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Inside The Bosch P-Pump - Diesel Tech

Plus: Do-It-Yourself Injection Pump Timing Tips

Photography by Ray T. Bohacz

The '94-'98 Cummins 5.9L engine has been the mainstay of diesel power in Dodge trucks, and has a reputation as an excellent performer with the ability to make some serious horsepower. One of this engine's features that makes it so desirable is the Bosch P7100 inline mechanical injection pump. This injection pump, often called a "P-pump," is the Holley Four-barrel carburetor of the diesel world. The P7100 pump has been around forever, is practically bulletproof, and is very tunable for high performance.

Cummins Injection Pump Overview
The Dodge Cummins engine has used four different Bosch-sourced injection pumps over the last 20 years. The early engines ('89-'93) employed a Bosch vE mechanical rotary pump, while the later applications ('94-'98) used the Bosch P7100 mechanical inline pump. When the engine moved to electronic controls ('981/2-'02), the injection system switched to a vP44 rotary-style pump before being converted to commonrail injection in 2003 and receiving the Bosch CP3 injection pump.

How Mechanical Injection Pumps Work
A mechanical diesel injection pump is designed to pressurize, meter, and distribute the diesel fuel that's injected into the engine to create the optimum power output.

A mechanical injection system needs to be capable of pressurizing the fuel sufficiently to open the injectors (nozzles) and deliver the fuel to the cylinders in an atomized form. Mechanical injector opening pressures are usually between 3,000 and 5,000 psi.

The amount of fuel metered by the injection pump is the precise control of fuel quantity. Diesel engines operate with an excess of air in the cylinders so there is always more air present than is necessary to completely combust the fuel.

Distribution of the fuel is the last job of the mechanical injection pump. The fuel system needs to be phased (sequenced) to deliver the fuel to each cylinder at the correct time and in the correct firing order. Proper fuel system phasing is required to balance the engine output (all cylinders producing the same amount of power). Thus, if the injection event timing is set at 3 degrees before top dead center (BTDC), all cylinders must be fueled at the same respective crank angle.

A complete cycle of a diesel engine is 720 degrees of crankshaft rotation, and the fueling pulse has only a maximum of 40 crank angle degrees to be accomplished. For example, a four-stroke diesel engine running at 2,000 rpm produces approximately 17 power strokes in each of its cylinders every second, so the pumping, metering, and injection cycle are all measured in milliseconds, which is 1/1,000 second.

How The P-Pump Works
The P7100 injection pump is flange-mounted to the engine and is driven directly from the engine's crankshaft through a gear train. The internal pump components are housed in a case constructed of cast aluminum, cast iron, or forged steel. The engine crankshaft drives the pump via a timed reduction gearset. The geardriven pump drive plate is connected to the injection pump camshaft (not the engine cam, but internal to the pump) with eccentrics designed to actuate the pump elements. Rotating the pump drive plate rotates the pump camshaft. The camshaft is supported by main bearings and rotates within the injection pump cambox. The cambox is the lower portion that houses the camshaft, tappets, and integral oil sump. In many ways, the P7100 pump has many of the components of an inline internal combustion engine.

The camshaft inside the P7100 is designed to have one cam profile dedicated to each engine cylinder. Riding each cam profile is a tappet assembly driving a pump element consisting of a plunger and a barrel. The barrel is stationary and drilled with two ports in its upper portion, which are exposed to the fuel-charging gallery.

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