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The M121 was a straight four-cylinder engine made by Mercedes-Benz during the 1950s and 1960s and used in their mid-size sedans, such as the 190, and in the 190 SL convertible. The M121 engine was also used for trucks such as the Unimog and L319 models. The M121 engine made its debut in 1955. It had a displacement of 1.9 liters and was the first generation of four-cylinder engines made by Mercedes-Benz. The original M121 engine was rated for 75 horsepower at 5500 RPM. This engine replaced the M136 1.8 Liter Mercedes engine introduced in 1953. The main improvement was the implementation of overhead camshaft design which replaced the M136's lateral camshaft layout.

History

The M121 engine was developed just after the end of World War II. It was built in the Sindelfingen Works factory, which assembled the Mercedes Ponton Model series to which the engine belonged to. The M121 engine made its debut in 1955. It was the first generation of 4 cylinder engines from Mercedes Benz. The M121 replaced the less efficient M136 1.8 liter engine, a pre World War II engine that was introduced 2 years before in 1953. The M121 benefited from innovations and technology from Mercedes' larger engines such as the M186 3.0 liter and M130 2.2 liter engines. One of these innovations was a new single over head camshaft design which allowed for more power and efficiency. It was developed by a team led by Hans Scherenberg. Within this team, Karl-Heinz Goschel was one of the key engineers who developed the engine design.

The M121 series was in production until 1967, when the M115 series engine took its place.

Profile

The M121 engine featured a single chain driven overhead camshaft layout with staggered valves pivoted via long and short rocker arms. The cylinder block was made from cast iron. The cylinder bore diameter for the M121 was 85 milimeters. The three ring crankshaft was forged. The stroke of the engine was 83.6 mm. This gave the M121 a displacement of 1897 cc, or 1.9 liters when rounded. The engine had a compression of 8:5:1. Through this configuration, the M121 engine had a initial rating of 110 horsepower at 5500 RPM and 114 lb-ft of torque at 4000 RPM. After testing of the final vehicle, the horsepower rating was lowered to 105. The crankshaft was carried by three main bearings. In the 190 SL, the engine had two parallel carburetors.

This picture shows the M121 engine in a 190 SL. (Photo author: Aida200400)

View of the Engine Bay

The M121 engine was distinctive during its time due to the fact that it stood upright in the engine compartment, and not at an angle like the 300SL's. Looking under the hood, one will notice a series of cast aluminum plenums. A circular air cleaner is mounted on the bulk head. The aluminum plenums channel air fro the air cleaner to the carburetors. Under the carburetors lie a 4 to 1 exhaust manifold.

This picture shows the M121 engine 4-Cyl. 1.9 litres / 80HP of a 1964 W110 190c. (Photo author: Jo664)

M121 BI

The newer Mercedes 190 Model (W210 Chassis) produced in the 1960's featured an updated version of the M121 engine. This version had an increase of 5 horsepower over the original version horsepower while fuel consumption stayed the same at 11.5 liters of premium fuel per 100 kilometers. The top speed also increased to 150 kilometers per hour, an improvement from the original's 139. These performance increases were possible despite the new model's bodywork which was significantly heavier due to new safety implementations to the vehicle.

M121 BII

An updated version of the M121 engine, called the M121 BII, was produced for the Mercedes 190SL. The four-cylinder unit has a single overhead camshaft and is regarded as the progenitor of an entire family of engines. The engine developed 77 kW at 5700 RPM. For the 190SL, acceleration to 100 km/h took 14.5 seconds, and the top speed was 170 km/h. This made the M121 BII one of the fastest engines on the road in its day. The fuel economy of the M121 BII was 8.6 litres per 100 km.

M121 BIX

The newest version of the M121 engine was the M121BIX. In this version, displacement increased from 1897 cc to 1988 cc and was thus designated as a 1.9 liter engine. This increase in displacement was possible due to a 2 mm increase in bore from 85 mm to 87 mm. The compression ratio was also increased to a value of 9:1. This engine also used a new Solex 38PDSJ carburetor. Through these various improvements, power was increased to 95 HP at 5200 RPM and 154 N-m of torque was possible at a reasonable 3600 RPM. The M121 BIX engine was produced from 1965-1968 for the Mercedes Benz W110 model series.

Applications

The M121 engine was first employed in the Mercedes Benz "Ponton" models such as the 180. The 190 models featured an updated version of the M121 which featured greater performance through variations in compression ratios and improved carburetor systems. The M121 engine has also been used in trucks such as the Mercedes-Benz L319 which was produced in 1967. 1967 1956. Some Unimog off-road models also implemented the M121.