M154

For 1938 the AIACR abandoned the weight formula for a capacity limit — three liters supercharged, four and a half without — which finished the 5.7-liter W 125 straight-eight at a stroke. Untertürkheim spent months on alternatives, among them a mid-engined car and a 4.5-liter naturally aspirated W 24 with three banks of eight, before the board resolved in July 1937 to design in-house. Albert Heess laid out a 60° V12 with 250 cc per cylinder, four camshafts and 48 valves worked through forked rockers, cylinders welded in threes inside sheet-steel water jackets, heads that did not come off, and glycol coolant good to 125 °C.

It was traditional Daimler-Benz practice taken to its limit: enormously durable, and enormously heavy at 500 kilograms, with a hundred liters of oil pumped through it every minute. Direct petrol injection was wanted and not ready, so a float carburetor of the sort proven on the M 125 fed two upright Roots blowers, now much smaller than before. Each engine was reputed to cost 89,700 reichsmarks. In the W 154 it gave Rudolf Caracciola the 1938 European Championship, and made Mercedes-Benz almost unbeatable in the first season of the new formula.