Mercedes-Benz released their new M278 V8 engine to much excitement and acclaim in the late 2010s. The M278 replaced the M272/3 and its V6 sibling, the M276. First introduced in the 2011 Mercedes S550, CL550, and CLS550, it quickly became a popular option for consumers.
The M278 is a 4.6-liter biturbo V8 that started off producing 429 horsepower and 516 pound-feet of torque, but by the conclusion of its production cycle, some cars had their horsepower increased to 449. Drivers consistently give the M278 high marks for its dependability, engineering, and performance. Since the 2020 model year, it has been discontinued, yet it continues to enjoy strong demand in the secondhand market.
This guide will focus on all we know about the Mercedes M278 engine, including its characteristics, design, performance, and common problems. Let’s begin with an examination of the engine’s evolution from Mercedes’ M273 predecessor and the changes that were made.
Updates From the M273 to the M278 Engine
As we’ve already established, the M278 engine replaces the M273 series. The M273’s final form was a 5.5-liter, V8 engine that produced 382 horsepower and 391 pound-feet of torque when properly tuned. Many drivers complained about serious problems with the timing components, the intake manifold, and oil leaks, therefore it does not have a great reputation for dependability. However, it gained a reputation for thrills and excitement thanks to its impressive power.
Several areas of the M273’s predecessor were addressed in the M278’s redesign. To begin with, despite its smaller engine size, it produced 12% more horsepower and 32% more torque. It also lowered carbon dioxide emissions by the same percentage as the fuel economy improvement it provided (22%). The conversion from natural aspiration to twin-turbochargers accounted for a large portion of the gain. The M278’s biturbo configuration, with one turbo for each bank, significantly boosts both efficiency and performance.
The M278 also borrowed substantially from the design of the M273 for its crankshaft, connecting rods, and pistons. Amazingly, the M278’s compression ratio of 10.5:1 is identical to that of the M273’s, which used naturally aspirated technology and was phased out. However, there was a significant update to the fuel system. The previous sequential fuel injection system was replaced by a direct injection system, which improved performance, reliability, and fuel economy.
MBZ M278 Specs
Specs for the Mercedes M278 engine are as follows:
| Engine | M278 |
| Configuration | V8 (90*) |
| Displacement | 4.6L (4,606cc) |
| Aspiration | Biturbo |
| Fuel System | Direct Injection |
| Block/Head Material | Aluminum |
| Bore & Stroke | 92.9mm x 86.0mm |
| Valve Train | DOHC 32V |
| Compression | 10.5:1 |
| Weight | 485 lbs |
| Boost Pressure | 0.9 Bar (13 PSI) |
| Horsepower Output | 402-449 hp |
| Torque Output | 465 lb-ft |
What Cars Use the M278 4.6L Turbo?
The M278 appeared in the following Mercedes-Benz models:
- 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:
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- 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
The M278 engine is a 4.6-liter biturbo V8 with an aluminum block, as was previously indicated. The M278 introduced a direct injection system that utilized a jet-guided combustion system with piezo injectors, both of which were novel developments. By injecting numerous doses of fuel into the combustion chamber at once, as is possible with direct injection technology, both power and efficiency can be improved. Injectors spray fuel at a staggering fuel pressure of around 200 bar (around 2,900 PSI), allowing for precision dosing and perfect atomization.
The M278 has a low pressure fuel pump and two high pressure fuel pumps, one for each cylinder bank, to accommodate the direct injection. The fuel is compressed and pressurized by the two high pressure fuel pumps, which receive their supply from the low pressure fuel pump. First, it is forced into the fuel rails, where it awaits further propulsion to the combustion chamber through piezo injectors.
The M278’s biturbo arrangement consists of a pair of Garrett MGT1752SM turbochargers operating at 13 pounds per square inch (0.9 bar of boost pressure). As a result of the turbos’ placement, the intercooler and heat exchanger can be installed inside the V of the engine. The engine’s peak torque is available at only 1,600 RPM thanks to its 10.5:1 compression ratio.
While the hyper-eutectic pistons, crankcase, and cylinder heads are cast aluminum, the connecting rods, crankshaft, and valves are all forged. A closed deck layout provides the engine with the protection it needs to handle high levels of power. There were Silitec cylinder liners in the engine as well. Cast within the aluminum cylinder is a hyper-eutectic aluminum-silicon alloy called Silitec, which serves to minimize friction, temperature, and wear. However, there are issues that the Silitec would cause that we shall discuss below.
M157 & M152 Variants
Mercedes-AMG developed the M157, a highly refined variant of the M278 engine capable of extreme speeds. The M157’s 5.5 liters of displacement are the result of an enlarged bore and stroke, which allows it to produce 536-577 horsepower and 515-664 pound-feet of torque. In comparison to the M278’s 10.5:1 compression ratio, this one is at 10.0:1.
The M157 is equipped with a pair of Garrett MGT2260SML turbochargers, which produce 13-14.5 PSI (0.9-1.0 bar) of boost pressure. It employs a direct injection system like the M278 and features sodium-filled exhaust valves for maximum heat reduction. When you get a moment, please read our earlier exploration of the five most frequent M157 issues.
The M152 is a 5.5-liter naturally aspirated V8 version of the M157 that was also developed by Mercedes-AMG. Although the bore and stroke remained unchanged, the compression ratio was increased to 12.6:1 to compensate for the absence of forced induction. The M152, on the other hand, had an all-new lubrication system, intake manifold, and cylinder head design. And since some of the cylinders may be turned off while under light load, fuel economy was also considerably enhanced.
The M152 was weaker than the M157 and the M278. The M152 produced 416 horsepower and 398 lb-ft of torque, however its peak torque wasn’t reached until 3,000 rpm later than the previous model.
Mercedes 4.6L Twin Turbo Performance
The M278 has tremendous potential right out of the box, and with some tweaks, it can become a true rocket. Power output ranged from 429 to 449 horsepower, and peak torque was 465 lb.-ft. at under 1,800 RPM in the performance variants. Better performance over the previous version is attributable to both the biturbo aspiration and the new direct injection technology. However, the redline was set at a modest 6,500 RPM, which was lower than its forerunners.
Almost universally, drivers have praised the M278-equipped vehicles. All three models of the E 63 AMG family—the CL 63 AMG, the SL 63 AMG—have received rave reviews from their respective drivers for their impressive power and durability. The biturbo layout provides rapid throttle response, while the enhanced VVT significantly boosts output at higher revs.
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.
- Tuning
- Intake
- Downpipes
*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
Tuning the engine control unit (ECU) is likely to yield the most benefits for the M278 engine, and these benefits can be quite substantial. A stage 1 tune will provide 60–100 horsepower and 100–130 pound-feet of torque to a stock vehicle. In addition to the 90-150 horsepower/170-230 lb-ft of torque gained by the intake and downpipes in the previous paragraph, a stage 2 tune can easily add an extra 30-50 hp and 70-100 tq. In order to increase boost by the minimum of 3 to 5 PSI that tuners recommend for the bigger power figures, they typically increase boost by a larger amount.
Tuning not only allows for the addition of a significant amount of power, but also allows for the installation of other bolt-ons that improve the vehicle’s efficiency and safety. Extra tuning, especially of the intakes and the exhausts, keeps the engine from running too lean and detonating.
The M278 is also E85 compatible, so your tuner can take use of that. In spite of the fact that the factory tune can only handle up to 25% ethanol, the fuel lines and high-pressure fuel pump can handle nearly full E85 without any performance degradation. Tuners can increase the M278’s performance by using ethanol as a fuel since it allows them to increase the air-to-fuel ratio and the ignition timing without causing the engine to detonate or pre-ignite. By far, the most effective method of increasing M278 power is by tuning with E85, which may easily provide an additional 30% in comparison to regular fueling.
4.6 Twin Turbo Upgraded Intake
Intakes are a well-liked modification for M278-equipped vehicles. Aftermarket intakes can increase induction noises and add 5-15 horsepower compared to the standard intake. Most full intakes for the M278 are quite pricy, making them a dubious purchase given the marginal advantages they provide. However, aftermarket intakes with increased airflow are strongly suggested for anyone thinking about changing their turbos.
Aluminum or carbon fiber tubes, which are physically superior and permit smoother flow under boost, typically replace the plastic OEM intake tubes. More air may enter the engine thanks to their larger size compared to the factory tubes. In order to maximize airflow, intakes and high-flow panel inserts can be utilized together (short of getting a full intake kit).
Mercedes M278 Upgraded Downpipes
As a last performance tip, we suggest newer downpipes for the M278 motor. The M278’s downpipes are equipped with two massive cats that severely limit exhaust flow and generate substantial back pressure. This lessens exhaust scavenging and occasionally causes reversion.
Replace the restrictive OEM ones with aftermarket downpipes to noticeably lessen the amount of pressure pushing back on the engine. Further, they reduce turbo lag, making the vehicle feel more lively. In the case of downpipes, you can choose between catless and high flow cat variants; however, catless downpipes are only authorized for use in competition or on a racetrack, and not on the street. For the M278, a downpipe will normally yield an increase of 20-30 whp/wtq, with an additional 5 hp available in catless configurations. Gains from the downpipes can be maximized by tuning.
An modified M278 downpipe not only improves performance but also alters the exhaust note. The effect is to make it louder, deeper, and raspier. The M278 is well-known for its amazing tone, which is complemented by the increased turbo sounds emanating from the exhaust.
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
The M278’s timing chain and tensioners were the vehicle’s Achilles’ heel in its early iterations. In older versions of the M278 engine, tensioners frequently failed. The engine uses three timing chains—one primary and two secondary. That was an issue with the M276 and M157, too, which are M278 offshoots.
Starting up created oil starvation problems due to poor design, and cold starts were especially hard on the secondary timing chains. The tensioners would prematurely wear down and make a horrible rattling noise until the oil pressure was high enough to silence them.
This was such a problem with the first generation of M278s that a manufacturer service bulletin was issued to address it. The solution to the rattling timing chains was uncomplicated. The secondary chain tensioners on the left and right sides are renewed, and new check valves are fitted. Oil leakage from the tensioners is reduced thanks to the installation of check valves.
2) Oil Leak onto Wiring Harness
Oil leakage onto the wiring harness is another prevalent M278 problem that can cause the vehicle to begin displaying fault codes. The camshaft position sensors/solenoids are notorious for being the source of oil leaks due to their poorly designed cam sensor body. In extreme cases where the leak has gone undiscovered and untreated for a long time, the entire wiring harness has had to be replaced.
The extent of the problem and the extent of the damage claimed to have been caused by the oil spill are also matters of some disagreement. Many people think this is a little, non-issue, while others think it’s a huge matter. Those who are concerned can utilize a wiring extension to relocate the harness away from the cam sensor, which is one solution to the problem. This prevents the oil from penetrating and causing problems.
3) Mercedes M278 Engine Valve Guide Wear
Valve guide wear is an issue with the M278 that is rarely discussed. This problem, along with the timing chain and tensioner problems, is unique to the first few years of the M278’s manufacturing run and was mostly fixed by the end of the line.
As was previously reported, the M278’s cylinders were lined with Silitec, a hyper-eutectic aluminum-silicon alloy that is cast into the aluminum cylinder to reduce friction, heat, and wear. The Silitec disappointedly fell short of expectations. It would create considerable wear on the valve guides because it couldn’t handle the high temperatures and occasional detonation events of the M278 engine. Due to Silitec’s cylinder scoring problems, Mercedes ultimately turned to Nanoslide coating.
4) M278 Engine Cam Adjusters Issues
The M278’s hydraulic cam adjusters are the last remaining problem. The M278’s cam adjusters are crucial since they regulate the engine’s variable valve timing. However, they frequently wear out too soon and begin to rattle loudly, a problem that is aggravated by cold starts. The M278 appears to share the M276’s problematic cam adjusters that plague other Mercedes powerplants.
Misfires, rattling noises from the valve train at starting, and eventually a host of timing codes are all signs of worn cam adjusters. The only option is to get brand new ones from the manufacturer and cross your fingers that they don’t break down again very soon. Thankfully, issues with cam adjusters only manifest themselves in a few unlucky engines.
M278 Engine Summary
The M278 engine, as a whole, is quite well-designed and can generate a lot of force. The low end is extremely robust and can withstand massive amounts of power, while the biturbo setup provides instant power across the board. Although there are a few problems with the engine’s timing chain and valve train, they are minor and were fixed before the engine was sent.
The M278’s innovative technology, including direct injection and a novel biturbo architecture, elevates the engine’s performance to a new level. The engine’s peak torque is reached at 1,600 RPM, and it continues to produce useful amounts of power up to its redline. While losing nearly a liter of displacement, it achieves better fuel economy and lower pollutants than its predecessor.
The M278’s performance variants produce 429-449 horsepower out of the box, but with little tuning and E85, that number can rise by the triple digits. Downpipes in particular provide a lot of responsiveness and a more intense sound to the M278 and are a nice upgrade overall.
