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.
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.
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.
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.
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.
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.
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.
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.