M256

The M104 closed the line of Mercedes-Benz petrol sixes, and everything after it was a V6 — chosen because a vee packs more easily behind a bulkhead. The M256 of 2017 overturned that on an argument nobody had been able to make before: electrification. Strip the ancillary drive off the nose of the engine and length stops being the problem. So there is no belt anywhere on it. A 48-volt system runs the coolant pump, the refrigerant compressor and an integrated starter-alternator sitting between engine and gearbox, worth up to 16 kW and 250 N·m and recovering energy on the overrun.

Underneath the electrics it is conventional in the best sense: 500 cc per cylinder, the same module as the four-cylinder and V8 families; NANOSLIDE arc-sprayed bores instead of iron liners; CAMTRONIC two-stage lift on the inlet side with a 70° camshaft adjustment range that allows Atkinson timing at part load; a zirconium-alloyed aluminum head; 120 bar of designed peak cylinder pressure. Lag falls to a BorgWarner electric compressor spinning to 70,000 rpm on the same 48 volts. AMG took the highest-output version for its 53-badged cars, and Aston Martin bought the engine outright for the DBX Straight-Six.