This Bosch injector was used on both the first- and second-generation 12-valve Cummins engines. The only difference is that the inlet size was slightly altered in the two Cummins models. These hydraulic injectors fire or pop off when they receive the correct quantity and pressure of fuel from the pump. The most common and simplest modification to any injector is to remove the nozzle and either increase the size of the holes, add more holes, or do both (in some cases). The aftermarket has a range of injectors available to suit customer needs. Typically, high-horsepower injectors are modified internally so the nozzle and pin are fed with a second fuel inlet. Modifications may also be made to most of the internal components of the injector.
The VP44 is a distributor-style, solenoid-valve-controlled, radial-piston pump that is electronically regulated. The Bosch VP44 is driven by the engine, and fuel is pressurized by several radial pistons. The internal radial piston pressurizes the fuel, and a high-pressure solenoid valve opens and closes the chamber outlet that distributes specific quantities of fuel to each of the six injectors. The VP44 has a built-in ECU that communicates via a CAN bus system with the main ECU and requires an electric lift pump to feed it diesel from the fuel tank.Hot-rodded VP44 pumps can add up to 100 hp thanks to different programming software on the pump ECU as well as internal mechanical modifications to adjust timing and capacity.
The 24-valve injector is very similar to the injector used on the older 12-valve engines. It looks different because it uses a stepped nozzle holder, but internally, it functions in a similar manner. The injector nozzles are modified using either an electrical-discharge machine (EDM) or an extrude-hone process-and sometimes both. The EDM process uses an electrode and electrolyte solution, whereas the extrude-hone process uses an abrasive fluid to increase the orifice size.
The HEUI was developed by Caterpillar and is used in the 7.3L Power Stroke V-8. This injector is significantly different from the Bosch injectors because it uses an engine-driven oil pump to feed high-pressure oil into the injector to pressurize the fuel. Because oil pressure is used to pressurize the fuel inside the injector, a high-pressure fuel pump is not needed. Fuel is fed to the injector at a relatively low pressure (50-70 psi) and a solenoid controls the high-pressure oil going into the plunging mechanism to ramp injection pressure up to 21,000 psi. To increase the flow of the injector, the aftermarket either extrude-hones or EDMs the injector nozzles, depending on customer requirements. Modifications are also made to the internal pumping mechanism in the injector; larger plungers are used, and the internals are machined differently. When aggressively modified injectors are used, Industrial Injection recommends using dual high-pressure oil pumps so the injector is not starved for oil.
The Duramax and Cummins engines use the same Bosch CP3 pump, so it makes sense that the injectors are also very similar.
Although the external body of the injectors looks different, the internals and functionality of these injectors are very similar. A solenoid valve on the top of the injector controls the fuel coming into the injector from the common rail. Most of the chips and downloaders available modify the duration that this solenoid stays open to add fuel and, thus, horsepower. To increase the fuel being injected, the size and shape of the holes in the nozzles are modified.
Cummins | 
Duramax |