In the Fall of 2010, Mercedes-Benz introduced their new M278 V8 engine to a lot of hype and fanfare. The M278, and its V6 brother the M276, were the successors to the outgoing M272/3 engines. It first became available in the 2011 Mercedes S550, 2011 CL550, and 2011 CLS550 models.
The M278 is a 4.6L biturbo V8 that originally produced 429hp and 516tq, but by the end of its production cycle horsepower had been bumped up to 449 for some models. Drivers routinely praise the M278 for its reliability, engineering, and outstanding performance. It has been out of production since the 2020 model year, but it is still highly sought after on the used market.
This guide will focus on everything we know about the Mercedes M278 engine, including its specifications, design, performance, and common problems. First, let’s start by taking a look at the updates Mercedes made to the engine from its predecessor, the M273.
*Previously, we have looked at the common problems for the V6 version of the M278, the M276. Make sure to check out the guide for any M276 related questions.
Updates From the M273 to the M278 Engine
As we mentioned, the M278 engine is the successor to the previous generation M273. The M273 was a naturally aspirated 5.5L V8 that produced 382hp and 391tq in its final iteration. It is not very well known for its reliability, as many drivers reported extensive issues with timing components, the intake manifold, and oil leaks. Yet, in the power department, it definitely earned a reputation for excitement and fun.
The M278 improved upon the outgoing M273 in a variety of ways. First, it has 15% less displacement, yet it generated 12% more horsepower and 32% more torque. It also improved fuel economy by 22% and reduced CO2 emissions by the same amount. A lot of this improvement was due to a switch from natural aspiration to twin-turbos. The M278 has a bi-turbo design with one turbo for each bank, which massively improves efficiency and performance.
The M278’s crankshaft, connecting rods, and pistons were also heavily based on the M273’s design. Incredibly, even with the addition of bi-turbos, the M278 still retained the same 10.5:1 compression ratio of the outgoing naturally aspirated M273. However, the fuel system underwent a massive change. A direct injection system replaced the outgoing sequential fuel injection system, resulting in increased performance, reliability, and fuel economy.
|Bore & Stroke
|92.9mm x 86.0mm
|0.9 Bar (13 PSI)
What Cars Use the M278 4.6L Turbo?
- 2011-2017 Mercedes-Benz S550
- 2011-2014 Mercedes-Benz CL550
- 2015-2017 Mercedes-Benz S550 Coupe
- 2011-2018 Mercedes-Benz CLS550
- 2012-2020 Mercedes-Benz SL550
- 2012-2014 Mercedes-Benz E550
- 2012-2014 Mercedes-Benz ML550
- 2013-2014 Mercedes-Benz GL450
- 2013-2019 Mercedes-Benz GL550/GLS550
There was also a high performance variant of the M278 tuned by AMG, the M157, which appeared in following models:
- 2011-2017 S 63 AMG (W221/222)
- 2011-2014 CL 63 AMG
- 2012-2018 CLS 63 AMG
- 2012-2015 ML 63 AMG
- 2015-2019 AMG GLE 63
- 2012-2016 GL 63 AMG
- 2013-2018 G 63 AMG
- 2012-2016 E 63 AMG
- 2012-2019 SL 63 AMG
A high-spec naturally aspirated version of the M157, the M152, also appeared in the following model:
- 2012-2016 SLK 55 AMG
M278 Engine Design
As mentioned before, the M278 engine is a 4.6L biturbo, aluminum block V8. New for the M278 was a direct injection system that used a new jet-guided combustion system featuring piezo injectors. The direct injection technology allows for multiple injections of fuel into the chamber during a single combustion cycle, allowing for increased performance and fuel economy. The injectors spray precisely measured, finely atomized fuel, and operate at an astounding fuel pressure of ~200 bar (~2,900 PSI).
Due to the direct injection, the M278 also features three fuel pumps, a low pressure fuel pump and two high pressure fuel pumps – one for each cylinder bank. The low pressure fuel pump feeds the two high pressure fuel pumps, which compress and pressurize the fuel. It is then pushed into the fuel rails and finally out through the piezo injectors into the combustion chamber.
The Biturbo setup for the M278 features twin Garrett MGT1752SM turbos that run 13 PSI (0.9 bar) of boost. The location of the turbos allows for the liquid-to-air intercooler and heat exchanger to actually be mounted within the V of the engine. The engine has a 10.5:1 compression ratio and makes peak torque at just 1,600 RPM.
The connecting rods, crankshaft, and valves are all forged, while the hyper-eutectic pistons, crankcase, and cylinder heads are all cast aluminum. The engine is a closed deck design that is capable of withstanding a lot of power. The engine also featured Silitec cylinder liners. Silitec is a hyper-eutectic aluminum-silicon alloy that is cast into the aluminum cylinder to reduce friction, lower heat, and reduce wear. However, the Silitec would lead to problems, which we will address later.
Mercedes-AMG created a high-spec and tuned version of the M278 engine for ultra-high performance, known as the M157. The M157 is a 5.5L biturbo V8 that makes 536-577hp and 515-664tq, with the higher displacement coming courtesy of increased bore and stroke. It has a slightly reduced compression ratio, 10.0:1, compared with the M278’s 10.5:1.
The M157 features twin Garrett MGT2260SML turbos that run 13-14.5 PSI (0.9-1.0 bar) of boost, depending on the application. It has sodium-filled exhaust valves for optimal heat reduction, and also uses a direct injection system like the M278. Previously, we have looked at the 5 most common problems with the M157, so make sure to check that out when you get a chance.
Mercedes-AMG also created a variant of the M157, the M152, a 5.5L naturally aspirated V8. Compression was raised to 12.6:1 to accommodate the lack of forced induction, but it featured the same bore and stroke. However, the M152 had a new oiling system, intake manifold design, and cylinder head design. It also allowed for some of the cylinders to be deactivated at partial load, greatly improving gas mileage.
Power was down the M152 compared with both the M157 and M278. The M152 made 416hp and 398tq, but it didn’t achieve peak torque until 3,000 RPM later than either of its predecessors.
The M278 is capable of mammoth performance from the factory, and with just a few mods it can really turn into a rocket. Performance models made between 429-449hp and 465tq, and peak torque was achieved at just 1,800 RPM. Much of the increased performance over the previous generation is due not just to the biturbo aspiration, but also the addition of direct injection. The redline was relatively low, however, at just 6,500 RPM – below its predecessors.
Drivers have given the M278 rave reviews in nearly all of the models that feature it. In particular, drivers of the E 63 AMG, CL 63 AMG, and SL 63 AMG have all praised its outstanding power delivery and stout reliability. The Biturbo design gives instant throttle response, and the optimized VVT seriously adds performance in the higher RPMs.
M278 Performance Upgrades
While the M278 is already a pretty fun car to drive, the performance can always be upgraded. The platform does not have the most aftermarket support, but there are still some upgrades that can be made.
*Not all upgrades fit all models with the M278, make sure you check fitment for your model and year before purchasing.
M278 Tuning & Flex Fuel
The mod with the biggest upside on the M278 engine is going to be ECU tuning, and it can net some pretty significant gains. Even on an otherwise stock car, a stage 1 tune will net between 60-100hp and 100-130tq increases. With an intake and downpipes, stage 2 tunes easily add an additional 30-50hp and 70-100tq on top of that (90-150hp/170-230tq over stock). Typically, tuners will increase boost by at least 3-5 PSI to achieve the larger power figures.
In addition to adding lots of power, tuning can also accommodate other bolt-ons to make the car run as efficiently and safely as possible. Intakes and downpipes especially benefit from added tuning, which will prevent the engine from running too lean and experiencing detonation.
Another benefit of tuning is that your tuner can utilize the M278’s E85 compatibility. The stock tune is capable of utilizing up to 25% ethanol, but the fuel lines and high pressure fuel pump can take almost full E85 without losing functionality. Ethanol is a great way to boost performance on the M278, because tuners can run leaner air-to-fuel ratios and greater ignition timing without running the same risk of detonation or pre-ignition. Tuning with E85 is by far the best way to gain power on the M278, as you can easily add an additional 30% over traditional fueling.
4.6 Twin Turbo Upgraded Intake
Another popular upgrade for M278 equipped cars is the intake. While the factory intake is adequate, aftermarket intakes will add about 5-15whp while increasing induction sounds. Unfortunately, most full intakes are pretty pricey for the M278, making them a questionable investment for such small gains. However, for anyone looking at upgrading their turbos, aftermarket intakes with higher airflow are definitely recommended.
Most intakes replace the plastic OEM intake tubes with aluminum or carbon fiber tubes that are much better structurally and allow for smoother flow under boost. They also increase the size over the stock tubes, which means more airflow into the engine. High-flow panel inserts can be used in conjunction with intakes for the best flow (short of getting a full intake kit).
Mercedes M278 Upgraded Downpipes
Our final performance recommendation for the M278 engine are upgraded downpipes. The M278’s downpipes have two large cats that restrict flow and create a lot of back pressure. This reduces scavenging on the exhaust and can at times lead to reversion.
Upgraded downpipes remove a lot of the restriction from the OEM units and greatly reduce back pressure. They also cut down on turbo lag, making the car more responsive. Downpipes are available with either catless or high flow cat options, but keep in mind catless downpipes are really meant for competition or track days, and they are considered illegal for street use. Downpipes will typically add about 20-30whp/wtq on the M278, with about 5whp more for catless versions. Tuning increases gains from the downpipes even more.
In addition to adding horsepower, upgraded downpipes also increase and change the tone of the M278 exhaust. It makes it much deeper, raspier, and louder. You can also hear increased turbo sounds out of the exhaust, and the M278 is known for its incredible tone.
Common Mercedes M278 Engine Problems
While the 4.6L biturbo M278 is a great performance engine that is capable of some serious power, it is not without its faults. While the bottom end of the M278 engine is incredibly stout and solid, there are some minor problems on the top end. Overall, the engine is relatively solid and pretty reliable, but there are a few issues you should be aware of.
- Timing Chain & Tensioners
- Oil Leak onto Wiring Harness
- Valve Guide Wear
- Cam Adjusters
1) M278 Timing Chain & Tensioners
By far the biggest issue with early versions of the M278 were the timing chain and tensioners. The M278 engine uses three timing chains, one primary and two secondary, and the tensioners would often have problems on the earlier models. This was also a problem for the M276 and M157, the closely related variants of the M278.
Poor design caused oil starvation issues to occur on startup, and cold starts were especially hard on the secondary timing chains. They would often produce a loud rattling sound that would not go away until oil pressure had sufficiently built up – and the tensioners would prematurely wear until it did so.
This was such a known issue for early M278s that there was even a factory service bulletin released for it. The replacement for rattling timing chains was pretty simple. Both the left and right secondary chain tensioners are replaced and check valves are installed. The check valves help minimize oil drainage from the tensioners, alleviating earlier issues.
2) Oil Leak onto Wiring Harness
Another common issue with the M278 is oil leaking onto the wiring harness, which can eventually start to throw several codes. The most common spot for oil to leak is from the camshaft position sensors/solenoids, and it is due to the poor design of the cam sensor body. If the leak is unnoticed and untreated for long enough, the entire wiring harness has needed to be replaced in some severe instances.
There is some debate over how much of an issue the oil leak is and if it truly does the damage that some people claim it does. Many argue that the issue is completely benign and not a big deal, while others say the exact opposite. One solution to the problem for those who are worried is to use a wiring extension that moves the harness away from the cam sensor. This effectively stops the oil from seeping in and causing issues.
3) Mercedes M278 Engine Valve Guide Wear
One problem with the M278 that is not always mentioned is the valve guide wear. Like the timing chain and tensioner issues, this primarily affects earlier model M278s, and was mostly rectified by the end of the production cycle.
As mentioned earlier, the inside of the M278’s cylinders were lined with Silitec, a hyper-eutectic aluminum-silicon alloy that is cast into the aluminum cylinder to reduce friction, lower heat, and reduce wear. Unfortunately, the Silitec far from lived up to its hype. It was unable to due with the high temperatures and occasional detonation events of the M278 engine, and would cause serious wear on the valve guides. Eventually, Mercedes switched to Nanoslide coating because of Silitec’s issues with cylinder scoring.
4) M278 Engine Cam Adjusters Issues
The final issue with the M278 is the hydraulic cam adjusters. Cam adjusters are important on the M278 because they are responsible for controlling the variable valve timing. Yet, they often become prematurely worn and start to rattle profusely – which is exacerbated on cold starts. Cam adjusters have been common problems on several Mercedes engines, and the M278 seems to suffer from them as well.
Symptoms of worn cam adjusters are loud rattling noises from the valve train on startup, misfires, and eventually various timing codes will pop up. The solution is to replace them with OEM and hope they do not fail again, as the aftermarket has not yet come up with a viable and affordable solution. Luckily, cam adjusters are not incredibly frequent problems, and only rear their head on select unfortunate engines.
M278 Engine Summary
Overall, the M278 engine is incredibly well-engineered and capable of producing some serious power. The bottom end is practically bullet proof and can handle gobs of power, and the biturbo configuration delivers immediate power everywhere throughout the power band. Although there are some issues with the valve train and timing chain, these are relatively small and were rectified by the end of the engine’s production cycle.
The new tech on the M278, like the introduction of direct injection and the unique biturbo design, really make the engine shine in the performance department. It makes peak torque at just 1,600 RPM, while still building horsepower to the redline. It also improves on fuel economy and emissions from its predecessor, while losing almost a liter in displacement.
From the factory, the M278 already makes 429-449hp on the performance models, but with some tuning and E85 you can easily see triple-digit gains. Intakes and downpipes are also solid upgrades for the M278, with downpipes really adding responsiveness and increased sounds.
What’s your experience with models containing the M278? Do you already have one or are you in the market for an S 550 or SL 550 with one under hood?
We do have a post on problems in Mercedes engine 256. If you are interested in reading, click here.
Let us know in the comments below!