PSD Tech

7.3L Injector Basics

Published in the April 2009 Issue April 2009 Injectors, PowerStroke, Column

How about a little Injector 101-a look at how a 7.3L Power Stroke injection system actually works. It's no question that aftermarket 7.3L injectors can be down right expensive, but when you take into consideration that the injector is the workhorse in this system, you'll soon understand why. You may be spending $3,000 on high performance injectors, but you won't be spending $4,000 on dual Cp3s like the Duramax guys have to or dumping money into modding an injection pump like the 12V Cummins guys do.

The 7.3L Power Stroke uses a HEUI-style injector, which stands for Hydraulically Actuated Electronic Controlled Unit Injection. Diesel fuel is supplied to the injectors through fuel rails inside the cylinder heads. A high-pressure oil pump, which is gear driven by the engine, is also used to supply high pressure oil through the heads to the injectors. When the computer sees that a cylinder needs to fire, a signal is sent to the Injector Driver Module (IDM). The IDM will send a pulse-width signal to the solenoid on the injector. When the solenoid is actuated, a poppet valve inside the injector opens, allowing the high pressure oil to flow inside the injector where it presses against an intensifier piston. This forces the intensifier piston down, which pressurizes the diesel fuel found inside the injector. When the fuel pressure inside the injector reaches a certain pressure the injector pintle will lift off its seat and allow fuel to be injected into the cylinder through the injector nozzle.

In a stock AD code Super Duty Power Stroke injector, the oil cavity above the intensifier piston is seven times the size of the fuel cavity below the piston, meaning the diesel fuel is pressurized seven times that of the high pressure oil, giving stock injectors a 7 to 1 injection ratio. So if the high pressure oil is running at 3000 psi, the fuel is being injected at seven times that or 21,000 psi.

There are many ways to modify a Power Stroke injector to increase performance and improve efficiency. Most common are modifications to the nozzles so more fuel can enter the cylinder quicker. Modifications to the intensifier pistons, plungers and barrels are also made to increase the amount of fuel that can be pressurized inside the injector. Two significant differences in Power Stroke injectors include the use of single shot or split shot firing pattern-single shot meaning there is one large burst of fuel into the cylinder while split shot means there is a small initial burst of fuel followed by a larger burst of fuel into the cylinder. Split shots were designed basically to quiet the engine. The single shot design is generally more efficient and better for making horsepower.

OEM Injectors Sizes

1994-97 7.3L use 98cc AA code single shots
1997 California and early 1999 models use 120cc AB code split shots
Late 1999-2003 use 140cc AD code injectors

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