Here’s Why The Mercedes-AMG M139 2.0-Liter Four-Cylinder Is An Engineering Gem

Here’s how Mercedes-AMG built the most powerful four-cylinder engine ever put in a production car.

Downsizing — it’s a trending phenomenon happening to most high-performance engines nowadays. It’s when automakers shift from using beefy power units to smaller-displacement engines with a turbo hybrid setup to meet emission regulations. And recently, this happened to the upcoming 2024 Mercedes-AMG C63 S, which for the first time, replaced the previous turbocharged V8 with a more compact four-cylinder plug-in-hybrid.

Of course, to replace the signature V8 of an AMG “63” model, Mercedes needed no ordinary four-cylinder engine. Hence came the Mercedes-AMG M139 series, comprising some of the most modern and high-powered 2.0-liter inline-4s today. Indeed with outputs ranging from 382 to 469 hp, the range-topper of the series currently holds the crown for the world’s most powerful four-cylinder unit on the market.

Needless to say, M139 engines are quite unique with their ability to squeeze such high outputs from their rather small displacement — especially when everything else seems to be turning electric nowadays.

So if you are interested in learning more about how the German manufacturer created the M139 engines and what makes them such engineering gems,  you are at the right place. This article will dive deep into the M139’s core and look at its background, performance numbers, and all of AMG‘s efforts required to make them the beast they are.

So without further ado, let’s start!

Overview

The M139 is a series of turbocharged 2.0-liter inline-4 engines produced by AMG since 2019, successors to the previous most powerful production four-cylinder engine series, the Mercedes-AMG M133. But while the predecessor was only specified with one output during a production period, the M139 offers multiple versions.

Initially, two variants were launched — the base model (285 kW) and the S model (310 kW). But with the recent release of the fifth-gen C-Class (W206) and seventh-gen SL-Class (R232) in 2022, Mercedes launched three more versions internally named M139L (L for longitudinal installation), including the 280 kW, 300 kW, and 350 kW variants.

To give you a slight overview, here’s what the current M139 lineup looks like, along with performance figures and model applications:

M139L (350 kW version)

Not Your Regular 2.0-Liter Turbo

You might have already noticed that carmakers are currently obsessed with putting a 2.0-liter four-cylinder turbo into their vehicles; it’s a pretty common setup nowadays. But looking at the M139’s performance numbers, you can tell what AMG did here was truly next level. Not only did it make the base M139 outperform its forerunner, but the fact there was still more power to be unlocked from the small engine is even more impressive.

Now, anyone in the automotive space knows that a 400+ hp output in a mass-produced 2.0-liter inline-4 is a lot of power — we’re talking about over 200 hp per liter/100 hp per cylinder here! To put it into perspective, even the hardcore 4.0-liter flat-6 in a Porsche 718 Cayman GT4 still makes less power and torque than the S-model M139.

As a result, many models adopting this four-cylinder monster simply dominate in their class. For instance, the Mercedes-AMG A45 S has been deemed the most powerful hot hatch since its launch. And with the same engine, the CLA45 S is currently the most potent four-cylinder car in the US. Not to mention the upcoming C63 S E-Performance will pair its mighty M139L to a 201-hp electric motor to smoke its six-cylinder powered rivals like the BMW M3 and Alfa Romeo Giulia Quadrifoglio.

AMG 101: How to Make More Power From a 2.0-Liter Engine

While it may sound like engineering witchcraft, it’s worth noting that, for the M139’s development, Mercedes actually took inspiration from the current M260 and preceding M133 engine — undoubtedly some of the brand’s most solid and matured compact power units. But in a typical AMG fashion, it still managed to crank them up to eleven to make the M139 as potent as possible.

Here are some of the key design and engineering changes implemented by the AMG folks at the Affalterbach plant to make the most out of the turbocharged four-cylinder:

Rotating the Engine by 180 Degrees

One major design change is the 180-degree engine rotation around its vertical axis. This configuration allows the exhaust manifold to be positioned at the rear instead of rounded underneath the engine from the front like its predecessor. Hence the engine can sit lower, reducing the height of the front hood and improving aerodynamics.

Moreover, turning the engine around also results in better airflow due to the shorter length and fewer diversions on the intake and exhaust, improving the overall engine response.

Adding Roller Bearings to the Turbocharger

The M139 is the first Mercedes engine to feature roller bearings within the turbocharger. Compared to the ball bearings in the previous setup, roller bearings can further reduce mechanical friction within the turbo, allowing it to spool up quicker, up to 169,000 rpm.

In addition, this engine also uses a twin-scroll turbocharger, with two separate scrolls in the turbine housing working in parallel to spool up the compressor. This design is optimized by arranging the ducts in the exhaust manifold, where two cylinders are paired to feed exhaust flow into one of the scrolls. So with a firing order of 1-3-4-2, you can ensure the exhaust pulses are constantly feeding into the turbine without overlapping, resulting in better turbo response and low-end torque.

Using a Closed-Deck Construction

It’s worth noting the M139 is running a considerably high boost level of 30.5 PSI (vs. 26.1 PSI in the M133), meaning there’ll be a lot of pressure generated within the cylinders. Thus, besides retaining the forged-aluminum piston and forged-steel crankshaft from its precursor, Mercedes also used a closed-deck engine block construction to maintain rigidity.

Of course, there are many advantages to such a strong block, including being able to withstand higher cylinder pressure of up to 160 bar (2320.6 PSI) and a higher maximum engine speed of 7200 rpm. However, a closed-deck construction also means the areas surrounding the cylinders are significantly tougher, with only a few little holes throughout the block for coolant and oil passages.

Enlarging the Exhaust Valves

Due to the higher pressure, Mercedes also had to find a way to fit larger exhaust valves to provide faster gas cycles, which involved relocating and slightly rotating the fuel injector in the cylinder head to make room for that larger exhaust.

Additionally, the exhaust features variable CAMTRONIC valve control, a clever system enabling variable valve duration within the two different cam profiles. As a result, the exhaust valves can be opened for shorter or longer periods depending on the engine load for better comfort and improved fuel economy under lower loads and better response and power delivery at high loads.

Adding a Two-Stage Fuel Injection System

For the M139 to make that extra power, it needs more fuel coming into the cylinders. To address the issue, Mercedes decided to add port injectors on top of the existing direct injectors, allowing for a true two-stage fuel injection system.

In the first stage, the piezo direct injectors supply fuel to the combustion chambers at up to 200 bar (2900.75 PSI) of pressure. And when needed, the second stage kicks in by triggering the solenoid port injectors in the intake manifold to deliver more fuel at up to 6.7 bar (97.2 PSI), enabling the engine to produce the highest possible output.

Coating the Cylinder Liners with NANOSLIDE

One of the main contributors to the M139’s outrageous power output is its ability to reduce friction using the brand’s patented NANOSLIDE technology. Indeed, this feature was brought over from the previous engine due to its superb capability to minimize friction between the pistons and cylinders by up to 50%.

In essence, NANOSLIDE uses the twin wire arc spray (TWAS) process to melt metal wires and spray thin, light, and microporous coating material to the cylinder’s inner surface, causing oil to stick longer on the cylinder walls and reducing both friction and wear.

Using Electric Exhaust Gas Turbocharger (M139L Only)

The M139L engines are basically the newer (and hybridized) M139 variants introduced in 2022 in some of AMG’s latest models. So aside from getting the longitudinal layout treatment, they also received a new F1-influenced high-tech turbocharger as part of the update.

This technology integrates a 1.6-inch thick electric motor on the turbocharger shaft to help spool up the compressor wheel before the exhaust gases arrive and takes over, resulting in lower lag, more boost available across the entire rev range, improved agility, and more brutal acceleration.

For the 2022 SL43 and 2023 C43, the electric turbo is operated via a 48-volt onboard electrical system that’ll spool the turbo up to 170,000 and 175,000 rpm, respectively. Meanwhile, the upcoming 2024 C63 S E-Performance will have a significantly more powerful 400-volt electrical circuit operating the turbo speed up to 150,000 rpm.

Final Words

Indeed, AMG’s decision to start downsizing its engines may’ve hurt many fans, especially when it added hybrid tech into the mix. Yet as numbers don’t lie, the German carmaker has proven its superbly engineered four-cylinder engines can generate as much (or even more) power as its V6s and V8s of the past while retaining the 2.0-liter’s lightweight form.

To give you an idea of how powerful current Mercedes’ 2.0-liter turbos are, the M139L that’ll be used in the upcoming 2024 C63 S will boast the same power as the 4.0-liter V8 biturbo of the previous gen C63 at 469 hp — that’s the same power for half the displacement and half the cylinder and turbocharger count! And about two decades ago, only those heavy V12 mills in the range-topping S-Class could achieve this kind of output.

Therefore, regardless of what you think about engine downsizing, Mercedes-AMG has done quite an evolutional job here with its M139 powerplants. Sure, the days may be over for its compact AMG vehicles to be equipped with roaring and charismatic V8s. Still, that also means that internal combustion engines are here to stay and will produce more power than ever before — even if it means being supported by electric power to some extent.

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