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Project Paw-Paw: Part 3

In Part 2, we laid the foundation for ensuring the 200,000-mile 7.3L Power Stroke in Project Paw-Paw could withstand the 370-percent increase in horsepower it was about to receive. This time, we’re diving headfirst into the most vital piece of our power-making puzzle: the fuel injectors. We’ve said it before, but that doesn’t make it any less true: if you want to make significant power with a 7.3L you have to run bigger injectors. In our case, we’re installing the second largest off-the-shelf set of injectors you can buy from Unlimited Diesel Performance. The company’s Stage 4 hybrid injectors flow nearly four times what the factory units do and their 200-percent larger nozzles allow them to get precious diesel in-cylinder in a much quicker fashion.

As a hybrid injector, the Stage 4 is designed to work sufficiently with a single high-pressure oil pump (HPOP). This is one of the biggest selling points behind hybrid injectors, as they save you the cost of having to add a second HPOP in order to maintain high pressure oil supply. But as one of the largest hybrids you can buy, not just any HPOP will do. For more on keeping the oil side of these big sticks happy, tune in for an in-depth look at the HPOP we’ve chosen to run in the next installment. For now, join us for our last trip under the valve covers. From here on out, a host of bolt-ons will be employed to get us where we need to be.

Stock Injector Vs. UDP Stage 4 Hybrid

001 Power Stroke Injector Comparison

The fuel injectors are at the tail end of the complex HEUI injection system, but it’s where we’re beginning our upgrades. As a general rule of thumb, the factory AA-code injectors in the ’94.5-’97 7.3L Power Stroke (left) can net you roughly 230rwhp, give or take, on good aftermarket tuning—but that’s it. The single-shot (one event per power stroke), AA-code injectors flow a maximum of 90 to 95cc’s of fuel in stock form. By comparison, the Stage 4 hybrids from Unlimited Diesel Performance (right) move 350cc’s of fuel on the flow bench.

Stage 4 Injectors Price: $2,475 (+$600 refundable core charge)

Justifying Their Cost

002 7.3L Power Stroke Injector Internals

With both fuel and high-pressure oil flowing into, through and then back out of each injector, there is a lot going on inside the units used in a 7.3L. This diagram is important in explaining why, unless you know all the ins and outs of a HEUI injector, it’s wise to fork over the cash for a set of performance injectors from a reputable company instead of building your own. Notice the poppet valve shown above. This is the piece that, when told to open, allows high-pressure oil from the rails to make its way into the injector. The nearby armature plate lifts simultaneously with the poppet valve.

What is a Hybrid Injector?

003 Hybrid Injector Plunger

For more than a decade, hybrid injectors have been the most cost-effective way to make big power with a 7.3L. They’re called hybrid injectors because they make use of a plunger and barrel assembly from larger, B-code injectors (specifically, the BD code injectors that were used in the I530E, a larger I6 Navistar engine with HEUI). However, thanks to the hybrids’ retention of the factory, A-code intensifier piston (the oil side amplifier that forces the plunger down on the fuel side), they require less high-pressure oil to fire the injector than a B-code injector.

Dual HPOP’s—Not A Necessity

004 HEUI High Pressure Oil Pump

Because hybrids use less high-pressure oil per volume of fuel delivered than any other type of HEUI injector on the market, they’re immensely popular. Their biggest draw is that you can get away with using a single high-pressure oil pump to fire any small to moderately-sized hybrid. In the case of our Stage 4’s (again, the second largest off-the-shelf injector you can buy), this is still the case, but a higher volume aftermarket HPOP is required to get the most performance out of them. More on that upgrade next time.

How Displacement is Increased

005 HEUI Injector Diagram

In addition to a larger plunger and barrel (which effectively increases each injector’s bore), Unlimited Diesel Performance adds fuel plates within the injector bodies of its Stage 4 units. On the oil side, the previously-mentioned intensifier piston is machined down to make it shorter, thereby increasing its stroke. These mods, along with several other proprietary ones, are what allow the injectors to flow 350cc’s of fuel vs. 95cc’s stock.

How to Make Use of the 7.3L’s Tiny Power Window

006 Power Stroke Timing Window

In a 7.3L with stock pistons and the OEM injector arrangement, you have approximately 24 degrees to inject fuel into the piston bowl before (and after) top dead center. In order to inject as much fuel as possible in this tiny window, you not only have to increase injector flow but also the injection rate. Just like an engine, boring and stroking inside the injector helps increase displacement, but that’s only half the battle. In order to get that added fuel volume in-cylinder quicker, a nozzle with larger orifices is required.

200-Percent Larger Nozzles

007 Diesel Injector Nozzle

To squeeze as much fuel as possible out of the injector and deliver it in the extremely tight window we have to work with, 200-percent larger nozzles got the call. Larger orifices mean more fuel is delivered per stroke and it’s generally accepted that 200-percent over units are as far as you want to go in street-driven applications (for both reliability and drivability purposes). Stock AA-code injectors come equipped with 7-hole nozzles, each orifice possessing a 0.006-inch opening. By comparison, our 200-percent over nozzles feature 0.012-inch orifices while retaining the same 7-hole pattern. In HEUI injector speak, where maximum flow is listed first and then followed by the nozzle size, this makes our Stage 4 injectors “350/200’s.” As far as widely-accepted nozzle speak is concerned, we went from 7x6’s to 7x12’s.

Housekeeping Measures

008 Power Stroke UVCH Glow Plugs

Any trip under the valve covers warrants inspection of the under valve cover harnesses (UVCH), UVCH gaskets and replacement of the engine’s glow plugs if it’s been a while. 7.3L’s are notorious glow plug eaters and hard-starters in winter. Plus, changing injectors is an opportune time to tackle these relatively inexpensive housekeeping items before they begin to show their age.

Injector Install

009 Power Stroke Diesel Fuel Injector Install

When the 350/200’s went in, we made sure to hit each injector body O-ring with a healthy coat of fresh engine oil and follow Ford’s torque specs to a “T.” The injector retaining (hold-down plate) bolts get torqued to 10 ft-lb and the oil spill spouts (or oil deflectors) see 9 ft-lb. After the injectors were secure, we tidied everything up under the valve covers, reinstalled the valve covers and then fed 14 fresh quarts of 15W-40 down the oil fill tube, installed a new oil filter and completed the job.

Supporting Mods Yet to Come

010 HEUI High Pressure Oil Pump

While this takes care of the injector portion of the HEUI system’s required mods to get close to the 600rwhp mark, there’s still a lot of work to do. To maximize the potential of the 350/200’s, we have to have adequate high-pressure oil on tap (also known as injection control pressure, or ICP) as well as at least 65 psi of fuel pressure feeding the injectors at all times. We’ll address high-pressure oil supply in Part 4 and then significantly upgrade the fuel supply system in Part 5.

Curious what we did in Part 2? We added stiffer valve springs, stronger pushrods and ditched the factory head bolts in favor of head studs. Check it out here.

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