skip to content
Driving Line Mark Logo

Performance Roadblocks of the ’94-’98 5.9L Cummins

With simple, cheap and mostly free upgrades, a few basic performance mods and 500,000-mile longevity go hand-in-hand with the P-pumped 5.9L Cummins. Quite literally, these things last forever—even at twice the factory power rating. In factory form, however, the 6BT that came in ’94-’98 Dodge Ram ¾-ton and 1-ton trucks was the definition of conservative, producing well under 200rwhp regardless of model year. But how could the engine that brought diesel performance to the masses be so docile in factory form? It all starts with the timid fueling of the Bosch P7100 injection pump. In addition to being conservatively timed, limited in its fuel volume and governed to just 2,700-rpm from the factory, the P-pump itself is the first performance roadblock a second-gen Dodge owner faces.

Once you’ve thrown the common AFC, fuel plate, smoke screw and star wheel mods at the P-pump and installed a higher rpm governor spring kit, your new horsepower wall will be turbo-related. In short, due to the factory wastegate opening at roughly 18-22 psi of boost, you won’t be able to see the extent of your fueling mods pay off without disabling it or altering when it opens. After that, you’ll either be at the edge of what the factory torque converter (47RH/RE automatic) or stock clutch (NV4500 manual) can take, or you’ll be lucky enough to realize the full potential of the stock injectors (as much as 325rwhp). While many of the components bolted to the P-pumped Cummins can support 700rwhp or more, there are plenty of barriers to cross before you get there. We’ll help you get started below.

Roadblock #1: Lack of Fuel

Bosch P7100 Injection Pump

The Bosch P7100 hanging off the side of the ’94-’98 Cummins is the equivalent of a sleeping giant. It can be woken up and aggravated (in a good way) with relative ease and with virtually zero dollars invested (a factory-based pump can support 700rwhp), but the P-pump is an absolute slouch from the factory. Its pre-configured air-fuel control (AFC), fuel plate positioning and conservative timing play a big part in the pump’s mild-mannered nature in stock trim.

Freebie Fueling Fix #1: Pre-Boost Screw

Cummins P-Pump Smoke Screw

There are various well-documented tweaks you can perform on the P-pump to increase its fueling, and most of them are free. All you need are a few simple hand tools and some spare time. Many get started with the pre-boost screw at the rear of the air fuel control assembly (AFC), the AFC being the device that controls the P7100’s fuel rate in low-boost conditions. The pre-boost screw (also referred to as the smoke screw) can be adjusted for significantly more off-idle fueling.

Freebie Fueling Fix #2: Star Wheel Adjustment

P7100 Bosch Star Wheel

This tweak takes some time to get perfect, but it allows you the ability to fine-tune your extra power for optimum drivability. The star wheel can be found on the top side of the AFC housing. Rotating it toward the passenger side of the engine helps open the P-pump’s fuel rack further (the rack being what allows fuel to enter the pump’s plungers). Incremental adjustments are recommended when turning the star wheel, as going too far results in excessive smoke and not rotating it enough won’t provide a noticeable difference in power.

Freebie Fueling Fix #3: Full-Forward AFC

AFC Bosch P7100 Adjustment

Adjusting the AFC assembly itself all the way forward brings fueling in sooner in the rpm range (hence why this mod can yield big torque gains). An additional trick you can perform during your AFC tinkering is to install a lighter AFC spring. The spring controls the AFC’s rate of movement, so ditching the stocker in favor of a spring with less tension allows the AFC to move forward sooner, and with even less boost on tap. Combining a full-forward AFC with the aforementioned star wheel rotation, it’s not uncommon to see a 60hp and 200 lb-ft gain.

Freebie Fueling Fix #4: Fuel Plate Delete

Fuel Plate Bosch P7100 Cummins

Similar to running a #0 fuel plate, removing the P-pump’s fuel plate provides full rack travel and as much as a 40hp gain. To pull the fuel plate, the AFC housing must be removed first, which calls for the removal of a tamper-resistant (not tamper-proof) bolt, followed by the two fuel plate mounting bolts shown above.

Cheapie Fueling Fix #1: Mack Rack Plug

Mack Rack Plug Cummins P-pump

It’s not free, but for $15 you can add a well-known, tried-and-true component that sends roughly 70cc’s more fuel into the P7100’s plungers. Called the Mack rack plug, it increases rack travel from 19mm to 21mm and (depending on your specific pump, its mods and injectors) can bring as much as 35 more horsepower into the equation. With the P7100 still on the engine, the Cummins’ front gear housing has to be massaged slightly to install it.

Cheapie Fueling Fix #2: Timing Bump

1994-1998 Cummins P-pump Timing

To yield the best combination of emissions, NVH, target power ratings and cold-start performance, the P7100 was set to approximately 12 degrees of timing (BTDC) by Bosch. Bumping up timing to 18-19 degrees has little effect on cold-start performance but can add an easy 30 to 40 hp to the mix. For trucks driven year-round and that see cold winters, most draw the line at 21 degrees of advancement. The process is involved and calls for the use of a dial indicator on the delivery valve holder to perform it correctly, but very rewarding given the solid gains that can be had.

Roadblock #2: Lack of RPM

Dodge Ram 2500 Cummins Tachometer

Not unlike the VE-pumped Cummins (’89-‘93), the P-pumped version is also governed at very low speed from the factory. Limited to approximately 2,800 rpm, there isn’t much of a power window to make use of, especially when you take into account that the governor actually starts to de-fuel as soon as 2,500 rpm on many engines. If you perform all (or even some) of the fueling mods listed above, your power band will remain limited until you address the factory governor springs.

RPM Solution: 3,000-rpm Governor Spring Kit

Governor Spring Kit Cummins

The best way to capitalize on all the free and cheap fueling mods you can perform on a P7100 is to install a 3,000-rpm governor spring kit. The collection of aftermarket springs extends the power curve of the engine, and with more rpm in the mix a much higher horsepower number can be achieved. It’s important to note that, while rpm is your friend on a P-pumped Cummins, a 4,000-rpm governor spring kit (which is also widely available and popular) should only be installed if you plan to pull the engine’s valve covers and install stiffer valve springs at the same time.

Roadblock #3: Lack of Boost

Holset HX35W Turbocharger Cummins

So you’ve added more than 100 horsepower, 250 lb-ft of torque and more rpm to the equation, but left the factory turbocharger alone so far. Even though the added fueling is making more power, the wastegate is still opening (and being overwhelmed at this point) at the stock boost threshold, which is somewhere between 18 and 22 psi. It’s time to bring more boost to the table while still keeping the Holset HX35W safe (WH1C on ’94 engines).

Boost Solution(s)

Boost Elbow 12-Valve Cummins

The ideal method of forcing the factory turbo to move more air is through the use of a boost elbow, a simple brass piece you can pick up for $25 or so. Adjustable, the boost elbow affords you the ability to fine-tune boost and get that magical 35-psi number out of the HX35. Limiting boost to 40-psi or less keeps the charger from over speeding, helps cool exhaust gas temperature and adds as much as 25 hp to your overall recipe. For $0 invested, you can disable the wastegate completely, but it’s wise to keep an eye on a boost gauge so you don’t overspeed the turbo (not to mention pop a head gasket if boost and drive pressure get too far out of hand).

Roadblock #4: Factory Torque Converter or Clutch

Chrysler 47RH Cummins Torque Converter

If you haven’t noticed that the stock torque converter or clutch hates you at this point, the time is coming. The factory converter in the 47RH (or 47RE) Chrysler four-speed automatic transmission is usually the biggest weak link and will begin to slip when asked to harness twice the factory torque rating. The same goes for the stock clutch behind the NV4500 five-speed manual, although with the hand-shaker all you’ll need is a stronger aftermarket clutch (and possibly an input shaft upsize) in order to keep going. Upgrading the converter in the automatic is just the tip of the iceberg, as the weak link will simply shift elsewhere (i.e. it needs to be properly built to deal with the Cummins’ immense torque output).

Is the P-pumped 12-valve really the best Cummins to ever grace Dodge trucks? Read up here and decide for yourself.

Return to beginning of article

Recommended For You

Loading ...