significantly reduces the risk of knock and pre-detonation, ensuring engine safety and longevity.

For more information on the subject of BMW S55 fueling modifications, take a look at our dedicated BMW S55 Water Methanol Injection Guide which also covers the subject of E85 blends.

BMW S55 Engine FAQ

While we covered the BMW S55 engine in detail above, we’ve also compiled some of the most frequently asked questions about the engine below.

BMW S55 Engine Guide Summary

From the initial skepticism to the nearly unanimous support that it has today, the BMW S55 engine has truly proven itself as one of the best powerplants to grace the BMW M-division. In the years since its release, the S55 has become a blueprint for the engines that power the current generation M-lineup.  

The S55 is a 3.0L turbocharged inline-6 engine with a closed-deck crankcase design, improved cooling system, and twin mono-scroll turbochargers. It borrowed many components from the non-M 3.0L turbocharged N55 engine but featured several upgrades, including a reworked forged steel crankshaft, revised connecting rods and pistons, and an enhanced oiling system. The S55 engine is known for its immediate throttle response, linear power delivery, and overall reliability.

While there have been some reported issues such as crank hub failure and charge pipe failure, these problems are relatively uncommon and can be mitigated with proper maintenance and, if necessary, aftermarket upgrades. Overall, the BMW S55 engine is highly regarded and has found its place in various M-series models, delivering impressive performance and driving enjoyment.