Cummins History, Lesson 2: ’94-’98 5.9L

• February 4, 2019
• Story By
  Mike McGlothlin

Thanks to its reputation for making cheap, easy horsepower, its anvil-like construction, long-term durability and simplistic nature, the ’94-’98 12-valve 5.9L Cummins remains one of the most highly sought after engines in the diesel world. In many ways it was similar to the original ’89-’93 5.9L, sharing the same block, head, rods and displacement. However, its biggest difference would be what made it so legendary: the Bosch P7100 (i.e. the P-pump). Believe it or not, the P-pump was introduced so the 5.9L could meet 1994 emission standards set to take effect on January 1, 1994. The higher injection pressures produced by the P7100 were used to cut down on particulate matter produced in-cylinder (and exiting out the tailpipe). In time, many end-users would take advantage of this new injection pump’s ability to flow considerably more volume than the VE pump ever could.

Other than the P-pump’s integration onto the 5.9L, different injectors were used, revised pistons were installed, a wastegated turbocharger was added and a larger intercooler was included. On top of that, the updated 5.9L Cummins was available in Dodge’s brand-new (“rules-changing”) Ram line of pickups. In 1994, the Dodge/Cummins partnership reached a new height—with Ram trucks even outselling Chevy Silverados at one point—and even though the inclusion of the 8.0L V10 stole most of the headlines, the Cummins was still revolutionizing the ¾-ton and larger diesel truck market. Case in point, before the release of Ford’s 7.3L Power Stroke midway through the 1994 model year, Dodge’s Cummins offering had gone unchallenged in terms of torque output for more than five years.

Interested in how the 24-valve Cummins came to be? Stay tuned for Part 3, coming your way next.

‘94-’98 5.9L Hard Facts

The Second-Gen

While the Cummins-powered ’89-’93 Dodges helped put Chrysler back on the map in the pickup segment, the ’94 Ram took a massive bite out of the market share. Though some of the new Ram’s success could be attributed to its bold look, Class 8-like grille and improved aerodynamics, the Cummins was the end-all, be-all for anyone looking to tow heavy and get impressive fuel economy while doing it. In 1995, Cummins produced engine number 100,000 for Dodge, followed by the quarter-million milestone being met just two years later. With that kind of exponential growth, it was clear for anyone to see that Cummins-powered Dodge Rams were in high demand.

Late ‘97s and Early ‘98s Were Treated to 24-Valve Parts

From ’94-’97, all 5.9L Cummins engines shared the same cast-iron block, crankshaft, main cap bolts and cylinder head found on the ’89-’93 mills. However, in 1997, when Dodge and Cummins were gearing up for the transition to the all-new 24-valve 5.9L ISB set to be released midway through the ’98 model year, the remaining 12-valve engines were built using the 24-valve’s block and crankshaft. As the 24-valve would utilize 12mm bolts to secure the main cap (vs. 14mm bolts on ’89-’97 engines), all late model 12-valves were equipped with the smaller diameter fasteners.

Revised, Emissions-Friendly Pistons

As part of the process of ensuring the engine met the impending 1994 emission standards (which emphasized a considerable reduction in particulate matter), Cummins switched to a different piston design. The pistons were still made from cast-aluminum, but the fuel bowl was reworked to improve swirl for more complete combustion. Cummins also narrowed the ring land above the top ring to cut down on emissions. The same, four-digit horsepower-capable, forged-steel connecting rods remained the rock-steady link between the pistons and the crank.

Improved Camshaft

With horsepower and torque being increased for ‘94, Cummins saw the need to treat the camshaft to a few subtle changes to improve its durability. Most importantly, the area between the front main journal and the first cam lobe, an area Cummins found to be a weak spot in the ’89-’93 camshaft, was shot-peened and treated to a rolled radius. The revised cam also featured hardened tappet faces and wider, lower-friction lobes.

Back When the Manual Option Got You More Power...

Beginning in 1994, if you wanted the higher horsepower, bigger torque version of the 5.9L Cummins you had to opt for a manual gearbox. For 1994 and 1995 model year Ram 2500 and 3500s, the five-speed NV4500 came bolted to the 175hp, 420 lb-ft variant, while the 47RH automatic was matched to the 160hp, 400 lb-ft engine. From ’96 to ’98, a more powerful 215hp, 440 lb-ft Cummins was available with the NV4500, while the new 47RE four-speed auto was joined with a 180hp, 420 lb-ft 5.9L.

Bosch P7100

When you hear folks refer to the “P-pump,” this is what they mean: the Bosch P7100. Thanks to the mechanical inline pump’s six individual plungers (vs. the one plunger in the VE pump), higher fuel volume and vastly quicker injection rates are made possible in the ’94-’98 5.9L Cummins application. Driven by the pump’s camshaft, each plunger pressurizes fuel within its own separate barrel and then sends it to its corresponding injector. On top of the obvious fueling advantage it possesses over the earlier VE, the P7100 is also much more imposing in physical size. In fact, its 52 pound overall heft required Cummins to design a stronger, wider front timing cover to support it.

P-Pump Tweaks

Whether it’s low-buck modifications or a completely re-worked pump built by an injection shop, nearly every internal component within the P7100 can be tweaked for added horsepower. For solid, homegrown gains using simple hand tools, the air fuel control (AFC) assembly can be adjusted, the pre-boost screw backed out, the star wheel turned and the fuel plate can be custom-ground or removed. Increasing rack travel, adding bigger delivery valves and installing higher rpm governor springs all offer substantial gains in power as well, and can be performed on the cheap. For all-out competition, more exotic items such as 13 mm plungers and barrels, custom high-lift camshafts and RSV governors are on the table.

Higher Pressure Injectors

Working in conjunction with the higher pressure P-pump, the ’94-’98 Cummins employs injectors with higher pop pressure (opening pressure). The difference is 260 bar (3,770 psi) on ’94-’98 engines vs. 245 bar (3,553 psi) on ’89-’93 units. The injectors remain 100-percent mechanically actuated with no electronics involved or computer telling them what to do, but the body is different from the ’89-’93 versions. The 7 mm outer diameter of the injector nozzle was the same as what was found on ’91.5-’93 5.9Ls.

Holset HX35

1994 model year Cummins mills were fitted with a wastegated version of the first-gen engine’s Holset H1C turbocharger, coined the WH1C, as well as a tighter 12cm² turbine housing (vs. the 18cm² housing). Then in 1995 the WH1C was scrapped in favor of Holset’s HX35W (better known simply as the HX35), a wastegated charger with a 12cm² turbine housing that would prove extremely reliable. Even when pushed to twice the factory boost rating (40 psi) the HX35’s failure rate is extremely low. This turbo would be bolted to all ’95-’98 engines and was even included on early 24-valve 5.9Ls.

KDP (Still a Problem)

The killer dowel pin, the tiny steel locating pin used to align the timing gear housing during engine assembly and that can back out and cause all sorts of mayhem at any time, is still a problem on these engines. In fact, KDP-related failures are more common on ’94-’98 engines—but this is due to significantly more ’94-’98 engines being produced than ’89-’93 versions. As is the case with all ’89-’02 5.9L Cummins power plants, the best way to rule out the KDP is to tear into the front of the engine and tab the pin in place. Several all-inclusive kits exist in the aftermarket to make this process as painless as possible.