The Mercedes-Benz M 100 engine was a or single overhead cam V8 produced by Mercedes-Benz between 1963 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 1963. 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.
The original displacement of exceeded that of the original, Rolls-Royce V8 engine by about 0.1 L. In the 1970s, AMG enlarged M 100 engines to 6.8 L, 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 M 100 engine was enlarged to for the 450 SEL 6.9 (1975-1981). Actually a 6.8 litre 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 valves 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 a complicated 8-piston mechanical fuel injection system designed and built in-house by Daimler-Benz. 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 6.8 L M 100 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 M 100 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 .