The Mercedes-Benz M 100 engine was a or single overhead cam V8 produced by Mercedes-Benz between 1964 and 1981. The successor to the M 189 version of the company's venerated straight-6 M 186, it was introduced in the flagship Mercedes-Benz 600 in 1964. From 1968 to 1972, it was fitted also into the S-class body for a special high-performance 300 SEL 6.3 sports sedan that was replaced by the 450 SEL 6 9 in 1975.

Design

The original displacement of exceeded that of the original, Rolls-Royce V8 engine by about 0.1L. In the 1970s, AMG enlarged M100 engines to 6.8L, for racing and road use. In the US, catalytic converters and unleaded fuel caused loss of power and Rolls-Royce accordingly increased the size of their V8 to . In response, the M100 engine was enlarged to for the 450 SEL 6.9 (1975–1981). Actually a 6.8litre engine, it was designated "6.9" to emphasize that its displacement still exceeded that of the enlarged Rolls-Royce V8 engine.

The M 100 featured a cast iron block, aluminum alloy heads, and aircraft-style sodium-filled valvess operating against hardened valve seats. As in all Mercedes-Benz automobile engines, the crankshaft, connecting rods and pistons were forged instead of cast.

Each hand-built unit was bench-tested for 265 minutes, 40 of which were under full load. When introduced, it utilized the complicated Bosch 8-point mechanical fuel injection. The later 6.9 used a simpler Bosch K-Jetronic Continuous Injection System.

The 6.3 L engine was conservatively rated at , with of torque helping to compensate for the 2.85 to 1 final drive ratio necessary for sustained high-speed cruising.

In non-US trim, the larger engine produced with of torque. The North American version, introduced in 1977, was significantly less powerful at and of torque due to more stringent emissions control requirements.

As a dry sump engine lubrication system reduces overall engine height, the 6.8L M100 used it in order to fit into the much flatter motor house of the W 116. As a side effect it enhanced longevity. Originally developed for racing as a way to prevent oil starvation during aggressive cornering, which in turn would create a serious drop in oil pressure, it allowed sustained high speeds at full engine power. The M100 system circulated a massive twelve litres of oil through the engine and a storage tank mounted inside the right front fender, as opposed to the usual five or six litres found in V8s with a standard oil pan and oil pump configuration. The dry sump system also had the benefit of extending the oil change interval to . This, along with hydraulic valve lifters which required no adjusting and special cylinder head gaskets which eliminated the need for periodic retorquing of the head bolts, made the 6.9 nearly maintenance-free for its first .

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