Mercedes-AMG has just made the most powerful four-cylinder engine in the world

With up to 421hp and 500Nm of torque, the new M139 powerplant will turn up the wick in the next generation of AMG 45 models


Of all the various auto makers’ in-house tuning arms, it is perhaps Mercedes-AMG that has the strongest reputation for building bonkers engines. From the 6.8-litre V8 in the first AMG – the 300 SEL “Red Pig” race car – to the humongous 7.3-litre V12 as used in the 1990s SL73 as well as the Pagani Zonda, to the more recent S 65 and SL 65’s twin-turbo 6.0-litre V12 with 1000Nm of torque, the Mercedes-AMG back catalogue has no shortage of thunderous motors endowing cars with sledgehammer performance.

The largest Mercedes road car engine, the M297 V12

The company’s latest work might be the smallest offering it makes, but that doesn’t make it any less impressive. Behold the M139, the most potent four-cylinder engine to ever come out of a factory. It’ll be available in outputs: a “basic” version with 387hp and 480Nm of torque, and an “S” with a whopping 421hp and 500Nm, and will be used in AMG 45 and 45 S models of the Baby Benz family, namely the A-Class, CLA, and GLA.

Now, dedicated petrolheads might remember that Mitsubishi released a limited-edition FQ-440 variant of the Lancer Evolution X, whose 4B11T had even higher outputs than AMG’s M139. But although that was fully backed up with a three-year warranty, that was a UK-market special tuned with aftermarket parts, not a complete package in full serial production.

In any case, the 2.0-litre turbocharged M139 is indisputably the most powerful four-pot available today. It’s worth noting that even the “basic” M139 is more powerful than the next closest contender, which was its immediate predecessor, the outgoing AMG A 45 and CLA 45’s M133, with 380hp.

If we’re talking about engines from other companies, then the next most powerful is the 365hp 2.5-litre flat-four that’s found in the Porsche 718 Boxster GTS and Cayman GTS.

More impressive than the new engine’s total output though, is its specific output. At 211hp per litre, the M139 is more power-dense than many supercar engines, including the Porsche 911 GT2 RS’s 3.8-litre flat-six (182hp/litre), the Ferrari 488 Pista and F8 Tributo’s 3.9-litre V8 (182hp/litre), and even the McLaren Senna’s 4.0-litre V8 (198hp/litre). This means the M139 is a pretty highly-strung unit continuously dealing with forces and pressures greater than most other engines.

This is why the engine block is of a closed deck construction, a form more commonly found in motorsport that’s able to withstand higher loads more reliably.

Apart from the lighter yet stronger block, other developments include low-friction roller bearings for the turbo which allow it to spin more freely (similar to the turbos on the AMG GT 4-Door’s 4.0-litre V8), both direct and port fuel injection to supply sufficient fuel to maintain the high output, Camtronic variable valve timing (similar to Honda’s VTEC), and enlarged exhaust valves.

There’s also low-friction Nanoslide liners in the cylinders, a technology also found in Mercedes-AMG’s Formula One car

So, objectively the new engine is faster, but AMG has also tuned it to feel faster too. Peak torque is developed at a high 5000rpm, which helps create a more exciting rush of acceleration as the revs rise that’s more akin to a normally aspirated engine, rather than the efficient but less dramatic flat torque plateaus of most modern turbo engines. The redline is also 500rpm higher, at 7200rpm, and the entire engine revs more freely, for a more “emotional driving experience” according to AMG.

No official details have been released for the new AMG 45 family, but it’s highly likely that the A 45 and CLA 45 will be the first to debut. And “emotional driving experience” should come naturally to these cars, because in addition to the new engine, the A 45 has been shown in a couple of teaser videos pulling off some epic powerslides, courtesy of a dedicated drift mode just like its E 63 S big brother.


about the author

Jon Lim
CarBuyer's staff writer was its fourth historical Jonathan. Old-fashioned in all but body, he thinks car design peaked in the '90s and is enthusiastic about vintage cars and old machinery.