When you think of big horsepower from the factory, you probably don’t think of four-cylinder engines. While there is no denying that four-cylinder powerplants can make huge amounts of horsepower by harnessing the latest and greatest technology from the aftermarket, usually you don’t see that coming from the OEMs.

Another thing you probably don’t associate with a four-cylinder engine is Mercedes-AMG. However, they are the force behind the most powerful production four-cylinder engine currently in existence. The 2.0-liter M139 engine is a single turbocharged inline-four that pumps out an astonishing 421 horsepower at 6,750 rpm and 368 lb-ft of torque at 5,250 rpm, with a maximum engine speed of 7,200 rpm.

When you are talking about a 2.0L four-cylinder making the same kind of power and torque numbers as a Gen-1 Ford 5.0L Coyote or the 6.2L Chevrolet LS3 engine, you are in rarefied territory. The question on most people’s minds at this point is, “How?” Well, luckily our favorite YouTube Engineer, Jason Fenske of Engineering Explained has done the heavy lifting in order to explain just what makes the M139 such a powerhouse.

“One of the first things [Mercedes] did was that they simply rotated the engine 180 degrees,” Fenske explains of the laterally mounted engine layout. “With the new intake routing, the air goes into the backside of the engine to enter the turbo, and then comes back up front through an air to water intercooler into the intake manifold.”

This little 2.0-liter four-cylinder packs a mighty punch from the factory, matching some of today’s most revered v8 engines with half the cylinders and less than half the displacement.

While at first you might be shaking your head, wondering what on earth the orientation of the engine has with its performance. Well, according to Mercedes-AMG, by reorienting the engine by half-a-rotation from the previous M133 engine, they have optimized the path for intake and exhaust gasses.

“Mercedes claims that by flipping the engine around, they have a shorter intake tract length, and fewer diversions for both the intake and exhaust,” says Fenske. By reducing the volume of the intake tract, post-turbocharger, the engine will be more responsive, and by straightening out the path of the air, there will be less restriction for the engine to overcome.

The Turbocharger is Mercedes-AMG’s Replacement For Displacement

While the list of improvements on the M139 engine is significant, the belle of the ball has to be the turbocharger. It’s the key to the little 2.0-liter inline-four-cylinder punching way above its weight class when it comes to horsepower. For the first time Mercedes is using roller bearings instead of journal bearings in the turbocharger,” explains Fenske.

“Versus journal bearings, the roller bearings are going to have less friction, and that means the turbocharger will be able to spool up quicker. That doesn’t necessarily mean that it’s going to reach a higher [impeller speed] or make more boost, it just means you’ll have the boost earlier in the engine’s RPM range. At a given engine speed, the roller bearing turbo will be at a higher impeller speed than an equivalent journal bearing turbo.”

When your peak turbocharger compressor wheel speed is 169,000 rpm and you’re making 30.5 psi of boost off the showroom floor, those little increases in efficiency add up to significant performance gains. In addition to the roller bearings, the turbocharger uses a twin-scroll turbine housing, which increases both the overall efficiency and responsiveness of the turbocharger.

“The two separate exhaust scrolls are parallel and feed the same turbine wheel which spins the compressor wheel,” Fenske explains. “What you’re doing is separating your exhaust pulses. By pairing-up cylinders two and three and cylinders one and four, and then giving them separate scrolls, all the exhaust pulses are separated. That leads to a better response and better low-end torque.”

A Beefed-Up Shortblock

When you’re making 421 horsepower and cramming over 30 pounds of compressed atmosphere into the engine on a regular basis, it’s no surprise that there were upgrades to the short-block required to accommodate the new pep in the M139’s step. “On top of forged pistons and a forged crankshaft, the engine uses a closed deck block,” Fenske reveals, about a strength feature that not everyone considers at first.

“This engine’s peak combustion pressure is about 160bar (2320psi). As a comparison, the 840 horsepower 6.2 Liter supercharged V8 from the Dodge Demon makes about 135bar (1960 psi) peak combustion pressure.” With that kind of pressure, it makes sense that Mercedes-AMG engineers looked beyond just the rotating assembly for increased strength.

In addition to increased strength, Mercedes-AMG designers found a little more power in the cylinders themselves. By using what they call “Nanoslide” cylinder coating (which is Mercedes tradename for their Twin Wire Arc Spray spray-on cylinder liner) the friction between the piston and cylinder wall is reduced.

“It’s very thin, very wear-resistant, and has extremely low friction,” says Fenske. “Even with its mirror-like finish, there is a micro-porosity there, which retains oil for extremely good lubrication. Mercedes claims this reduces piston, piston ring, and wall interaction friction by as much as 50 percent.”

Cylinder Head Improvements

It only stands to reason that with this massive redesign, the cylinder head would see revisions as well. Using the previous M133 cylinder head as a base, engineers made a few key alterations in order to support the extra boost and horsepower production. “They changed the location of the fuel injector and rotated it a bit, and are now able to use larger exhaust valves.” Says Fenske.

“They have also added port-injection to the previous direct-injection. The direct-injectors operate at 200 bar (2,920psi) and the port injectors fire off at 6.7 bar (97psi).” While that might seem like high pressures for the port injectors, keep in mind, with 30-plus-psi of compressed intake charge, that’s only about 10 psi higher pressure differential in the manifold than a factory LS engine.

In addition to the extra fuel, the M139 engine has also incorporated variable valve duration on the exhaust camshaft, in addition to the standard variable camshaft timing on both the intake and exhaust cams.

Together, all these upgrades and increases in efficiency work together for an approximate 50-horsepower boost from the previous model, and the title of the world’s most powerful production four-cylinder engine.

Another feature that might be overlooked by a casual observer is the closed-deck engine bock design employed by Mercedes-AMG. The design keeps more material around the cylinder walls to increase both stiffness and strength. Also, note the factory multi-layer steel head gasket — a must in a high-boost application.