The Mercedes M278 Engine Manual. Mercedes-Benz debuted their new M278 V8 engine in the fall of 2010 to much fanfare. The M278 and its V6 sibling, the M276, were the M272/3 engines’ replacements. It was first available in the 2011 Mercedes S550, CL550, and CLS550 cars.
The M278 is a 4.6L biturbo V8 with 429hp and 516tq at launch, but by the conclusion of its production cycle, horsepower had been increased to 449 for select variants. Drivers consistently appreciate the M278 for its dependability, engineering, and exceptional performance. Although it has been out of production since the 2020 model year, it is still in high demand on the used market.
This guide will cover all we know about the Mercedes M278 engine, including specs, design, performance, and typical issues. First, let’s look at the changes Mercedes made to the engine from its predecessor, the M273.
*Previously, we looked at the most typical issues with the M276’s V6 version. Check out the instructions if you have any M276-related questions.
Engine Upgrades from the M273 to the M278
As previously stated, the M278 engine is the replacement for the previous generation M273. In its last incarnation, the M273 had a naturally aspirated 5.5L V8 engine that generated 382hp and 391tq. Many drivers reported extensive issues with timing components, the intake manifold, and oil leaks, so it is not well known for its dependability. However, in terms of power, it has earned a reputation for excitement and fun.
The M278 improved on the previous M273 in a number of areas. For starters, it has 15% less displacement but produces 12% more horsepower and 32% more torque. It also increased fuel economy by 22% while decreasing CO2 emissions by the same amount. The conversion from natural aspiration to twin-turbos accounted for a large portion of this gain. The M278 has a biturbo design with one turbo for each bank, which enhances efficiency and performance significantly.
The M278’s crankshaft, connecting rods, and pistons were also greatly influenced by the design of the M273. Despite the inclusion of biturbos, the M278 had the same 10.5:1 compression ratio as the previous normally aspirated M273. However, the fuel system was drastically altered. The outgoing sequential fuel injection system was replaced by a direct injection system, which resulted in improved performance, reliability, and fuel economy.
MBZ M278 Specifications
The Mercedes M278 engine specifications are as follows:
What Vehicles Make Use of the M278 4.6L Turbo?
The M278 was used in the following Mercedes-Benz vehicles:
- Mercedes-Benz S550 (2011-2017)
- Mercedes-Benz CL550 (2011-2014)
- Mercedes-Benz S550 Coupe 2015-2017
- Mercedes-Benz CLS550 (2011-2018)
- Mercedes-Benz SL550 2012-2020
- Mercedes-Benz E550 2012-2014
- Mercedes-Benz ML550 2012-2014
- Mercedes-Benz GL450 2013-2014
- Mercedes-Benz GL550/GLS550 2013-2019
The M157 was a high-performance variation of the M278 tuned by AMG that came in the following models:
- 2011-2017 S 63 AMG (W221/222)
- 2011-2014 CL 63 AMG
- 2012-2018 Mercedes-Benz CLS 63 AMG
- 2012-2015 ML 63 AMG
- 2015-2019 Mercedes-Benz GLE 63
- 2012-2016 GL 63 AMG
- 2013-2018 G 63 AMG
- 2012-2016 E 63 AMG
- 2012-2019 SL 63 AMG
The M152 was a high-spec naturally aspirated version of the M157 that also debuted in the following model:
- 2012-2016 Mercedes-Benz SLK 55 AMG
Engine Design M278
The M278 engine, as previously stated, is a 4.6L biturbo, aluminum block V8. The M278 received a direct injection system that utilised a new jet-guided combustion system with piezo injectors. Direct injection technology allows for numerous fuel injections into the combustion chamber within a single combustion cycle, resulting in improved performance and fuel economy. The injectors spray precisely measured, finely atomized fuel and function at an amazing 200 bar (2,900 PSI) fuel pressure.
Because of the direct injection, the M278 has three fuel pumps, one low pressure and two high pressure, one for each cylinder bank. The low pressure fuel pump supplies the fuel to the two high pressure fuel pumps, which compress and pressurize it. It is then forced through the fuel rails and into the combustion chamber through the piezo injectors.
The M278’s biturbo configuration includes twin Garrett MGT1752SM turbos that provide 13 PSI (0.9 bar) of boost. Because of the location of the turbos, the liquid-to-air intercooler and heat exchanger can be mounted within the engine’s V. The engine has a compression ratio of 10.5:1 and produces peak torque at just 1,600 RPM.
The connecting rods, crankshaft, and valves are all cast aluminum, while the hyper-eutectic pistons, crankcase, and cylinder heads are all forged. The engine has a closed deck design that can handle a lot of power. Silitec cylinder liners were also used in the engine. Silitec is an aluminum-silicon alloy that is cast into the aluminum cylinder to reduce friction, heat, and wear. However, the Silitec would cause issues that we will discuss later.
Variants of M157 and M152
Mercedes-AMG developed the M157 engine, which is a high-spec and optimized variant of the M278 engine. The M157 is a 5.5L biturbo V8 with 536-577hp and 515-664tq, with the greater displacement due to increased bore and stroke. It features a slightly lower compression ratio, 10.0:1, than the M278’s 10.5:1.
The Garrett MGT2260SML turbos on the M157 produce 13-14.5 PSI (0.9-1.0 bar) of boost depending on the application. It includes sodium-filled exhaust valves for maximum heat reduction and, like the M278, has a direct injection system. Previously, we looked at the five most prevalent issues with the M157, so make sure to check it out when you have a chance.
Mercedes-AMG also produced the M152, a 5.5L naturally aspirated V8 variation of the M157. The compression ratio was increased to 12.6:1 to compensate for the lack of forced induction, although the bore and stroke remained same. The M152, on the other hand, had a redesigned oiling system, intake manifold design, and cylinder head design. It also enabled several cylinders to be deactivated at minor load, significantly boosting gas mileage.
Power was lower on the M152 than on the M157 and M278. The M152 produced 416 horsepower and 398 torque, although it did not reach peak torque until 3,000 RPM later than either of its predecessors.
Performance of the Mercedes 4.6L Twin Turbo
The M278 is capable of massive performance right out of the box, and with a few modifications, it can truly transform into a rocket. Peak torque was attained at only 1,800 RPM in performance vehicles with 429-449hp and 465tq. Much of the improved performance over the previous generation is due to the addition of direct injection as well as biturbo aspiration. However, the redline was relatively low, at only 6,500 RPM – lower than its predecessors.
The M278 has received fantastic ratings from drivers in practically all of the versions that include it. Drivers of the E 63 AMG, CL 63 AMG, and SL 63 AMG, in particular, have commended its exceptional power delivery and robust reliability. The biturbo architecture provides fast throttle response, and the improved VVT significantly improves performance at higher RPMs.
M278 Performance Enhancements
While the M278 is already a fun car to drive, its performance can be improved. Although the platform does not have a lot of aftermarket support, there are some upgrades that can be made.
*Not all M278 improvements are compatible with all models; please confirm fitment for your model and year before purchase.
Flex Fuel & M278 Tuning
ECU tuning is the upgrade with the most potential for the M278 engine, and it can result in some major gains. Even on a stock car, a stage 1 tune will result in gains of 60-100hp and 100-130tq. Stage 2 tunes easily add 30-50hp and 70-100tq on top of that (90-150hp/170-230tq above stock) with an intake and downpipes. Tuners will often raise boost by at least 3-5 PSI to attain the higher power statistics.
Tuning can accept various bolt-ons in addition to providing a lot of power to help the automobile run as effectively and safely as feasible. Intakes and downpipes, in particular, benefit from additional tuning, which keeps the engine from running too lean and causing detonation.
Another advantage of tuning is that your tuner can take use of the M278’s E85 compatibility. The stock tune can use up to 25% ethanol, however the fuel lines and high pressure fuel pump can handle nearly full E85 without compromising functioning. Ethanol is an excellent option to improve performance on the M278 since tuners may utilize thinner air-to-fuel ratios and higher ignition timing without risking detonation or pre-ignition. Tuning with E85 is by far the best way to increase power on the M278 because you can easily add an extra 30% above standard fueling.
4.6 Twin Turbo Intake Upgrade
The intake is another popular improvement for M278 equipped vehicles. While the factory intake is sufficient, aftermarket intakes will add 5-15whp while also improving induction sounds. Unfortunately, most full intakes are quite expensive for the M278, making them a dubious investment for such marginal gains. However, if you’re thinking about upgrading your turbos, aftermarket intakes with more airflow are a must.
The majority of intakes replace the plastic OEM intake tubes with aluminum or carbon fiber tubes, which are more stronger structurally and allow for smoother flow under boost. They also expand the size of the tubes over standard, allowing for greater airflow into the engine. For the best flow (short of obtaining a full intake system), high-flow panel inserts can be utilized in conjunction with intakes.
Upgraded Downpipes for the Mercedes M278
Upgraded downpipes are our final performance advice for the M278 engine. Two huge cats in the M278’s downpipes block flow and cause a lot of back pressure. This reduces scavenging on the exhaust and can occasionally result in reversion.
Upgraded downpipes remove a significant amount of restriction from the OEM systems and significantly reduce back pressure. They help reduce turbo lag, making the vehicle more responsive. Catless or high flow cat downpipes are available, however keep in mind that catless downpipes are actually geared for competition or track days, and they are banned for street use. Downpipes typically add 20-30whp/wtq to the M278, with catless variants adding around 5whp. Tuning boosts downpipe gains even further.
Upgraded downpipes boost and vary the tone of the M278 exhaust, in addition to improving horsepower. It becomes considerably deeper, raspier, and louder as a result. Increased turbo sounds may also be heard coming from the exhaust, and the M278 is recognized for its amazing tone.
Typical Mercedes M278 Engine Issues
While the 4.6L biturbo M278 is a great performance engine with serious power, it is not without flaws. While the M278 engine’s bottom end is extremely strong and solid, there are some minor issues on the top end. Overall, the engine is rather solid and dependable, although there are a few drawbacks to be aware of.
- Tensioners and Timing Chain
- Oil Spill on Wiring Harness
- Wear of Valve Guides
- Adjustable Cams
1) Timing Chain & Tensioners M278
The timing chain and tensioners were by far the most problematic aspects of early M278 models. The M278 engine has three timing chains, one primary and two secondary, and the tensioners would frequently fail on previous models. This was also an issue for the M276 and M157, two variations of the M278.
Due to poor design, oil starvation issues occurred during startup, and cold starts were especially difficult on the secondary timing chains. They would frequently make a loud rattling sound that would not stop until enough oil pressure was built up – and the tensioners would prematurely wear until that happened.
This was such a well-known problem with early M278s that a factory service bulletin was issued to address it. The solution to rattling timing chains was straightforward. Check valves are added and both the left and right secondary chain tensioners are changed. The check valves reduce oil drainage from the tensioners, resolving previous difficulties.
2) Oil Spill on Wiring Harness
Another typical problem with the M278 is oil leaking into the wiring harness, which can eventually cause multiple errors to be thrown. Because of the poor construction of the cam sensor body, the most typical source of oil leakage is from the camshaft position sensors/solenoids. If the leak goes unreported and untreated for a long enough period of time, the entire wiring harness may need to be replaced in very extreme cases.
There is some disagreement about how serious the oil leak is and whether it genuinely does the damage that some claim it does. Many argue that the issue is completely harmless and unimportant, while others argue the exact opposite. For those who are concerned, one alternative is to utilize a wiring extension that extends the harness away from the cam sensor. This efficiently prevents oil from seeping in and causing problems.
3) M278 Mercedes Engine Valve Guide Wear
One issue with the M278 that is frequently overlooked is valve guide wear. This, like the timing chain and tensioner problems, predominantly affects early model M278s and was mostly resolved by the conclusion of the production cycle.
As previously stated, the M278’s cylinders were lined with Silitec, a hyper-eutectic aluminum-silicon alloy cast into the aluminum cylinder to reduce friction, heat, and wear. Unfortunately, the Silitec fell far short of expectations. It couldn’t because of the M278 engine’s high temperatures and infrequent detonation events, causing severe wear on the valve guides. Because of Silitec’s cylinder scoring concerns, Mercedes eventually switched to Nanoslide coating.
Related : The Mercedes M156 Engine Manual
4) Problems with the M278 Engine Cam Adjusters
The hydraulic cam adjusters are the M278’s final flaw. Cam adjusters are crucial on the M278 because they are in charge of controlling variable valve timing. However, they frequently become prematurely worn and begin to rattle excessively, which is exacerbated by cold starts. Cam adjusters have been a prevalent issue on various Mercedes engines, and the M278 appears to be no exception.
Symptoms of worn cam adjusters include loud rattling noises from the valve train on startup, misfires, and the appearance of various timing codes. Because the aftermarket has yet to develop a viable and affordable solution, the solution is to replace them with OEM and hope they do not fail again. Fortunately, cam adjusters are not a common issue, and they only appear on a few unfortunate engines.
M278 Engine Overview
Overall, the M278 engine is quite well-engineered and capable of delivering a significant amount of power. The bottom end is extremely bulletproof and can withstand massive amounts of power, while the biturbo design offers instant power over the whole power spectrum. Although there are some minor issues with the valve train and timing chain, they were resolved by the end of the engine’s production cycle.
The advent of direct injection and the unique biturbo architecture on the M278 really made the engine shine in terms of performance. It achieves peak torque at 1,600 RPM while producing horsepower to the redline. It also outperforms its predecessor in terms of fuel economy and pollutants while losing about a liter in displacement.
The M278 currently produces 429-449hp on performance models out of the factory, but with some tune and E85, you can easily see triple-digit improvements. Intakes and downpipes are also good modifications for the M278, with downpipes adding a lot of responsiveness and volume.