In recent years, it's become increasingly harder to conceal the fact that diesels can be made into formidable sleepers. As more and more truck owners uncorked the hidden potential within their engines, the stigma that diesels were slow and only useful for work slowly disappeared. But now that the secret is out, how do you keep an oil burner in stealth mode? First, you pick an ancient platform that's both lightweight and unassuming. Second, you make your power with one of the most underrated diesel engines in existence. And finally, you make damn sure that, no matter which tires you choose, you never get rid of the factory wheels. Meet our latest exploit: Project Paw-Paw. It's a 1997 model year F-350 crew cab 4x4 with the 7.3L Power Stroke under the hood.
The 216,000-mile, old body style Ford once spent its days working a farm in central Ohio. Now, in retirement, we're going to more than triple the HEUI-injected V8's horsepower without touching the original short-block—and with surprisingly few top-end mods. The parts we do add won't be cheap or free, but they will be affordable and provide a straight-forward path toward 600rwhp (along with 1,100 lb-ft of torque). To wrap up the build, we'll visit the dyno and collect low-13-second time slips to confirm our work, and then we'll saddle the old Ford with a trailer and make it earn its keep.
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Move over, grandpa. It's our turn to drive…
Sticker Shock
Prices vary regionally, but for a clean, near-rust-free OBS Ford with a 7.3L Power Stroke under the hood, you're going to spend a five-digit figure. It may not be $25K but it will be every bit of 10 to 15-large for a truck in the 150,000 to 200,000-mile range. As old age, salt and accidents inevitably remove them from North America's roadways, the '94.5-'97 Fords aren't getting any cheaper with the passage of time.
Why a 7.3L?
Bullet point numero uno: they're cheap. Unlike a Duramax, the demand-driven Cummins mills or even late-model Power Strokes like the 6.7L and especially the 6.4L, these things are dirt cheap in comparison. Thanks to being plentiful (nearly 2 million of them were produced), you can find 7.3L's in the junk yard, on eBay and even under the hood of Uncle Rob's old plow truck (along with many other workhorses that do a lot of sitting). Usable core long blocks usually go for anywhere between $200 and $700, with complete, running take-outs bringing $1,200 to $2,000.
The Power of Diesel
Thanks to its solid construction at the OEM level, a forged-rod 7.3L Power Stroke ('94.5-'97 and '99-'00 engines) can be pushed to 600rwhp and still be reliable, so long as you opt for good engine tuning and drive sensibly. This series will highlight how to do that to a 'T.' Although our engine will undergo a lot of changes during the course of this build, it will still use a single turbocharger, a single high-pressure oil pump and the HEUI injection system to make its power.
Summary: forged-steel rod bottom ends can handle considerable power, but if you do grenade a 7.3L they're relatively inexpensive to rebuild or even outright replace with a take-out engine.
The Often-Misunderstood HEUI Injection System
Due to the fuel side of the injection system requiring hydraulic actuation via engine oil, the HEUI system is often misunderstood by many, but it can be dumbed down and easily understood in the following way. Instead of utilizing a traditional high-pressure fuel pump (or fuel injection pump), a high-pressure oil supply pump (HPOP) is employed, and thanks to the injection pressure regulator (IPR) the engine oil in the high-pressure circuit is pressurized between 500 and 3,000 psi. Instead of the fuel rails you'll find in a traditional common-rail system, there are oil rails integrated within the cylinder heads. High-pressure oil routes through these rails to the oil side (top side) of the injectors where—after entering the poppet valve, acting on the intensifier piston and forcing the plunger downward—it actuates the fuel side, culminating in fuel being sprayed in-cylinder.
Hydraulically Activated, But Electronically Controlled
Despite the various limitations that exist within the HEUI system, the 7.3L can still be made to make respectable power and run cleanly. This is due in large part to the engine being completely electronically controlled, which means the 7.3L has a brain (i.e. the powertrain control module, or PCM) that can be infiltrated and recalibrated. The module on the left is the injector driver module (IDM). It takes its cues from the PCM and (when commanded) is responsible for sending the precise, 110-volt pulse to the injector solenoids in order to fire them. In this project all of our power gains will be obtained through the PCM. The IDM will be left completely alone.
First Step in Breaching the HP Wall
The biggest hindrance in adding horsepower to a 7.3L, especially the '94.5-'97 versions, is the factory fuel injectors. On the flow bench, the stock units in our '97 F-350 flow a maximum of 95cc's of fuel and even with aftermarket tuning can only support 230rwhp. Needless to say, we'll be upgrading injectors, along with adding application-specific PCM programming at the same time. As far as the injectors are concerned, we'll be installing the second largest off-the-shelf hybrid injectors you can buy. Their overall flow will be nearly quadruple the stockers and the 7-hole orifices on the nozzles will flow 200-percent more than OEM, providing for a much quicker injection rate.
Docile on the Dyno
Strapped to a DynoJet 248C inertia chassis dyno, the truck made a weak 159 hp at the rear wheels—not an atypical number for a bone-stock yet mechanically sound 7.3L-powered '94.5-'97 Ford with an automatic transmission. In this condition, Paw-Paw needed more than 19 seconds to crawl through the quarter-mile and couldn't even hit 70-mph by the time it made it to the stripe. In traffic, it didn't only feel underpowered. It was painfully slow.
Why It's the Ultimate Sleeper
On appearance alone, the old body style Ford represents the quintessential 'grandpa truck' and the factory 16x7-inch forged-aluminum Alcoas will no doubt aid the art of deception. After all, it looks like it belongs on a farm and indeed that's where the truck came from. As previously mentioned, the fact that we are running the original, 23-year-old 7.3L Power Stroke plays a key role in the sleeper recipe. We're not swapping a Cummins between the frame rails and we're certainly not tearing into the engine to add better rods, pistons or throwing in a cam until we have to. Those that know about diesels will write the truck off for being just another slow 7.3L, and those that don't follow diesels won't have the slightest apprehension when lining up next to it.
Our last bit of trickery exists in the truck's weight, or lack thereof. When most people hear the word diesel truck they think ‘heavy.' They're both right and wrong on that one. But what a lot of folks don't know is that the OBS Fords weigh considerably less than a comparable diesel-powered Super Duty. In fact, on race day Project Paw-Paw will tip the scales roughly 1,000 pounds lighter than a late-model F-350 of the same configuration.
Scope of the Build
Before, during and after adding big power to any 7.3L Power Stroke, several areas of concern will need to be addressed. We'll get started on that in Part 2, where we explain and install the top-end hard parts essential in keeping the valve train happy. After that, we'll move on to the injector, high pressure oil pump, electric fuel system and turbo upgrades, followed by the transmission build and the installation of traction bars and gauges. Then we'll hit the track, revisit the chassis dyno and put some weight behind us.
*Bear in mind that there are literally dozens of power recipes for these trucks. Whether you want 300, 400, 500 or 600rwhp there is a combination of parts for you. We decided to go all-in with Project Paw-Paw due to: 1) the fun factor, 2) knowing the truck can still be reliable at 600rwhp and 3) we only use it to tow lighter loads (8,000 to 11,000 pounds), so it's more of a play toy than a working rig or a daily driver.
Looking for a milder setup for your 7.3L? Check out the proven power recipes listed here!
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
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
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?
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
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
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
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
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
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. Prefedit 4 31. Plus, changing injectors is an opportune time to tackle these relatively inexpensive housekeeping items before they begin to show their age.
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.
Paw 3 1 1000 Lbs
Supporting Mods Yet to Come
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.
Paw 3 1 1000 Grams
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.
