Back in Time, Part 2: 7.3L Power Stroke vs. P-Pumped 5.9L Cummins

• July 26, 2019
• Story By
  Mike McGlothlin

As we steer our diesel-powered DeLorean back to 1994, American manufacturers were on deadline to meet new EPA-mandated regulations intended to tighten up diesel emission standards. To the end-user’s direct benefit, Chrysler and Cummins cleaned up the 5.9L’s particulate matter output by changing its injection system. The change entailed scrapping the Bosch VE rotary pump in favor of the venerable Bosch P7100 (a.k.a. the “P-pump”), the mechanical plunger pump that would go on to make the ’94-’98 Cummins legendary. Downwind of the P-pump, the injectors were revised and the pistons reworked to further optimize combustion.

But while Dodge and Cummins stuck with mechanical injection, Ford and Navistar turned to an all-new (and radically different), electronically controlled 7.3L V8 with hydraulically operated fuel injectors. Christened the Power Stroke, its HEUI injection system relied on highly pressurized engine oil to actuate the fuel injectors and the elaborate arrangement called for integrated oil rails in the cylinder heads, a gear-driven high-pressure oil pump, an injection pressure regulator (IPR), powertrain control module (PCM) and an injector drive module (IDM) in order to work. In quieter news, GM fitted its 6.5L turbo IDI engine with an electronically controlled Stanadyne DS4 injection pump for 1994.

Shortly after Dodge revealed its redesigned Ram, complete with its big-rig style front-end, aerodynamic body lines and the more powerful 5.9L Cummins as an engine option, Ford dropped a bombshell of its own: the 7.3L Power Stroke. It topped the Cummins in both horsepower and torque production and would become available mid-year ’94 in F-250 and larger trucks. So how exactly did these two heavyweights stack up back in 1994, and how long would it take for Dodge to retaliate? Read on (and stay tuned) to find out!

Toe-to-Toe Comparison

Revamped 5.9L Cummins

Though much of the components employed on the ’94-’98 version of the 5.9L Cummins carried over from the first generation of the 6BT, meeting emissions with the P-pump and updated injectors in the mix required reworking of the pistons. The cast-aluminum units found in ’94-’98 mills featured a revised fuel bowl to improve combustion (and cut down on particulate matter leaving the tailpipe). Cummins also addressed the weak area in the camshaft between the front main journal and the first lobe by adding a rolled radius and subjecting it to shot-peening. The updated cam was also treated to hardened tappet faces and wider, lower-friction lobes.

P7100

The biggest change on the ’94 rendition of the 5.9L Cummins was the addition of the Bosch P7100. Not only does the P7100 pressurize the fuel intended for each cylinder individually (in its own dedicated barrel), but more volume and pressure is supplied to each injector. The higher pressure facilitated better atomization with the higher pop-off pressure mechanical injectors (260 bar vs. 245 bar previously) and the P-pump’s hidden potential could be taken advantage of with a few simple hand-made adjustments. A beefier front timing cover was also necessary to accommodate the hulking P7100, with its weight doubling that of the VE pump it replaced.

Turbo Change

While the P-pumped 5.9L debuted with the carryover WH1C Holset turbocharger on board, for ’95 it was replaced with the HX35. Like the WH1C, the HX35 was a wastegated and fixed geometry turbo, but its 12cm² exhaust housing made ’95-up Rams much more drivable thanks to quicker spool up. The aforementioned wastegate, which relieves excessive drive pressure in order to preserve the life of the turbo, was typically set to open around 20 to 22 psi of boost from the factory.

Intercooled

One item the Cummins had that the Power Stroke didn’t was an intercooler. Dodge began intercooling the Cummins on ’91.5 model Rams, which has continued through to present day. Use of an efficient air-to-air unit on all ’94-’98 Cummins mills helped keep exhaust gas temperature well in check, even under heavy load. It also allowed the engine to see cooler intake temps. The Power Stroke wouldn’t receive a factory intercooler until ’99.

The Manual Was Mandatory

Not wanting to outsource an automatic transmission to place behind the Cummins, Chrysler used its own four-speed slushboxes behind the 5.9L (the 47RH, followed by the 47RE) and they were adequate, at best, when tasked with harnessing the 5.9L’s plentiful low-end grunt. For this reason, only manual transmission-optioned trucks could be had with the 175hp, 420 lb-ft version of the 6BT in ’94. When power increased for the ’96 model year, only the five-speed trucks saw the 215hp, 440 lb-ft engine.

Ford’s New 7.3L: Nothing in Common With the Turbo IDI

Even though it shared the same bore and stroke, the 7.3L Power Stroke shared nothing else in common with the 7.3L turbo IDI mill that preceded it. Cast from gray iron, the 7.3L Power Stroke’s block housed a forged-steel crankshaft that’d had its main and rod journals and fillets hardened to resist wear. The connecting rods and camshaft were forged from steel, too, while the direct injection pistons were cast from aluminum and featured Ni Resist ring inserts up top and oil cooling nozzles underneath. The cast-iron cylinder heads anchored to the block via six bolts per cylinder, featured two valves per cylinder, self-adjusting hydraulic lifters and high-pressure oil and low-pressure fuel galleries to accommodate the HEUI system. Reusable valve covers called UVCH’s (under valve cover harnesses) featured pass-through electrical connectors for the injectors.

HEUI

Navistar had Caterpillar to thank for the HEUI injection system. After developing the hydraulically actuated, electronically controlled unit injection system, CAT leased the technology to Navistar for use on several of its engines (the 7.3L Power Stroke for Ford and its own T444E, DT466E and I530E). Instead of a traditional injection pump, the Power Stroke relied on a high-pressure oil pump (HPOP), which was fed oil from the low-pressure oil pump in the crankcase, to provide oil volume for the injectors. The HPOP (pictured above) is a seven-piston, fixed displacement axial piston pump that mounts to the front cover. Its drive gear is driven by the camshaft gear but is not timed to the camshaft.

HEUI Injectors

With both oil and fuel running through them, the HEUI injectors in the 7.3L Power Stroke are fairly complex. The most important thing to remember is that the oil side (top side) actuates the fuel side below it, the result being as much as 21,000 psi worth of fuel injection pressure spraying in-cylinder. Once the injector solenoid is energized via the 100-volt pulse sent its way courtesy of the injector drive module (IDM) and as commanded by the powertrain control module (PCM), the internal poppet valve opens and allows high-pressure oil to enter the injector. From there, the high-pressure oil forces the intensifier piston, a component that possesses roughly seven times more surface area than the plunger beneath it, downward until the nozzle opens and fuel flows into the combustion chamber.

Electronically Controlled

Thanks to its advanced (at the time) computer modules, fast-reacting injection pressure regulator (IPR), a plethora of sensors constantly gathering information and the high-pressure oil pump not having to be timed to the engine, the 7.3L Power Stroke’s fuel pressure and injection timing could be varied independent of engine speed. This precise control over the HEUI system meant the 7.3L was the cleanest engine in the pickup market at the time, peak performance was available at any rpm and it was better suited for cold weather performance. Among the key components in the HEUI control system were the aforementioned PCM, IDM and IPR (in charge of varying HPOP output pressure), along with the injection control pressure (ICP) sensor.

Non-Wastegated Turbo

Packaged close to the valve covers at the back of the lifter valley, the 7.3L Power Stroke made use of a fixed geometry Garrett TP38 model turbocharger. Its oil feed and return circuits were built into the turbo pedestal and sealed by O-rings, which meant no external oil lines were required. An exhaust back pressure device (or EBP valve) was used for cold-weather warm up. Mounted between the turbo’s exhaust housing and the downpipe, the EBP valve would restrict exhaust flow and thereby load the engine in order to bring it up to operating temp quicker. It was hydraulically-actuated according to the engine oil temperature sensor. The TP38 was void of a wastegate but it didn’t need one. In stock form the most boost the 60mm compressor could produce was 17 to 19 psi.

The Torque War Begins

Determined to keep its new 7.3L workhorse on top of the Cummins and confident in the engine’s ability to handle added horsepower and torque in the future, Ford gladly entered into the cat-and-mouse game that would become today’s endless numbers battle. After the 7.3L Power Stroke’s 210hp, 425 lb-ft introductory rating was surpassed by Dodge’s 215hp, 440 lb-ft version of the Cummins in ‘96, Ford answered with an uprate of its own: a 215hp, 450 lb-ft Power Stroke for the same model year. In ’97 Ford would increase the 7.3L’s horsepower again, to 225hp.

By 1998 Dodge and Cummins were ready to sit atop the diesel segment once more. Find out what they came up with in Part 3…

Want to know more about the first Power Stroke? Enjoy some light reading here. Looking for more info on the legendary P-pumped 12-valve Cummins? Check out the '94-'98 6BT’s full backstory in this history lesson.