Mercedes-Benz M139

AMG's hand-built 2.0-litre four, and for years the most powerful production four-cylinder engine in the world — the successor to the M133, with the whole engine rotated so the turbocharger sits at the rear.

The number to keep in mind is specific output. It makes more than 175 hp per litre in standard form, and everything about its behaviour follows from that: boost pressure, charge temperature and oil condition are not maintenance details here, they are the operating envelope. A tired or incorrect oil, a marginal intercooler or an intake leak produces knock and heat-related faults far sooner than it would on an ordinary 2.0 turbo.

It is also among the most heavily modified engines on the road, so the standing question on any fault is what software and hardware the car is actually running. A knock or over-boost code on a modified M139 is usually reporting the modification rather than a component failure.