Mercedes-AMG made a big splash with the unveiling of the A45 and CLA45 high-performance models earlier this year and their newly developed M139 engine. Delivering up to 416 bhp in the S versions, the 2.0-litre turbocharged engine is the most powerful mass-production four-cylinder in the automotive industry, and could grow even stronger in the years to come.

While Ford’s 2.3-litre EcoBoost engine inside the Focus RS “only” makes 350 bhp, Mountune has developed several packages that lift output to 400 bhp, 444 bhp, and even 513 bhp courtesy of the m520 package. Rather than resting on its laurels, the tuner continues to extract more muscle from the engine, and they’ve managed to nearly double the stock output.

With 684 bhp on tap, the four-banger has a whopping 334 bhp more than the stock EcoBoost mill, prompting Mountune to say they have created what is possibly the most powerful four-cylinder direct-injection engine out there. Torque is also substantially up from the series 350 pound-feet to a mountain-moving 566 lb-ft.

The engine featured in this dyno test had a BorgWarner EFR 8374 turbocharger and Mountune’s own MRX cylinder head, but it remains to be seen whether it will become an actual upgrade package for owners of the Blue Oval’s all-paw hot hatch. There’s also the matter of reliability as people have expressed their concerns about AMG squeezing over 400 bhp from only a small four-cylinder unit, questioning how long it would last. The same folks would likely have a thing or two to say about a future Mountune kit with almost twice the power of a standard Focus RS.

As you might have heard, the aftermarket specialist is no longer only about fast Fords as it’s now dipping its toes into the Volkswagen tuning scene by coming out with upgrade kits for the Golf GTI and Golf R. A modified version of the latter was recently pitted against a five-cylinder Audi RS3 and managed to beat it in a drag race.