M25

The AIACR's 1934 formula capped Grand Prix cars at 750 kg dry, which retired the seven-liter SSK at a stroke. Daimler-Benz answered with a supercharged straight-eight drawn by Hans Nibel, Max Sailer, Albert Heess and Max Wagner: twin overhead camshafts driven by spur gears, four valves per cylinder, five main bearings, two updraft carburetors and a Roots compressor. It began at 3.4 liters — roughly half the SSK's displacement — and within the year was beating that car's 300 hp. The W25 scratched from its intended debut at AVUS with fuel trouble, then won the Eifelrennen a week later.

Over four seasons the M25 was bored and stroked repeatedly, through 3.7 and 4.0 to 4.3 liters, the last of those carrying Rudolf Caracciola to the 1935 European Championship in a year the W25 won nine Grands Prix. Then it ran out of room. Taken to 4.7 liters it turned unreliable; a V12 built to replace it proved too heavy for the weight limit, and shortening the chassis to accept it only spoiled the handling. Rudolf Uhlenhaut, sent to find out why Auto Union was winning, drove the car himself and diagnosed suspension too stiff for a frame too soft — and built the W125.