M133

AMG had never built a four-cylinder before. Rather than shrink a V8, the M133 started from the ordinary transverse M270 and kept its bore, stroke and 90 mm cylinder spacing, then was rebuilt around peak firing pressures of up to 150 bar: a closed-deck crankcase, a forged steel crankshaft with induction-hardened journals, forged pistons in a racing alloy, compression dropped to 8.6:1. A twin-scroll turbocharger runs 1.8 bar of boost through indirect water-cooled charge cooling, and the whole path from air intake to inlet port measures under 1.2 meters, chosen for response.

At its launch in the A 45 AMG in June 2013 it was the most powerful series-production four-cylinder in the world, and 181 hp per liter put it on terms with supercars; a 2015 revision took it to 381 PS and 475 N·m. Piezo injectors fire up to five times per cycle while the engine warms, to hold particulate emissions down. Every unit was assembled by hand on a dedicated line at Kölleda under AMG's one man, one engine rule, the builder signing the plate. The M139 replaced it in 2019.