Originally rated at 56 kW (76 PS; 75 hp) at 5500 rpm, it replaced the 1.8 liter M136 introduced in 1935, offering improved performance over the M136's side camshaft design.

Applications included mid-size sedans, such as the 190, the 190SL roadster, and trucks such as the Unimog and L319 models.

It was built in the Sindelfingen Works factory, which assembled the Mercedes Ponton Model series to which the engine belonged to.

One of these innovations was a new single over head camshaft design which allowed for more power and efficiency.

The M121 engine featured a single chain driven overhead camshaft layout with staggered valves operated by long and short rocker arms.

After testing of the final vehicle, the power rating was lowered to 105 hp (78 kW; 106 PS).

These performance increases were possible despite the new model's bodywork which was significantly heavier due to new safety implementations to the vehicle.

[3] The four-cylinder unit has a single overhead camshaft and is regarded as the progenitor of an entire family of engines.

The M121 BIX engine was produced from 1965 to 1968 for the Mercedes Benz W110 model series.

The 190 models featured an updated version of the M121 which offered greater performance through variations in compression ratios and improved carburetor systems.