Although Mercedes-Benz has only given us glimpses of the CLA 45, GLA 45, and A 45, the new engine powering these three cars has been fully revealed and cleared for production. It will be available in two guises, with one version producing 382hp and 480Nm.

The S version of the engine is the real head-turner, putting down 416hp and 500Nm. The kind of power figures that Mercedes AMG has managed to extract from such a small mill is nothing short of impressive. These specs make it the most powerful production four-cylinder engine ever.

As is the norm with all AMG engines, the factory follows the "One Man, One Engine" routine.

Where does so much power come from?

The 2.0-litre inline-four block is made of aluminium with a closed-deck design. The steel crank and aluminium pistons are all forged. The twin-scroll turbocharger takes advantage of an air-to-water intercooler setup and both port and direct fuel injection. The four cylinder engine is cooled with the help of a primary radiator behind the grille and an auxiliary one behind one of the wheel wells.

A drivetrain this small is bound to generate heat disproportionate to its size. It needs aggressive cooling to preserve its block and gearbox. To that end, the transmission shares the engine’s cooling circuit and has its own heat exchanger mounted directly to it. Interestingly, the engine is oriented 180 degrees opposite its predecessor with the exhaust manifold and turbocharger facing the firewall. This arrangement helps reduce the complex turns needed for intake and exhaust piping, presumably improving gas flow.

Being an Mercedes AMG engine (albeit the smallest), each unit is hand assembled by one person entirely. And the list of premium features doesn’t stop here. While final details on other parts of the ‘45’ powered cars are still scarce, we do know that they will feature an eight-speed dual-clutch transmission. All-wheel-drive will be standard, as will be a rather smart ‘Drift’ mode.

While not official et, a '45' derivative of the recently previewed Mercedes-AMG CLA 35 Shooting Brake would spice things up in the segment massively.