Power Stroke History, Lesson 1: 7.3L
While General Motors didn’t offer a viable light-duty diesel engine platform until the Duramax came along in 2001, Ford all but revolutionized the segment in 1994. In an effort to stay ahead of GM at the time, but (more importantly) also gain back the ground lost with the release of the 5.9L Cummins for ’89 model year Dodge trucks, Ford released the cutting edge 7.3L Power Stroke. Built by Navistar, the 444 ci V8 featured direct injection, a notable departure from its indirect injection predecessor, debuted the hydraulic electronic unit injection system better known as HEUI, provided the cleanest emissions footprint ever offered in a diesel truck and topped Dodge’s Cummins-powered, newly-redesigned Rams with best-in-class horsepower and torque figures.
With hard parts like forged-steel connecting rods and six head bolts per cylinder, the heavy-duty makeup of the 7.3L was obvious. However, what wasn’t so obvious at the time was how well the newfangled HEUI system, which relied on highly pressurized engine oil to make the engine run, would hold up over the long haul. Twenty-five years later, the method of injection employed on the 7.3L—though complex—remains one of the most reliable technologies Navistar and Ford ever used. It’s not uncommon to climb aboard a 7.3L-powered ’99-’03 Ford truck and find more than 400,000 miles on the clock, or an odometer that has since rolled over that (and gone back to 300,000) on a ’94.5-’97 model. This and much more helps explain why Navistar built nearly 2 million of them. Find out all you need to know about the time-tested, revered workhorse that is the 7.3L Power Stroke below.
7.3L Hard Facts
|Production:||1994.5 to 2003||Valvetrain:||OHV, two-valves per cylinder, single cam, hydraulic lifter|
|Bore:||4.11 inches||Injection System:||Hydraulic electronic unit injection (HEUI), direct injection|
|Stroke:||4.18 inches||Injectors:||HEUI (under valve cover)|
|Displacement:||444 ci||High-Pressure Oil Pump:||Bosch 15-degree (’94.5-‘99), Bosch 17-degree (’99.5-‘03)|
|Compression Ratio:||17.5:1||Turbocharger:||Garrett TP38 fixed geometry (’94.5-‘99), Garrett GTP38 (’99.5-‘03)|
|Rods:||Forged-steel (’94.5-‘00), powdered metal (’01-‘03)||Horsepower:||210hp at 3,000 rpm (initial release in ’94.5) to 275hp at 2,800 rpm (‘03)|
|Pistons:||Cast-aluminum||Torque:||425 lb-ft at 2,000 rpm (initial release in ’94.5) to 525 lb-ft at 1,600 rpm (‘03)|
|Heads:||Cast-iron with six head bolts per cylinder (with sharing)|
7.3 Liters: The Original Power Stroke
Producing 210hp at 3,000 rpm and 425 lb-ft of torque at 2,000 rpm when it debuted, the 7.3L Power Stroke represented a revolutionary break from the lazy, underpowered diesels that had formerly powered Ford’s ¾-ton and larger trucks. Although it shared the same displacement as its predecessor, the 7.3L IDI V8, it shared nothing else in common. The connecting rods were beefier, the crankshaft had bigger mains, direct injection was employed, six bolts were utilized per cylinder to anchor the heads to the block and higher pressure fueling was electronically (and precisely) controlled for more power output and cleaner emissions.
Forged-steel connecting rods (above) are believed to be found in all ’94.5-’00 Power Strokes. However, in ’01-‘03 model year engines there was a back-and-forth along the assembly line between powdered metal rods and forged-steel units. Per insider knowledge from Hypermax Engineering, 14,965 “test” sets were installed in engines before Navistar fully committed to using powdered metal rods for the duration of the 7.3L’s production run. After the test-run of powdered metal rods, Navistar reverted back to forged-steel units in order to use up the remaining inventory on-hand at the manufacturing plant. The engine serial breakdown for forged vs. powdered metal rods (PMR) is as follows:
Start of production–1425746=Forged
Forged-Steel Over Powdered-Metal
In the performance enthusiast niche, finding out which type of connecting rod an ’01-’03 owner has can determine how far he or she decides to push the horsepower limit of the engine. Forged rods are capable of surviving 600rwhp and tend to bend rather than break, while those with the weaker powdered metal units (which generally break rather than bend) are advised to stay at or below 500rwhp.
Of all the advancements to debut on the 7.3L Power Stroke, its use of direct injection arguably did the most to usher it into the modern diesel age. Direct injection means that fuel is sprayed directly on top of the piston (the combustion chamber) on the power stroke (no pun intended). No pre-combustion chamber exists (as it does on IDI engines). Each cast-aluminum 7.3L piston features a traditional direct injection, “Mexican Hat” design, a Ni-Resist top piston ring insert for superb wear and corrosion resistance and a plasma-coated top piston ring to prevent cylinder wall scuffing when exposed to sustained high temperature. Beginning in ’99 the piston featured a deeper intermediate ring groove to incorporate a larger intermediate ring width.
Holding Down the Fort
For the type of head-to-block sealing seen in commercial duty applications, the 7.3L Power Stroke benefits from six 12mm diameter head bolts per cylinder (18 per bank). By comparison, its IDI predecessor featured five head bolts per cylinder and its successor (the 6.0L Power Stroke) made use of just four fasteners. The six-bolt arrangement makes the 7.3L capable of handling boost well in excess of 40 psi before head gasket issues are of any concern. Above, a 400,000-mile 7.3L out of an ’02 Super Duty is being fitted with ARP head studs in place of the factory head bolts during an overhaul at San Jacinto, California’s Diesel Tech.
At the heart of the hydraulically-activated HEUI injection system lies this component: a fixed displacement axial-piston pump called an HPOP (or high-pressure oil pump). Its job is to introduce oil volume into the oil rails within the cylinder heads. As oil exits the HPOP, the injector pressure regulator (IPR) pressurizes it as high as 3,000 psi. The stroke of the pump’s internal swash plate determines how much oil volume it can put out. From ’94.5 to early ’99, an HPOP with a 15-degree swash plate was used. Starting in ’99.5 however, the HPOP’s swash plate was increased to 17 degrees, which better supports higher horsepower and larger injectors. We’ll note that timing of the HPOP drive gear is not required. Only the cam and crank are timed with each other.
Physically demanding in size, there are a lot of things happening inside a 7.3L Power Stroke injector. When prompted by the IDM (more on that below), the electronic solenoid at the top is used to pull the internal poppet valve off of its seat, effectively allowing high-pressure oil to enter the injector. The high-pressure oil then forces the intensifier piston beneath it downward, and the nozzle needle beneath that to lift, which pressurizes the fuel present in the plunger cavity. Finally, the nozzle opens and—through a process of multiplication thanks to the intensifier piston possessing a surface area roughly seven times larger than the plunger—the 3,000-psi high-pressure oil figure effectively becomes 21,000 psi worth of fuel pressure in-cylinder.
These two modules are the injector drive module (IDM, left) and the powertrain control module (PCM, right). In terms of the injection system on the 7.3L Power Stroke, the PCM calls upon the IDM to energize the injector solenoids via a 100 to 120-volt current pulse (voltage varies depending on the model year of the engine), with the PCM also determining the timing and duration of the pulse. The MAP sensor allows the PCM to determine engine load in order to calculate the amount of fuel quantity required of the injectors.
Early Vs. Late Lift Pumps
Instead of utilizing a conventional fuel supply system where the injection pump receives a steady low-pressure volume of diesel, the lift pump aboard the 7.3L Power Stroke sends fuel directly into the heads for the injectors to use (remember, fuel is pressurized for the combustion event inside the injector itself). Early engines (’94.5-‘97) came with a cam-driven mechanical lift pump located in the lifter valley that supplied 40 to 70 psi to the injectors (but it usually checks in around 45 psi). Later 7.3Ls (’99-‘03) came with a chassis-mounted electric lift pump that produced 60 to 65 psi, better supported additional horsepower and proved slightly more reliable.
Three different fixed geometry Garrett turbochargers made it onto the 7.3L Power Stroke throughout its production run. First, a non-wastegated, T4 flange Garrett TP38 with a 1.15 A/R exhaust housing was used on the non-intercooled ’94.5-’97 models. In ’99, a tighter 0.84 A/R exhaust housing and V-band turbine inlet flange was added, along with a wastegate and the use of an air-to-air intercooler. The wastegated TP38 was then replaced by the Garrett GTP38 for the ’99.5-’03 model years, which added a 1.0 A/R exhaust housing, a larger wastegate and a map width enhancement groove. Each version used a 60mm inducer compressor wheel, a 70mm exducer turbine wheel and a 270-degree thrust bearing and journal bearing center section. Despite their differences however, all turbos were oil cooled via the pedestal positioned directly on top of feed and return ports located in the block. This meant that the turbocharger required no external oil lines.
Another device that seemed ahead of its time in 1994 was the 7.3L Power Stroke’s use of an exhaust back pressure valve (EBP) that served as a choke of sorts for quicker engine warm up. Activated according to engine oil temperature, a butterfly within the EBP housing that’s bolted to the turbocharger’s exhaust housing closes and restricts exhaust flow until the engine is up to operating temp. The PCM oversees the warm up process by way of the exhaust back pressure sensor and slowly adjusts the butterfly back to fully open as oil temperature rises.
Robust Oil Cooler
Another component on the 7.3L Power Stroke that attests to its durability lies in its externally-located fluid-to-fluid oil cooler (mounted below the driver side exhaust manifold). Locating this key component externally aids longevity in virtually any engine, but especially on one that relies on engine oil to fire its injectors and sees considerable engine load by nature of being used as a workhorse. These things virtually never fail. The only things they ever really need are fresh O-rings to quell a slight oil drip, which happens about every other decade. By direct comparison, the internal oil cooler on the 6.0L Power Stroke that was buried in the crankcase would prove extremely problematic, in more ways than one.
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