The new Mercedes A45 AMG will have the world’s most powerful four‑cylinder engine

With an output of up to 421hp and 500Nm – 41hp and 25Nm more than its previous most potent M series powertrain – the Mercedes-AMG 2.0-litre M 139 engine has been developed from the ground up, and even outranks some sports car engines in terms of output per litre. Each unit is entirely assembled by hand on an innovative production line in Affalterbach, which applies the ‘One Man, One Engine’ principle.

There will be two power outputs for the new A45 AMG, however, a 'basic' version with 387hp and an 'S' model with 421hp, so you'll need to spend a little extra money if you want the full-fat motor.

The transverse-mounted engine promises immediate response, with a carefully balanced torque curve where the peak of 500Nm (480Nm for the basic version) is available from 5,000-5,250rpm (4,750-5,000rpm), before a redline of 7,200rpm.

Tobias Moers, Chairman of Mercedes-AMG, commented: “We already set the benchmark in the segment with the preceding engine. This fundamentally new four-cylinder presented us with the challenge of doing even better. And we succeeded with a number of sometimes revolutionary solutions.

“With the M 139, we have once again impressively demonstrated the engine expertise of Mercedes-AMG. Not only is the output per litre unrivalled for a turbocharged engine, the high level of efficiency also demonstrates that the internal combustion engine still has further potential.”

Given how quick the old A45 AMG felt with only 376bhp, we can't help but get a little giddy at the prospect of getting behind the wheel.