How The 7.3L Power Stroke HPOP Works

In the mid-1990s, tightening particulate matter emissions standards gave birth to the hydraulically actuated electronic controlled unit injector system we all know as HEUI. One of the engine’s to debut the new injection technology, which Caterpillar leased to Navistar, was the 7.3L Power Stroke. In the past, we’ve detailed the makeup of an oil-fired HEUI injector at length, but what about the other piece of the puzzle, the high-pressure oil pump? As we close in on 30 years since HEUI was introduced, many still don’t know the inner workings of an HPOP. What’s more is that a lot of 7.3L enthusiasts have never even seen inside a high-pressure oil pump.

 This month we’re taking an in-depth look at the 7.3L HPOP. We’ll show you all the components packed within its compact housing, how they work together to provide oil volume, and highlight the electrically operated dump valve that controls injection control pressure (the IPR). Ever wondered what the difference is between a “15-degree” and a “17-degree” HPOP? Keep reading. After we distinguish the difference between the early and late high-pressure oil pumps, we’ll cover some of the modifications that allow an HPOP to produce more oil volume in order to support larger injectors. Let’s dig in…

Also known as the high pressure oil supply pump, the high-pressure oil pump (HPOP) employed on the 7.3L Power Stroke is charged with providing adequate oil volume to operate the injectors. It is gear-driven via the camshaft, though its drive gear is not timed to the cam or keyed or pinned to the pump itself, and it mounts to the 7.3L’s front cover at the front of the lifter valley.
Physically, the HPOP mounts to the 7.3L’s front cover via two bolts, both of which call for a 10mm socket to remove or install them. We’ll also note that the driver side HPOP-to-front cover mounting bolt is the longer of the two fasteners. The HPOP mounts to the front cover by way of a reusable gasket.
It should be noted at the outset that the HPOP can never perform its job without an efficient low pressure oil pump in the mix. The LPOP (or lube oil pump, gerotor pump) routes low pressure engine oil through a check valve and ultimately into the high pressure oil pump reservoir. Additionally, a short circuit oil gallery is connected to the gerotor pump’s discharge, which provides a quick fill of the high pressure oil pump reservoir for timely cold starting.
Of the axial piston variety, the 7.3L HPOP is a fixed displacement pump that utilizes seven pistons. A brass cylinder block houses all seven pistons, which ride on what’s known as a swash plate and also utilizes a piston slipper. The pump’s drive shaft is made from a special, hardened steel.
The HPOP’s role in the HEUI system is to route high pressure oil to the oil galleries machined into the cylinder heads and, ultimately, the top side (oil side) of the injectors. And while oil pressure leaving the HPOP can be pressurized as high as 3,000 psi in stock form thanks to the IPR (more on the IPR later), in-cylinder injection pressures can reach as much as 21,000 psi. This is because the intensifier piston in each HEUI injector multiplies the hydraulic force applied to the fuel plunger by a ratio of 7:1.
This is the hardened drive shaft that turns the internals that rotate inside the HPOP. The drive gear, which again is driven off of the camshaft, is bolted to this shaft. The drive gear bolt calls for an 18mm socket and 95 ft-lb of torque. And though it isn’t always performed, checking the pump gear to cam gear back lash is recommended any time an HPOP is installed on a 7.3L.
Even if you only know a few of the components contained within an HPOP, you’ve likely heard the term “swash plate.” This piece sits stationary at an angle and is what the pistons move against as the cylinder block rotates, causing the pistons to reciprocate within their respective bores. The angle of the swash plate determines the stroke of the pistons. The higher the angle, the longer the stroke—and the larger the displacement and the more volume the pump puts out.
During the suction cycle, or one half of a revolution, the pistons create a vacuum that forces oil in. On the other half of the revolution, the pistons discharge oil. In the 7.3L HPOP, the pistons measure 0.436-inches in outer diameter. A 15-degree pump’s internal cc size is roughly 6.8 cc’s, while the 17-degree pump’s is 7.2 cc’s.
Piston springs sit under each piston within the cylinder block. During the suction cycle (also called the intake stroke), the piston spring ensures the piston is pulled back and maintains contact with the swash plate, which allows oil to fill the empty cavity that’s left behind. Then on the discharge stroke the swash plate forces the piston back into the cylinder block, which compresses the spring and discharges the oil.
Another vital part of the HPOP is the reservoir, which keeps a constant supply of engine oil available for the pump to use thanks to a passage located in the 7.3L’s front cover. The high-pressure oil reservoir tower is sealed via O-ring on the pressure side and RTV sealant on the splash side. It holds approximately one quart of engine oil.
The rear cover snap ring is essentially what holds all the contents of the high-pressure oil pump together, and we’ve seen aftermarket pumps incorporate a 360-degree spiral lock ring here for added insurance. Unfortunately, an oil leak from the rear cover is discovered here from time to time, most often due to a rear plate O-ring seal failure.
High pressure hoses feed the oil galleries that are integrated into the 7.3L’s cylinder heads. Not only are the hoses rated to handle the fluctuations in pressure they constantly see, but for the temperature differentials they regularly experience. Beginning with ’99 model year (Super Duty) 7.3L engines, the high-pressure oil lines feature quick-disconnect fittings. Prior to that, they were threaded.
IPR stands for injection pressure regulator, and this electrically operated dump valve is what controls ICP (injection control pressure, also known as high pressure oil pressure). It’s controlled by the PCM, mounted on the rear of the HPOP, and precisely varies its outlet pressure in a range between 450 psi to 3,000 psi (in stock form). Its excess flow is dumped into the return circuit.
The IPR essentially increases ICP by restricting the path to drain. In order to increase ICP (again, high pressure oil psi), the PCM will command a higher duty cycle from the IPR. It does this by sending an ICP-based electrical signal to the IPR solenoid, which creates a magnetic field that applies a variable force on the poppet to control pressure. Duty cycle on the IPR is measured in a percentage.
While IPR duty cycle can be a fun parameter to watch, it’s important to remember several key things in relation to it. As a general rule of thumb, less than 30-percent duty cycle should be observed in a crank-to-start scenario (and also note that it takes 450 to 500 psi of ICP, minimum, to start a 7.3L). At idle, 8 to 16-percent is normal with the engine up to operating temp (shown). At full load, less than 50-percent is typical. And finally, the maximum commanded IPR duty cycle on a ’94.5-’97 model will be 54-percent, and 65-percent on ’99-newer 7.3L’s.
IPR replacement isn’t always necessary for a rough running 7.3L suffering from poor performance or an oil leak in the valley. Sealed to the back of the HPOP housing via an O-ring and with on additional O-ring being serviceable as well, the IPR can be given a new lease on life with a simple, $13 reseal kit like the one shown here, and offered by Riffraff Diesel Performance.
Similar to an engine, additional displacement can be achieved by increasing the bore and/or stroke of an HPOP. A longer stroke is most common, and as mentioned can be executed by increasing the angle of the swash plate. However, over-stroking the pistons can lead to excessive side loading. Larger diameter pistons is the other means of adding displacement, which in the interests of ensuring optimum wall strength can call for a whole new (thicker walled) cylinder block.
Of course, there are other parts and practices that go into some aftermarket HPOP’s that make them better than the rest. Due to their stellar track records, Terminator Engineering’s T-500 HPOP and DieselSite’s Adrenaline HPOP (shown) represent these kinds of high-quality pumps. Tighter tolerances, decreased internal clearances, improved pump efficiency, consistent pump output, and a slightly increased swash plate angle (again, for added stroke) are all built into an Adrenaline, which—like the T-500—is a direct replacement performance HPOP that makes use of the factory housing but little else.
It goes without saying that ICP is an important part of the equation, but specifically how well your HPOP can build adequate ICP and then maintain it is most critical. By and large, AA code injectors (’94.5-‘97) run great with anything higher than 2,600 psi worth of ICP, AB and AD code (split-shot) injectors (’97 CA and ’99-‘03) run better with at least 2,900-3,000 psi, and hybrid injectors (which sport a 5:1 injection ratio) tend to run best at 3,200 psi. Although most HPOP’s can support in excess of 3,400 psi, anything higher than 3,200 psi is said to offer very minimal gains while also being exceptionally hard on injectors.
From stock injector engines up to a7.3L running 285cc AD hybrids, DieselSite’s Adrenaline HPOP is a proven performer. Not only can it support 550-rwhp, but it can last as long as a factory pump (if not longer) while doing it. The Adrenaline’s great reputation for reliability likely stems from the company’s stringent validation process that’s performed before a pump is allowed to leave its facility.
As was mentioned earlier, larger pistons residing in a redesigned (bigger) cylinder block is one way to improve the displacement of a 7.3L’s HPOP. The SRP1.1 from yesteryear was one such pump, and these upsized pistons were key pieces in what made the pump capable of supporting a 300cc hybrid injector equipped with a 200-percent over nozzle. They measure 0.500-inches in diameter (left) vs. 0.436-inches stock .
Opening up the output ports of the HPOP increases the volume of oil leaving the pump. On the aforementioned SRP1.1, output puts were upsized to 0.360-inches in diameter (right) vs. 0.210-inches stock (left). And taking a step further in improving reliability, all fittings were increased to #8 SAE ORB from #6 SAE ORB to prevent stripping of the threads and/or the potential of them blowing out.
Early model 7.3L HPOP’s featured a rear ball bearing for optimum support, but it was replaced with a bushing on later model pumps. Some aftermarket HPOP’s (the Adrenaline included) re-incorporate dual ball bearings into the mix. Though the bushing was never a major longevity concern, exchanging it for dual ball bearings illustrates a company’s investment in its product.
When it comes to the kinds of high-flow hybrid injectors common in the 7.3L performance world, there is no better high-pressure oil solution than running two pumps. Think of it in terms of dual CP3’s on a common-rail, where twin pumps help maintain consistent volume to the rails at all times. The major players in the dual HPOP game are Full Force Diesel and Terminator Engineering. Full Force’s Dual HPOP Pump kit, which was acquired from BTS years ago, stacks two pumps on top of each other, while Terminator’s “Big Oil” dual HPOP system locates pumps in-line in the valley. The big takeaways of running two stock displacement HPOP’s are abundant high-pressure oil volume and factory-like reliability.
A third dual HPOP option that is highly popular in the big injector segment is the Gen3 from Swamps Motorsports. Based around the use of an industrial-grade, Haldex hydraulic pump, it’s paired with your existing HPOP and works in conjunction with the factory pump’s drive gear. The Gen3 mounts above the factory HPOP, doesn’t require a second IPR, and ships 90-percent assembled.



Full Force Diesel

Riffraff Diesel Performance

Swamps Motorsports

Terminator Engineering




You May Also Like

How Much Horsepower Can Your Stock Power Stroke Handle?

If you’re looking to more than double the horsepower your Power Stroke brings to the table, we’ve got you covered. If you’ve ever asked yourself much hp can a 7.3 (or a 6.0l, 6.4l, or 6.7l)…

Diesel Conversion Kit: Automotive Aftermarket

The questions we usually have when going to pull off a diesel conversion is to start off with matching an engine’s power output to a project’s needs. That’s one of the most…