Mercedes M155: Everything You Need To Know

In the early 2000s, McLaren and Mercedes planned a joint venture project that will recreate and marks a new beginning of generational engines, primarily to power the grandest tourers ever built, the SLR concept.

Though these engines do not bear any AMG badge, the machine was made, emphasis hand-made, in Affalterbach. Presently, this engine has got to be one of the most impressive supercharged V8 to be developed in the European region.

What are Mercedes M155 Engines?

The special edition Mercedes M155 engine started out as the 5.4-liter M113K V8 with slightly different modifications and development on the concept cars and was intensively improved for its final production versions.

At that particular period, Mercedes owned 40% of the McLaren Group and decided to create a world-renown project – the SLR.

Mercedes M155, as we mentioned earlier, is the brainchild of the AMG experts. This supercharged version of the 5.4 Liter M113 is exclusively designed to fit for the Mercedes-Benz SLR McLaren, earning the badge of the M155 SLR engine code.

The project was first unveiled in the Vision SLR concept inspired by the 300 SLR Uhlenhaut Coupe from 1955.

It has the same all-aluminum lineup. The displacement and even the SOHC three-valve per cylinder design of the flagship E55 AMG performance models were adapted but had more upgrades.

The most noticeable change is the heavily reinforced closed deck engine block, which was technically different from those of the M113 K. The compression rating was decreased to a lower 8.8 due to the redesigned pistons and high-strength forged connecting rods.

The rev limit was raised to 7,000 RPM, contrary to the prior 6,500 RPM.

Engine Specifications and Design:

  • Production Run: 2003 – 2006
  • Cylinder Head Material: Aluminum
  • Cylinder Block Material: Aluminum
  • Configuration: V8
  • Bore: 97 mm
  • Stroke: 92 mm
  • Valvetrain: SOHC three valves per cylinder
  • Displacement: 5.4 L (5439 cc)
  • Compression Ratio: 8.8
  • Weight: 365 lbs.
  • Maximum HP: 641 HP at 6,500 RPM
  • Maximum Torque: 605 lb-ft at 4,000 RPM

The Mercedes M155 engine is more closely related to the M113 E55 gasoline engine since the M155 originated from the version of the M113K – and some attempts of some tuners to replicate that one.

Many enthusiasts assert that the M113K was Mercedes’ latest attempt to create the great ‘K’ Kompressor engines until the recent takeover of the 6.2 Liter.

M155 engines share the same cylinder bank angle of 90-degrees across the M113 family and uniform cylinder spacings of 106 mm. It has a cylinder bore of 97 mm and a piston stroke of 92 mm.

The M155 cylinder block is a closed-deck design made from lightweight aluminum with cast-in Silitec (Al-Si-alloy) cylinder liners and cylinder banks arranged in a ‘V’ configuration.

It is equipped with sequential fuel injection, a larger 80-mm throttle body, dry-sump lubrication, iron-coated aluminum pistons, magnesium dual-length intake manifold, and high-strength forged steel connecting rods.

It has an optimized light aluminum alloy cylinder head with a single overhead camshaft per cylinder bank; the camshaft is also the balancer shaft, the same as the M112 engine, and is driven by a double roller chain.

The crankcase was also reinforced; inside is a precision-balanced crankshaft designed to significantly improve the weight distribution and balance.

For each cylinder, the M155 engine has two intake valves and one exhaust valve, which were actuated by light metal roller rocker arms with hydraulic valve clearance compensation.

A twin-spring assembly for the valve train was installed to increase the maximum engine speed to 7,000 RPM.

Mercedes chose a single exhaust valve to result in a smaller surface exhaust passage that minimizes cold heat losses and enables the catalytic converter to reach the operating temperature faster.

A thin-walled sheet metal exhaust manifold also contributes to this by absorbing only small amounts of heat. To add, on the crankshaft in the cylinder, a balancer shaft is placed that rotates inversely to the crankshaft at the same rate to equalize first-order rocking motions.

The M155 engine, the same as the M113 family, has two spark plugs arranged on the right and left of the exhaust valve for the combustion chamber – it seems like a face-to-face orientation.

These valves are controlled via a time-shifted mechanism and arranged in a symmetrical manner with a double ignition operation to have fast and uniform combustion, higher tolerance for a lean mixture, greater safety against combustion misfiring, and promotes better exhaust gas recirculation.

The double ignition affects the piston action due to a higher heat load imposed on the piston, which is cooled by oil spray nozzles placed below the piston crown.

It has a Lysholm-type IHI twin-screw supercharger placed between the cylinder bank, providing a maximal boost pressure of 13 psi. The compressed air is then cooled through two large intercoolers, each with a designated water pump.

The supercharger has two Teflon-coated aluminum shafts that rotate at 23,000 RPM and pushes a maximum of 4080 lbs. of air per hour directly into the combustion chambers.

However, the scroll compressor only operates at particular engine speeds and load conditions to minimize fuel consumption activated by a separate poly-V-belt and electromagnetic coupling.

The M155 SLR engine was rated at 617 HP at 6,500 RPM with 575 lb-ft of torque at 3,250 RPM in all these upgrades. The uprated version was rated at 641 HP at 6,500 RPM and 605 lb-ft of torque at 4,000, which is featured in the 722 editions.

The Mercedes M155 marked its end of production with a total of 2,157 units sold; all of them were assembled at McLaren’s Surrey plant in the United Kingdom.

Applications of Mercedes M155 Engine:

  • 2004 Mercedes-Benz SLR McLaren
  • 2006 Mercedes-Benz SLR McLaren 722 Edition
  • 2004 – 2010 Mercedes-Benz SLR Coupé
  • 2007 – 2010 Mercedes-Benz SLR Roadster
  • 2009 – 2010 Mercedes-Benz SLR 722 S Roadster
  • 2009 – 2010 Mercedes-Benz SLR Stirling Moss Speedster

Engine Tuning, Upgrades, And Modification

With its stronger internals and a lower compression ratio, the supercharged M155 engine is a much friendlier machine to be honed with any kind of forced induction. To add to that, the AMG heads also contributed to better power output.

You can also opt to upgrades some parts of your engine, such as a performance exhaust system, replacing the intercoolers, internals, and ECU tuning.

Problems Surrounding Mercedes M155 Engines:

Being on the higher-end list of the widely-recognized automaker will set a massive expectation on the engine’s party. This engine is also considered one of the best in the industry.

Blissful as it sounds, however, M155 machines are far from perfect kind of engines. They have some weaknesses and shortcomings that we need to address so that the future owners – or those who already owned, will have prior knowledge of what’s to come with the M155 engine.

Here are some troubles that you might encounter with the M155:

The supercharged and twin-intercooled version, 5.4 L M113K, is almost identical to the high-performance M155 engine. This engine is equipped with intercoolers that can also be found on the M113K engine.

As such, this issue only affects higher-performance machines, hence the AMG engines.

A short description of the intercooler is that intercoolers are responsible for cooling compressed air. Without intercoolers, the intake air temperature will rise drastically and prompts the ECU to disable the supercharger.

The issue with M155 is the failure of the intercooler pump. If the pump fails, it causes the intercooler system to run under its rated capacity or just simply shuts down. When this happens, the intake air temperature rises that can lead to weaker engine performance. Once the air got too hot, the computer will shut down the supercharger.

The next issue focuses more on the effects of the age and mileage of the M55 engine – oil leaks. However, this is not exclusive to the M155 as it affects many machines. Please note that the M155 is almost two decades since its arrival and may have unattended issues like this.

The next issue is the primary seal deterioration.

The rear primary seal’s main purpose is to seal the crankshaft where it connects to the transmission. When cracks develop, the oil began to leak, which becomes worse and worse.

This is not an urgent problem, but you might want to replace the seals before the leaking becomes flooding and affect the total performance of the engine.

This issue can be noticed by the visible leaks towards the rear of the engine bay; valve cover gasket leaks might show symptoms like burning oil smells and light smoke.

Lastly, this is not a common problem but can count as a preventive measure to counter any adverse effects regarding huge numbers of spark plugs.

Since the M155 engine V8 runs on two spark plugs per cylinder to a total of 16 spark plugs, spark plugs need to be replaced because they are standard maintenance items that need to be replaced like engine oil.

These spark plugs are crucial to a smoother running engine. Worn-down spark plugs can no longer ignite the cylinder’s air-fuel mix properly, which generally leads to misfires and drivability issues.

This issue affects this supercharged version due to its high boosting abilities that cause high cylinder pressures that put more stress and strain on ignition components. Power loss can be a telltale sign, but it is hard to notice unless multiple cylinder misfiring happens.

Always remember this engine needs quality engine oil and fuel.


The story of this engine on how it was developed is historic by itself. Mercedes themselves paved the way and revolutionized how superchargers work on the machine’s behavior, which is on full display in the M155 engine.

Its closed-deck design is planned to carry out a more efficient engine with higher temperature and pressure resistance. The engine’s ‘V’ profile also fits to sit in a lowered position, which contributes to better weight distribution and balance.

The power output is rightfully balanced to the amount of torque it produces. The lower and higher-end power band provides decent operation and can be felt in almost any speed range.

It is similar, if not, efficient to be driven at the city and track session. Hence, the respect it gets is earned and proven. The potential for this engine is endless, the tremendous amount that only waits for the right hand to do so.

Overall, this engine is highly reliable with the power it gives. I hope that this brief discussion helped you understand and cleared M155 engine design, applications, troubles, reliability, and its overall impact on the automotive industry.

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