Mercedes-Benz M133

AMG's first hand-built 2.0 four, in the original A45, CLA45 and GLA45, and the engine the M139 replaced. When it launched it was the most powerful production four-cylinder in the world.

Its specific output is the operating envelope, not a statistic. At well over 150 hp per litre, boost pressure, charge temperature and oil condition decide whether it behaves — a tired or incorrect oil, a marginal intercooler or an intake leak produces knock and heat-related faults far sooner than on an ordinary 2.0 turbo.

And it is not the M139 under an older number. The successor turned the engine around to put the turbocharger at the rear; this one does not, and the layout, the plumbing and the access are different. A repair procedure written for one does not describe the other.