Part Eight: Injecting some Horsepower
The OBS Resto-mod project just keeps moving right along now into our eighth installment of the buildup of this 1996 F350 Ford truck. We’ve made a host of upgrades and have already really turned this big white school bus into quite the multi-use daily driver. New reverse shackle front suspension, heavy-duty steering parts, some interior upgrades, minor power bolt-ons with an intake, exhaust, and a chip. At this point, its time to silence some naysayers about the potential of that old 7.3L Power Stroke to make a respectable horsepower number. We’re going to dive into the fuel injection system and pump up the potential volume of fuel injected with some modified Hybrid injectors from Full Force Diesel of Murfreesboro, TN.

Full Force Diesel of Murfreesboro, TN has been making a living specializing in the 7.3L Power Stroke for well over a decade now. Throughout that time, they’ve been perfecting their signature fuel injectors to produce some of the cleanest running and most efficient modified injectors on the market today. For Project Obsessed, a dedicated daily driver and tow rig we’ll be installing their 205cc Hybrid’s.

This truck doesn’t show any real signs of bad injectors, as it idles fine, no white or gray smoke when it’s cold, but at 230,000 miles they’re tired, and who can resist the potential for more power? Full Force Diesel has been specializing in the 7.3L platform for over a decade and has fine-tuned its options for upgraded performance injectors for the HEUI (Hydraulic Electronic Unit Injector) system. The factory injectors in the 94-97 7.3L was a single shot 90cc AA-code, which means it has one injection event per combustion cycle and can move a total of 90cc of fuel in that time. In California, model trucks (like ours), were outfitted with a larger 120cc AB-code ‘split-shot’ injector. This means it has a small pilot and followed by the main injection per cycle, which made the engine run quieter and cleaner. In an OBS truck, you are limited to around a 70hp max increase with AA’s and maybe 140hp max with the splits.

While an injector swap may sound like a daunting and overwhelming task, if you’ve ever done a glow plug job on your end, injectors don’t add to many extra steps and can still be accomplished in a few hours’ time with the right tools and knowledge.

Full Force Diesel modifies all their injectors to the single-shot design, which offers better performance and injector efficiency. First, they will increase the CC’s of fuel delivery and change the nozzle sizes to achieve more power and performance to meet your specific power and drivability goals. Their modifications can increase the flow rate up to 450 cc’s, but that’s way more than we’d ever need in a daily driver and towing application. By increasing the CC’s of flow, the injector will inject more fuel into the cylinder, meaning a bigger explosion in the combustion chamber for more power. On the nozzle side of things, the larger the nozzle allows all that fuel to empty faster, making for more precise timing for that explosion.

With the valve covers removed from the engine, you’ll get your first real look at the HEUI injectors that power the 7.3L engine. Pulling the injectors requires just disconnecting the electrical connector and the removal of the lower hold down bolt. The injector hold down can then be slid upwards to clear the top hold down bolt and a small pry bar to pop it up and out of the injector cup bore.

For a tow rig wanting more power than stock, you’ll need more CC’s with a mild nozzle upgrade. This will equate to lower EGT’s and less strain on your drivetrain. For more of a hot street truck, you can get away with even more CC’s since EGT’s are less of a concern on short bursts. A competition truck will not have CC limits since you’re trying to produce the most power and performance available without the concern of keeping it running clean on the street.

With the injectors removed from their bores, you’ll see and hear engine oil from the oil rails draining down into the cylinders, which will need to be evacuated and cleared upon reassembly. You’ll also notice the glow plugs which will need to be removed at this point as well.

After speaking with the crew at Full Force and our tuner, Andrew at A&A Design, they both steered us towards the Stage 3 205/30 Hybrids. This injector will deliver the same, if not better power than a Stage 2 180/100 injector, but do it more efficiently. The 205 is a hybrid injector design that uses considerably less high-pressure oil than a big A-code injector. This means the engine can maintain higher injector control pressures on a stock high-pressure oil pump. By utilizing the smaller nozzle with a higher flow rate, with the right tuning, you gain more fuel volume at the nozzle for better atomization. That improved atomization means more power with cooler EGT’s and less smoke. Both ideal outcomes when towing. The 205/30 injector is going to max out around 425-450hp, which is going to be more than enough for this basically stock engine. Besides, at the beginning of this build, we hoped to make the kind of power a brand new truck does and 450hp put us right there in the ballpark.

The entire driver side bank of injectors is out and on the bench. While the truck was still running fine, at 230,000 miles they are definitely tired and should be showing signs of wear inside which can affect performance and efficiency. It is also common at higher mileage that the O-rings can leak causing poor running conditions and hard starting as the high pressure oil system can’t build pressure like it should.


Alright, so now to the good part. Anytime we talk about turbos or injectors, the first question asked is how much power will they pick-up? Most 94-97 7.3L trucks were equipped with the single-shot 90cc AA-code injectors, but since this truck was a California model with 120cc split shot AB-codes it was no surprise to see it make a bit more power than the average OBS truck does. With nothing more than a cold air intake, 4” exhaust, and the custom tuning, we made 307hp last spring. After trading out that factory injector for the new Hybrid 205cc 30% injectors from Full Force, we were lucky enough to get it back on the same dyno a month later. We made no other performance changes to the truck, except retuning our chip for the new injectors. Still running the stock turbo, stock mechanical fuel pump, and stock high-pressure oil pump, our numbers jumped clear to 399hp.

Our new Full Force 205cc Hybrid injectors come ready to install with new O-rings and the copper washer to seal it in the injector cup. Since we’re upping the volume of fuel the injector can move from that stock 90cc to 205cc, we also opted to upgrade to a 30% over nozzle, which can help that fuel exit quicker.

That’s 92-horsepower to the wheels with just an injector change. We should note that in larger tunes the truck is rather smokey and the EGT’s will get out of hand in a hurry if you aren’t paying attention. The stock turbocharger just can’t supply enough air to adequately burn the fuel from this injector size and our fuel pressure drops significantly as the factory mechanical pump can’t keep up either. However, in our lower power levels, the truck is extremely drivable and smoke-free. This combination towed the fifth wheel quite well and has had no issues running around daily. One other note worth mentioning, our fuel mileage has increased from an average of 14.2 to around 15.5 consistently in mixed city/highway driving.

The passenger side will take a little more time due to some of the space constraints in the engine bay, but you follow the same process. This is also a great time to check your valve cover gasket/harness as they are prone to fail at the connectors, so the injectors and glow plugs may not receive the power and signals they should.

Overall, we are super happy with our decision to upgrade the injectors, and our concerns about doing them before the turbocharger and fuel system were upgraded have been washed away. Yes, we still plan to upgrade those parts to get the full potential out of these 205/30’s but for now, the truck drives and tows great if we keep in the lower power levels of the Hydra chip. In our following installments of the build, we’ll start to address that outdated mechanical fuel pump with a complete electric pump conversion, regulated fuel return, new filtration, and larger in-tank pick-up tube. The turbocharger system will be addressed as well with a larger 63mm drop-in unit, pedestal, high flow outlet and some bellowed up-pipes. We have already made mention of some new exterior upgrades that are planned along with a new set of tires, so stay tuned.

When the new injectors are reinstalled into their cups, the injector hold down bolt will need to be reinstalled and it is important to torque these to the proper spec. If that bolt were to come loose, the combustion pressure could push the injector back up out of the cylinder head and lead to some major issues.
With the injectors torqued down and the glow plugs removed, we used a small vacuum pump to suck as much oil up and out of the cylinders as possible. Then, the valve covers can be slid over and held in place with just a couple bolts so the engine can be barred over by hand off the damper bolt. Cycling the engine over a few times will push any leftover out through the glow plug holes.
With the injector swap complete, we of course had to have our custom Hydra chip retuned by our tuner, A&A Design. The bigger single shot injectors would need the ICP, fueling, and timing tables adjusted to take full advantages of the increased fuel volume entering the cylinders. After a few hundred miles of driving, we were happy to an average of 15.6mpg daily driving, that is up from an average of 14.2 with the stock injectors.
With the oil evacuated from the cylinders the glow plugs can be installed, valve cover harnesses connected to the plugs and injectors, and the valve covers torqued down. At that point, it is a good idea to fill your high pressure oil reservoir through the small port directly on top of it. This will help refill the oil rails quicker while cranking, hopefully leading to an easier first start-up.
If you recall back to the March issue of Diesel World, we had installed a 4” Banks Monster exhaust and the A&A Design custom tuning and dyno tested the truck on the mobile chassis dyno from the Northwest Dyno Circuit and made 307hp.
On that dyno run in the spring, with nothing more done to the truck but an S&B Filters cold air intake, 4” exhaust and custom tuning we made 307hp. That is stock injectors, stock turbo, stock high pressure oil pump, and stock mechanical fuel pump. Numbers we were quite surprised and happy with.
Fast forward a few months to our next follow-up dyno run with the addition of the Full Force Diesel 205cc 30% injectors and tuning to match them. Nothing else was changed on the truck, performance wise at least. Still stock turbo, fuel pump, and oil pump, and on the same dyno we jumped to 399hp. And increase of 92-horsepower!

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