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The M139's output of 416 hp makes it the world's most potent four-cylinder engine in serial production.

Introduced in 2019, the relatively new Mercedes M139 turbocharged 2.0-liter inline-4 is still the world’s most powerful 4-cylinder mill. It is the successor to the M133 that Mercedes produced between 2013 and 2019. Also, like its successor, the M133 is an award-winning turbocharged inline-four engine offered in 1.8-liter and 2.0-liter models. It is a 3 times Engine of the Year winner in the Best New Engine 1.8-liter and 2.0-liter categories.

The Mercedes' latest inline-4 is, among other things, is the first four-cylinder engine by AMG to be specified with two outputs. It is also the first to be assembled by hand on par with the "one man, one engine" philosophy. This engine is equally the first to be equipped with a combined port and direct fuel injection system. How is the M139 the most powerful series-production four-cylinder in the world?

The answer is due to its output of 416 horsepower, specifically, 208 horses per liter or 104 horses per cylinder. We currently have three models of the M139; a 382 horsepower at 6,500 rpm model making 354 lb-ft of torque at 4,750 – 5,000 rpm, and a 416 horsepower at 6,750 rpm model making 369 lb-ft of torque at 5000 – 5250 rpm. Both are still in production. The third is a 402-hp and 369 lb-ft of torque model introduced this year.

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The badass M139 is currently powering eight Mercedes cars. These include the Mercedes CLA (C118) 45 AMG 4MATIC+, a luxury compact car that Mercedes has been making since 2013; Mercedes A-Class (W177) 45 AMG 4MATIC+, an entry-level subcompact executive hatchback and sedan first introduced in 1997 and currently in its fourth generation; Mercedes GLA 45 AMG 4MATIC+, a two generations subcompact luxury C-SUV manufactured by Mercedes since 2013 and the smallest Mercedes SUV;

Mercedes-AMG SL 43, the refreshed 7th-gen Mercedes roadster replacing the R231 in 2022 and marketed under the AMG branding; Mercedes CLA (C118) 45 S AMG 4MATIC+; Mercedes A-Class (W177) 45 S AMG 4MATIC+; Mercedes GLA 45 S AMG 4MATIC+, and the Mercedes-AMG C 63 e-Performance 4MATIC+; a fifth generation C-Class replacement for the W205 C-Class produced since 2014 with an integrated 15 kW electric motor and 48-volt electric system.

The first four on this list have the 382-hp version of the M139 engine, while the last four have the 416-hp. The upcoming Mercedes-AMG C43 4MATIC scheduled for release later this year for the 2023 model year will feature the newest 402-hp version of the M139 engine. The last gas-powered Lotus, the Emira that was released in 2022, comes with a Toyota-sourced 3.5-liter supercharged V6 and a tuned version of the M139 engine and DCT gearbox in a mid-engine/rear-wheel drive installation producing around 360 hp.

Why would Mercedes switch down instead of up? Switching from four to six seems the natural order, but a 6-cylinder is apparently unnecessary when specific output has increased significantly. Consider that modern engines are typically 500cc/cylinder, with a specific output of 125 hp/L even for small engines. The older 2011 Mercedes-Benz 2.0-liter turbocharged inline-4 M274 engine churns up to 255 horsepower, which some two decades ago is output for V8 mills, and still boasts a fraction of the V8’s weight.

Now, the newest up to 416-hp M139 equip cars like the AMG CLA 45 S 4MATIC+ with 421 horsepower, and it’s not even a supercar. That said, switching from 6- to 4-cylinder is also Mercedes’ way of reducing its carbon footprint, especially with electrified models like the Mercedes-AMG C 63 e-Performance 4MATIC+.

Fuel efficiency is another factor as European automakers raced to meet the EU-wide regulations deadline. Four-cylinder mills consume less fuel, especially at low loads, a compelling reason for the marque’s addition of mild hybrid systems in many of its vehicles to achieve greater torque and counteract the loss of basic engine power, for example, at startup or when the car is climbing a hill.

Cost is another argument. It's cheaper to develop a modular engine that can be built with four or six cylinders than to develop a new inline-4 and V6 that are often redesigned down the road to improve efficiency.

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The design of the new Mercedes M139 engine is based on the current Mercedes-Benz M260 engine, which is an upgraded version of the outgoing M270 we mentioned earlier. Thus, it shares its predecessor’s bore and stroke dimension as well as the block.

Deviating from the design of the M260, the AMG engineers rotated the engine block 180 degrees, placing the intake system in the front of the engine, with the turbochargers and exhaust headers between the engine and firewall. This arrangement optimizes ventilation to optimize performance with a shorter distance and fewer diversions. The 180° rotation also supports what the guys at Affalterbach (the 4-cylinder’s ultra-modern production facility) called “the flattest possible and aerodynamically advantageous front section design."

You might wonder how the engine can put up with nearly 30.5 pounds per square inch of turbo pressure. The engine block is made from a chill-cast aluminum process, resulting in a dense, high-strength closed-deck engine block which AMG engineers say mimics race cars and maximizes the crankcase's rigidity-to-weight ratio, and allows higher peak combustion pressure.

Other ingenious features like an electric water pump that reduces parasitic drain and can be more accurately controlled to manage engine temperatures and the air-to-water intercooling system also improve the engine’s power output, stability, and longevity. The exhaust valves are bigger to improve higher flow, and the compression ratio has risen to 9.0:1, compared to the previous M133's 8.6:1.

The M139 features a historic-first two-stage fuel injection system where piezo injectors deliver fuel directly into the combustion chambers as the first stage and solenoid valves in the manifold as the second stage. This is the first time in Mercedes-Benz history a production 4-cylinder engine has a 2-stage fuel injection system, which increases specific output.

As stated earlier, specific outputs - for the base version of the engine are 382 horsepower and 354 lb-ft of torque; and 416 horsepower and 369 lb-ft of torque for the S-model. In other words, the M139 isn’t just the most powerful production 4-cylinder but also boasts a specific output higher than any modern supercar we can think of right now. It doesn't really need more than 400 horsepower, but the gearheads at AMG are obviously from another world.

Philip Uwaoma, this bearded black male from Nigeria, is fast approaching two million words in articles published on various websites, including toylist.com, rehabaid.com, and autoquarterly.com. After not getting credit for his work on Auto Quarterly, Philip is now convinced that ghostwriting sucks. He has no dog, no wife- yet- and he loves Rolls Royce a little too much.