Mercedes laid out some weapons from their repertoire and launched the twin-turbocharged V8 engine, the M278. It quickly gained popularity, being on the radar of most engine enthusiasts roaming to find and witness one of the “truths” engine styles.
Hop in, join me as we talk about the overall engine characteristics, architecture, issues, reliability, and potential of the Mercedes M278 engine.
What are Mercedes M278 Engines?
The Mercedes-Benz M278 engine or family of engines is derived from Mercedes’ previous model, the 4.7 Liter M273 V8 engine. These two engines share the same bore pitch, Silitec aluminum/silicon low-friction cylinder liners and an aluminum engine block.
Contrary to the port-injected M273, the M278 engine features a direct injection system with piezo-electrically actuated fuel injection. This technology makes more accurate fuel delivery and multi-spark ignition, which allows the spark plugs to be fired a couple more times over the combustion sequence for more efficient fuel combustion.
Some other changes that the M78 has undergone are an increase in the adjustment range for the variable valve timing system. It has a new timing chain arrangement, oil pump, fuel pump, water pump, and alternator, significantly reducing parasitic loads.
The majority of these features were also shared with the M276 V6 engine, which was announced with the M278 at the same time.
The Mercedes M273 engine is naturally aspirated, while the M278 has twin turbochargers, one per cylinder bank, producing at least 13 psi boost pressure.
Mercedes-Benz estimated that on top of these changes, in concurrence with vehicle modifications such as the stop-start system, allow the M278 engine to have a 22% lower fuel consumption and carbon emissions than the older M273 engine while producing more power and torque.
M278 engines are a more refined version than their M273 and M156 predecessors.
Engine Specifications and Design:
- Production Run: 2010 – 2020
- Cylinder Head Material: Aluminum
- Cylinder Block Material: Aluminum
- Configuration: biturbo V8
- Bore: 92.9 mm
- Stroke: 86.0 mm
- Valvetrain: DOHC four valves per cylinder
- Displacement: 4.7 L (4663 cc)
- Compression Ratio: 10.5
- Weight: 485 lbs.
- Maximum HP: 429 HP at 5,250 RPM
- Maximum Torque: 516 lb-ft at 1,800 – 3,500 RPM
The basic Mercedes M278 engine has a displacement of 4,663 cc with a cylinder bore of 92.9 mm, a piston stroke of 86 mm, and a compression rating of 10.5. This machine is also referred to as the 4.7 Liter bi-turbo V8 engine designed for Mercedes-Benz vehicles.
The cylinder block of M278 is made from lightweight aluminum material. The cylinders are arranged in a ‘V’ configuration with 106 mm space intervals between them. Inside the cylinder bore is cast-in Silitec cylinder liners.
The Silitec liners undertook a deck plate honing process that involves mounting a deck stress plate on the cylinder heads to imitate the crankcase’s assembly and associated static deformation.
Mercedes M278 has two oil separators for its ventilation system. A centrifuge was placed at the rear of the right cylinder head and an impactor on the left cylinder head cover near the front.
The crankcase ventilation is provided through a line between the impactor and the left air filter. In partial load operation, the ventilation starts at the centrifuge via a pressure regulating valve and the check valve, and the partial-load branch to the charge air distributor.
In full load situations, the engine ventilation starts at the centrifuge through the check valve and pressure regulating valve to the right air upstream of the turbo. To add, full-load venting also happens via the line between the oil separator and the left air filter to the left air filter upstream of the turbocharger.
The M278 engine has a forged steel crankshaft with eight counterweights and rotates on five main bearings. Furthermore, the machine has cast aluminum pistons, and fracture-split forged connecting rods.
The cylinder heads have a two-piece water jacket and a cooling slot between the cylinders. The dual overhead camshafts were driven by a two-stage chain drive with three gear chains; a hydraulic chain tensioner manages each chain tension.
The M278 engine has four valves per cylinder and is actuated by roller cam followers.
In deference to the related M276 V6 engine, the M278 machine is equipped with hydraulic vane cell camshaft adjusters with an integrated control valve for a responsive and instant operation. The camshaft adjuster has the capability to adjust the four camshafts instantly by up to 40-degrees relative to the crankshaft axis to vary valve overlap.
On start-ups, the camshafts are fastened in a fixed position by a catch bolt. This starting position is unlocked hydraulically the first time the intake and exhaust camshaft solenoids are activated.
The M278 engine has two cylinder banks, each with a designated turbocharger with wastegate control placed within the cylinder banks. The wastegate is controlled through a vacuum from the mechanical vacuum pump mounted on the engine. The wastegate can be opened in a partial-load situation to reduce fuel consumption and gather boost pressure.
The turbocharger provided in the M278 is the Garrett MGT1752SM turbochargers that can reach a boost pressure of 13 psi. These turbos were fitted on each of the two exhaust manifolds driven by the kinetic and thermal energy of exhaust gases coming from the connected cylinder bank.
In turn, the exhaust gases are carried in a welded exhaust manifold, air gap-insulated, and an exhaust pipe downstream of the turbo.
Mercedes M278 engines also have a charge air cooling system. This system enhances the air for a greater density for power production, reduces the formation of nitrogen oxides, and mitigates the tendency for uncontrolled detonation.
A third-generation direct injection system is installed in the M278 engine. It has a non-return high-pressure supply with two separate fuel rails. From those rails, the piezo injectors spray refined atomized fuel into the combustion chambers and are capable of providing up to five injections per cycle.
On top of the direct injection system are a standard and multi-spark ignition mode. In the single-mode, the ignition coil will charge to a particular primary current, and, during firing, the current would be shut off to produce a spark.
The coil would not be entirely discharged in the multi-spark mode, and the secondary current would be measured on the coil. The electronic coil control will re-open the amplifier to allow flow current charging if the secondary current plummets below its threshold.
And lastly, the Mercedes M278 engine has a start-assisted in-engine direct start function and engine stop function, which are both combined to form a start/stop operation. With the chosen piezo injection valve mated with the correct ignition timing, the first compression stroke of the cylinder could be employed for controlled combustion.
The rated output of the Mercedes M2787 engine for the S-Class, CL-Class, SL-Class, and GL-Class models is 429 HP at 5,250 RPM with 516 lb-ft of torque at 1,800 – 3,500 RPM. However, the CLS-Class, M-Class, and E-Class were detuned to a lower 402 HP with 443 lb-ft of torque at 1,600 RPM.
All of the abovementioned vehicle models are still badged as “550”. The GL-Class also features the GL450 trim that carries a detuned 4.7 Liter version making 362 HP and 406 lb-ft of torque.
Applications of Mercedes M278 Engine:
- 2011 – 2017 S 500/S 550
- 2011 – 2014 CL 500/CL 550
- 2015 – 2017 S 500 Coupe/S 550 Coupe
- 2011 – 2018 CLS 500/CLS 550
- 2012 – 2020 SL 500/SL 550
- 2012 – 2014 E 500/E 550
- 2012 – 2014 ML 500/ML 550
- 2013 – 2014 GL 450
- 2013 – 2019 GL 500/GLS 500/GL 550/GLS 550
- 2016 – Present BAIC BJ90
Engine Tuning, Upgrades, and Modifications
The Mercedes M278 engine is an absolute beast, but it is capable of reaching greater numbers and has a lot of potential in its stock form.
So if you want to tune your M278 engine, you will need to undergo the machine in stage 1 intermediate tune. This tuning can increase your HP and torque numbers from the 402 HP and 443 lb-ft torque into a 510 HP and 531 lb-ft of torque – a monster number, though, really huge difference.
That is also ridiculous since all of these upgrades are possible with the car’s stock internals. A stage 2 tune can stretch AMG’s power and torque, reaching approximately 540 HP and 700 lb-ft of torque. Ample amount of increase that can rival even the sports cars today.
Problems Surrounding Mercedes M278 Engine:
Even the best engines have weaknesses or issues that need to be addressed. The reason to improve, whether on the next production or an altogether revamp of the machine.
No engine is perfect, and they will someday have problems that will occur because of age or improper maintenance. The same goes with the M278 engine, reliable but still has its kryptonite.
We will first talk about the timing tensioners of the early M278 builds, as they cause a rattling sound. However, the challenge here is that the engine has a pretty complicated timing chain drive arrangement. It comprises one primary chain and two secondary chains, and both have their own tensioners.
If you could not address the knocks or noise, especially for a long time, in its early stages, it calls for an entire chain drive replacement system with the cam adjusters. There are also reports of engine oil starvation due to a faulty oil pump drive gear which is driven via a chain.
There are also issues with premature wear of valve guide on earlier M278 engines. The Silitec coating inside does not cut it since high temperatures and detonation during operations require resistance, and the material cannot contain it. Hence, the wear of the guides.
Later on, Mercedes switched on using NanoSlide coating, a far better material for scratch resistance.
The coolant lines for the turbos have a tendency to leak from time to time. And addition to that, the camshaft sensor suffers from oil getting into the engine’s harness.
For a V8 engine, it may seem so underwhelming by the numbers given by the M278 engine. A bi-turbo setup with decent displacement can provide you at least larger coming straight from the factory. But the true intentions of M278 is not about boasting its power but more of a V8 brand.
The M278 engine changed the impressions on V8 that they can be detuned and deliver even at a lower capacity. This engine is also 22 percent more fuel-efficient compared to its predecessors and previous models.
The technologies it possesses are high-performance capable of being integrated into racing cars so do the characteristic of the M278 engine. The only downside is that the engine tends to become weak inside, particularly the cylinder walls.
Even with those, M278 is still a solid, reliable engine with the power and a decent amount of torque. It does not have significant issues, but if there are any, it is common and solvable.
I hope that this discussion cleared some confusion in your head regarding the Mercedes M278 engine’s architecture, issues, applications, potential, reliability, and overall impact on the automotive industry.