7.3L Hybrid Injectors

Building Power With Hand-Built Injectors

If you are serious about diesel performance, replacing the stock fuel injectors with high-performance injectors should be a consideration. We’re looking to push 400 rear wheel horsepower with Project 7.3, so an injector upgrade, along with our other modifications, is a must. Fortunately, Unlimited Diesel Performance, in Bremen, Ohio, specializes in high-performance fuel injection, and much of the company’s expertise and reputation was built around delivering 7.3 Power Stroke fuel injection applications.


Hybrid Injectors

In Part 4 of Project 7.3, Unlimited Diesel Performance removed all eight stock injectors so that the headwork could be completed. The next stop was a trip to the parts washer for initial cleaning and disassembly.

That fuel injection expertise is really what sold us on having Unlimited Diesel Performance handle the mechanical work on our 2000 Super Duty. On our first trip to the shop, Nate Bailor gave us a tour of Unlimited Diesel’s injector room, which looks more like a medical facility than a diesel shop. Stainless steel work surfaces and sophisticated equipment, such as the large ultrasonic parts cleaner, allow Unlimited Diesel to custom build injectors to better-than-new condition.


Stage 3 Hybrids


Because we were after stout, streetable performance, Bailor recommended we go with the company’s Stage 3 250/100 Hybrid injectors for increased fuel flow. The factory 7.3 injectors are a split-shot design and have a 6mm plunger and barrel. Unlimited’s Stage 3 Hybrid design replaces that with a 7.1mm plunger and barrel used in the DT466E / I530E International engines. The Stage 3 design also converts the injector to a single shot to eliminate the small, high-pressure oil pressure drop experienced between the pre-injection and main shot, with the split-shot injector.


climate controlled and superclean injector room

The climate-controlled and superclean injector room at Unlimited Diesel Performance looks more like an operating room than a diesel shop. Stainless steel counters and well-organized bins and racks full of injector parts make the rebuild process more precise and efficient.


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Here’s something you won’t find in many shops: Unlimited Diesel uses this Pro Ultrasonic cleaner to sanitize injector parts. An alkaline base soap with rust inhibitor and 175-degree water are used, along with ultrasonic waves, to clean parts without damaging the metal or affecting tolerances.

To accommodate the increased fuel flow, the Stage 3 Hybrids also increase the tip hole size from .006 inch to .0085 inch while retaining the factory, seven-hole spray pattern for best efficiency. The final selling point on the Unlimited Diesel injectors is the fact that they come with a limited lifetime warranty. As long as you return the warranty card with the injector cores and follow a regular oil change and maintenance schedule, Unlimited Diesel has you covered.


Dumping Fuel


Injector flow is determined by a mathematical equation that multiplies the surface area of the plunger diameter and stroke of the intensifier piston to come up with cubic millimeters , or [[mm3]]], and then each injector is fired 1,000 shots to measure cubic centimeters, or cc.


After cleaning nozzle nut

After cleaning, the nozzle nut (top) looks like new. The rest of the parts to convert our stock injectors to Stage 3 Hybrids are organized on the assembly table for easy access.

Nate Bailor removes the nozzle nut assembly

With the injector secure in the vise, Nate Bailor removes the nozzle nut assembly from the business end of the injector. Unlimited Diesel uses a special tool that safely latches onto the nut without damaging it.

Our stock 7.3 injectors had a maximum flow rate of 140cc. The Stage 3 Unlimited Diesel Performance injectors flow 250cc, for an increase in displacement of 78 percent while using less high-pressure oil than the company’s stock AD injectors. Unlimited Diesel currently offers injectors ranging from mild to wild.


internal parts stack up inside the body of the injector

The nozzle nut is removed, so now you can see how the various internal parts stack up inside the body of the injector. At the top is the stock nozzle. It will be replaced by a 100 percent-over-stock nozzle (.0085 inch) in the Stage 3 Hybrid assembly.

Power Stroke

This exploded view of the internals show that even though this Power Stroke only had 80,000 miles, there is some buildup and wear.

Unlimited Diesel currently offers every injector, from stock to 455cc, and everything from stock OE flow nozzles to 30, 80, 100, 200, 400, 600 and 800 percent increases in flow. Everything from 400 percent up is considered to be for competition use. Some truck owners are never satisfied, so Unlimited Diesel Performance is currently working on an injector that will dump up to 750cc of fuel for extreme pulling applications.


Building Better Injectors

Since the stock injectors were already out of the Super Duty from last month’s headwork, Nate agreed to let [Diesel World] follow along and see some of the magic that happens in the injector operating room. We’ve detailed much of the procedure in photos and captions that illustrate this article, but they can only tell part of the story. Rebuilding or tricking out fuel injectors is a job best left to a professional. A good mechanic can do it, but one mistake can mean very expensive repair bills and more downtime.


Clean Is Mean


There are also some specialized tools involved. In addition to a parts washer with a 3 micron filter, Unlimited Diesel takes things one step further and treats the components to a spa treatment in its Pro Ultrasonic cleaning tank. The Ultrasonic uses a combination of 175 degree (F) water and an alkaline-based detergent with rust inhibitor to sanitize the parts with the magic of ultrasonic waves or pulses. The parts that come out of the tank at the end of the process literally look better than new.


plunger diameters

Here you can see the difference in the plunger diameters—stock 6mm is on the left, 7.1mm used in the Stage 3 Hybrids is center, and a huge competition unit developed by Jason Hoffman, of Mingo, Ohio, is on the far right.

Nate checked the clearance between the adapter plate and the armature

With the solenoid spacer removed, Nate checked the clearance between the adapter plate and the armature. If it is less than .002 inch or 0.05mm, it is worn and won’t function properly

Disassembly and rebuild of the injectors also require a few tools that you won’t find in many mechanics’ toolboxes. To safely remove the nozzle nut without damage, for example, Unlimited Diesel uses a special, socket-like fixture that grips the nut and applies pressure evenly without the risk of damaging it. There’s also a special alignment tool that keeps everything true and straight when torqueing down the adapter plate to the body of the injector.


Trouble Shooting


Inspection of the individual components during tear-down and after cleaning is a critical step. Dirty oil, for example, can score critical surfaces inside the oil side of the injector. The poppet valve is particularly susceptible to wear from dirty oil. Fuel can also leave deposits that hurt performance and can lead to wear. One of the most critical tolerances inside the injector is between the adapter plate and the armature. It shouldn’t be less than .002 inch, or 0.05mm.


Diesel World

Dirty oil will cause components inside the injectors to wear. You can see scoring on the surface of this poppet valve, which will need to be repaired.

internal components

All internal components are carefully inspected during disassembly and again when reassembled. This is the inlet side of the piston and valve body.

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Reassembly begins at the solenoid end of the injector, working toward the nozzle end.

Unlimited Diesel also bench tests the solenoid by running a resistance test with a multimeter before assembly. Resistance should be around 3.2 to 3.4 ohms. Once assembled, the company also bench tests the psi required to pop the injector open to make sure it is functioning properly before installing in the truck. For this application, it should be 2,750 psi.


The Build


With all the cleaned and replacement parts at hand on the bench, the assembly starts with the body clamped in the bench vise with the solenoid side down. We won’t go into a detailed description here, because there are a ton of steps to the process. Essentially, the various components inside the injector body stack from the solenoid end to the tip. Many are aligned with dowel pins, while others are fastened with machine screws.


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A special alignment tool is screwed onto the adapter plate to ensure that everything is square and true before tightening. Once in place, the four bolts can safely be torqued with an inch/pound torque wrench.

The 7.1mm plunger-and-barrel assembly and higher-flow nozzle are the keys to the Hybrid injector build. Unlimited Diesel uses other “tricks” to make its custom injectors unique. Understandably, the company didn’t share all its trade secrets. One item Nate insists on is the highest quality O-rings he can find. He prefers to only use Alliant Power External O-rings.


Diesel World

Bailor then torques the bolts to 85 inch/pounds. Once tight, the alignment tool is removed so the armature can be installed.

As you might imagine, proper fastener torque is critical during reassembly. Many of the torque settings inside the injector are measured in inch/pounds. He also uses a torque screwdriver set to just 13 inch/pounds for the armature screw. The big nozzle nut that holds everything together is torqued to 75 pound/feet with that expensive fixture we talked about earlier.


Go With the Flow


The final test of the new injectors is to check the proper and matched flow on the flow bench. Unlimited Diesel Performance built its own flow bench to closely replicate real-world conditions in the truck. It uses actual fuel and oil, rather than the calibrating fluid some benches use.


Diesel World

Proper torque is also critical for the armature screw. A loose armature screw is one of the most common failures seen in the 7.3 injector. A torque screwdriver is set to 13 inch/pounds to snug down the armature screw.

Utilizing a factory Ford IDM (injector driver module), Unlimited Diesel made the voltage input adjustable so that different model years, as well as competition injectors, can be tested. The flow bench can run at 100, 110, 120 and 140 volts.


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After installing the shim, sleeve, spring and poppet valve, the piston and valve body are dropped in place. Here, a special collar is used to protect the body O-ring from the threads during assembly.

Diesel World

The intensifier piston O-ring is then installed inside the bore of the valve body. A pick is used to carefully seat it in place.

Our Stage 3 Hybrid injectors flowed perfectly at both idle and high rpm. Max flow was exactly 250cc, with all collection beakers filling to identical levels.




Installing the Stage 3 Hybrid injectors is a simple matter of popping off the valve covers, lining up the injector in the head and pushing it down into the bore. The new injectors with fresh O-rings were a nice, snug fit in the new injector cups we installed in the last [[DW]] issue.


Diesel World

With the O-ring properly seated, the intensifier piston is dropped into the bore of the valve body assembly and filled with fuel additive.

The final step is to install the injector hold-down bolt and oil deflector. The torque on these two bolts is 140 inch/pounds. It is important to remember to connect the injector harness wires to the connectors on the solenoids before reinstalling the valve covers.


Two dowel pins

Two dowel pins are used for alignment of the plunger-and-barrel assembly. The spring end of the plunger goes down inside the intensifier piston, and the dowel pins align with the holes in the top barrel end.

We can’t wait to fire up Project 7.3 and see how the Unlimited Diesel Performance Hybrid injectors perform. Unfortunately, that will have to wait until we make the rest of the under-hood performance modifications and slide the engine back into the Super Duty.


stop plate and check ball housing

The deposits on the stop plate and check ball housing will cause leakage (right) if not cleaned. Unlimited Diesel’s ultrasonic cleaner and a little hand polishing make them look better than new.

check valve assembly with the barrel

Dowel pins also align the stop plate and check valve assembly with the barrel. There’s a ball-bearing check valve between these two pieces, so they must be held together until seated on the stack.

check valve assembly

This spacer sleeve is then installed on the check valve assembly. The unit is beginning to look like a fuel injector at this point.


The valve opening pressure (VOP) spring is dropped down into the bore in the spacer sleeve.

A lift pin is installed inside the valve-opening pressure spring

A lift pin is installed inside the valve-opening pressure spring to prevent coil bind on the spring. A VOP shim will ride on top of the pin and valve to put push on the needle in the injector tip.

needle inside the tip

The short dowel pins align the spacer sleeve with the nozzle assembly tip. That disc in the center is the valve opening pressure shim that rides against the base of the needle inside the tip.

top of the injector stack

With the needle inside, the nozzle assembly tip is aligned with the pins at the very top of the injector stack.

The trick nozzle nut tool

The trick nozzle nut tool is installed on the nut, and the assembly is torqued to 75 pound/feet while still in the vise.

check the solenoid before installing the new injector

It’s also a good idea to check the solenoid before installing the new injector in the truck. Here, Nate checks resistance in the solenoid; it should be between 3.2 and 3.4 ohms.

tests injector performance on a flow bench

Unlimited Diesel Performance tests injector performance on a flow bench before they go out the door. Our Stage 3 Hybrids flowed great, with matching ccs of fuel at both idle and 3,000 rpm.

stock injector on the left and our new Stage 3 injector on the right

The stock injector on the left and our new Stage 3 injector on the right. The 250/100 hybrid should deliver up to 300 additional horsepower at the rear wheels, with the proper supporting modifications. With all the modifications Unlimited Diesel is making, total rear-wheel power should be 500 or better.

The final step

The final step is to install the new Unlimited Diesel Stage 3 injectors into the head. They slipped perfectly into the new injector sleeves (cups) we install last month. The hold-down bolt and oil deflector should be torqued to 140 inch/pounds.

The waiting is the hardest part.












Bob Boyd Ford




Smith Bros. Pushrods




Unlimited Diesel Performance




Whitaker Tools




By Todd Kaho

Photography: Todd Kaho


Posted in Tech
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  1. Juan Gutierrez says:

    Thanks for the info. would be nice to see if we can buy the sockets to open the injectors.

    juan gutierrez