Mercedes-Benz M129 engine

The Mercedes-Benz M129 is the fuel-injected version of the 2.5 litre M108 inline-six, built by Mercedes-Benz from 1965 for the W108 250 SE sedan, the matching W111 coupe and cabriolet, and later the W113 250 SL roadster. A mechanically controlled six-plunger Bosch pump fed injectors that were repositioned to aim more directly at the inlet valves, and the cooling fan gained six blades with a viscous coupling. It produced 150 hp at 5,500 rpm. The engine proved controversial and short lived: the cylinders were still spaced unevenly in pairs, which cooled them unequally, and the stroked design reached a peak piston speed of 14.4 m/s, which led to premature failures. Mercedes-Benz answered with a block with symmetrical bores, giving the M114 and the larger M130.