M125

The 750-kilogram formula limited weight, not displacement, so Mercedes-Benz built as much engine as the rules would carry. The M125 measured 94 × 102 mm for 5,663 cc, with four valves per cylinder, twin gear-driven overhead camshafts, a nine-bearing crankshaft, dry-sump lubrication and a single upright Roots blower. It burned a blend of methyl alcohol, benzene, ethyl alcohol and petrol, weighed 222 kg — close to a third of the whole car — and gave between roughly 560 and 595 hp in race trim, with a best test-bed reading of 646 PS at 5,800 rpm.

It ran rough, and the mechanics learned to distrust smoothness: when an M125 started running clean, its crankshaft was cracking. Tire wear rather than the dynamometer decided how much of the output could actually be used. Rudolf Caracciola took the 1937 European Championship with it and W125 drivers filled the first four places in the standings. Then the formula changed — from 1938 supercharged Grand Prix engines were capped at three liters — and one season was all it had. Nothing in Grand Prix racing made comparable power again until the turbo era of the early 1980s.