The 855 Big Cam is regarded as one of the last true variable timing mechanical engines on the heavy duty side before electronics came into play with the N-14. The engine was first introduced in 1976 and mass produced up until 1985. After ’85 the engine was engine was still supplied by Cummins up until 2000, with engineering supersessions from 1988-1991. The Big Cam 855 replaced the Small Cam 855 of the 60s and in fact was the first engine designed to meet the new updated mandates of the Clean Air Act of 1976.

Old school truckers and farmers all loved the Cummins 855 Big Cam because of its simple design and reliability all the while producing some major horsepower and torque to get the job done in the field. The over the road truck application is designed to run for 700,000 before a major overhaul.

Notable aspects of the design is the demand flow cooling system which only cools parts of the engine when needed. This saved energy is this used at the crankshaft to deliver more horsepower to the task at hand. This additional horsepower is what made the engine so popular over the previous smaller cam models. A larger camshaft diameter and larger top stop injectors really pushed up the HP on these engines. Top-Stop Injectors are all mechanical but are designed to using a zero lash setting on the base circle on the cam injector lobe. This zero lash setting became known as the Inner Base Circle or “IBC” and increased fuel flow significantly over the small cam models.

Reaching 400+ HP was now attainable and stable without having to do any performance upgrades to the fuel pump. The drawback to the fuel injection system is the lower 2,200 psi it runs at which negatively affects power to the oil pump, water pump, variable timing system and valve spring pressures. In cold climates the engine can be difficult to start. However this can be remediated with block heaters or oil immersion heaters. Overall, you’re not going to have a whole lot of issues with this classic diesel engine.