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Cummins P-Pump Tweaks: A Triple-Digit Horsepower Gain For $0

Posted in How To: Engine on February 6, 2018
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Free horsepower has a nice ring to it, doesn’t it? In this day and age, it’s almost a foreign concept to think that you can add power to any vehicle without first opening up your wallet. However, there was a time when turning wrenches led to solid performance gains. For the ultimate example of low-buck power-adding, look no further than the ’94-’98 3/4- and 1-ton Dodge Rams equipped with the 5.9L Cummins. Thanks to being graced with the Bosch P7100 injection pump (aka the P-pump), 100 hp or more can be unleashed once a few key areas of the mechanical pump are modified. All that’s required is a little bit of your time and a few simple handtools.

If you’re still using one of these old Dodge workhorses around the farm or are thinking of picking one up because it’s cheap and will last forever, don’t be surprised when it struggles to keep up with traffic. These mild-mannered mid-’90s Rams only sent 135-150 hp to the pavement in stock trim, which is all the more reason to break out the wrenches and turn up the P7100. With the fueling tweaks listed here, you can darn-near double the factory horsepower rating while tripling torque output—and it won’t cost you a thing! And for those of you who don’t plan on stopping with the freebies, we’ve also included a few subsequent steps you can take in your pursuit of additional power.

*Note: all performance gains mentioned in this article assume that the truck you’re starting with is mechanically sound (i.e., a P7100 in good overall health, a strong lift pump, and injectors with adequate pop-off pressure).

Thanks to their hard-to-kill nature, thousands of 5.9L Cummins engines are still hanging around. When it comes to diesel engine swaps, the 12-valve ’94-’98 version is the most sought after mill of the bunch. Other than this inline-six powerplant’s simple yet robust design, it’s the P-pump hanging off the side that gives it its luster. While we’re showing you how to add an extra 100 to 120 hp in this article, a reworked and highly modified aftermarket variation of this pump (larger plungers, quick-rate cam, high-rpm Ag governor) is capable of supporting more than 1,500 hp.
No image better represents diesel performance than the P7100: a 100 percent mechanical, inline plunger-style injection pump manufactured by Bosch. The P7100 is flange-mounted to the Cummins engine block and the pump is gear-driven via its camshaft—the camshaft driving each plunger up and down within its respective barrel. The pump’s governor springs, air fuel control, injection timing, rack, plungers, and barrels can all be altered, modified, or upgraded to improve its fueling performance. The P-pump might feature 19th century technology, but there is no denying its performance potential.
First things first—even before cracking the pump open—you can slide the air fuel control (AFC) assembly full-forward for immediate gains. Located at the back of the P7100, the AFC is responsible for handling the P-pump’s fuel rate under low boost pressure. By moving the entire assembly forward, backing out the pre-boost screw, and adjusting the internal star wheel (see next caption), an extra 150 to 200 lb-ft of torque and as much as 60 hp is instantly on the table. Internally, a lighter aneroid spring can be installed to allow for full AFC linkage travel, and some folks even massage the AFC foot to provide more rack travel (note that the rack is what allows fuel to flow into the P7100’s plungers and barrels). Fortunately for the DIYer, virtually every piece of the AFC can only be reassembled one way.
The aforementioned star wheel adjustment can be seen here. The star wheel is accessed by removing the plug on top of the AFC housing via an 8mm Allen wrench. With the pump in place along the block, turning it toward the engine reduces tension on its respective spring, thereby opening the fuel rack further. Adjusting the star wheel is best done in smaller increments if you’re concerned about smoke. As with anything, everyone’s opinion of “too smoky” varies, so it’s up to you to discern which amount you can live with.
Removing or modifying the P-pump’s fuel plate—the component that controls the maximum fueling output of the P7100—requires you to pull the AFC housing. This calls for the removal of the factory tamper-proof bolt (hello, chisel) and the extraction of the two fuel plate mounting bolts (an impact screw driver works best). After loosening and carefully removing the fuel plate mounting bolts and washers, you have four performance choices: slide the fuel plate forward, modify it, buy an aftermarket plate, or remove it altogether. The latter option, combined with a full-forward AFC and turned star wheel, will result in an easy 80 to 100 hp gain.
Just as the P7100 was set up to fuel conservatively from the factory, the Holset HX35 turbocharger was configured to produce a fairly timid amount of boost. In stock trim, the HX35 only builds between 18 to 22 psi of boost, depending on when its internal wastegate opens. By disabling the wastegate or running an adjustable boost elbow (shown), it’s possible for the HX35 to create 35 psi of boost or more. The added boost not only brings more power into the equation, but it also helps reduce smoke and keep exhaust gas temperature (EGT) in check. When you combine the boost increase with the AFC sitting full-forward, the star wheel turned, and the fuel plate removed, 100 to 120 hp more than stock can be realized.
One of the best ways to wake up a 12-valve is to advance the P7100’s timing (BTDC). From the factory, timing advance is set on the conservative side, with a lot of trucks checking in near the 12-degree range (although this number varies from pump to pump). Bumping up to 17 or 18 degrees nets you lower EGT, improved fuel economy, and as much as 50 more hp, depending on your combination of parts and other pump tweaks. To set your P-pump’s timing quickly and accurately, Power Driven Diesel offers a timing kit, which thanks to including a gear puller, degree wheel, and great instructions, allows you to advance the timing of your P7100 in a matter of minutes.
Why is 18 degrees the magic number in timing a P-pump? For street-driven trucks, this is the diesel performance industry–imposed limit, and is about as far as you want to advance your timing before cold starts and drivability issues surface. The earlier P7100’s (’94-’95 model year Rams) respond very well to timing being bumped into the 18-degree range. This is due to these lower horsepower versions (160 hp and 175 hp, respectively) being equipped with a much less aggressive cam than what was employed on ’96-’98 models (180 hp and 215 hp). In extreme cases such as truck pulling, it’s not uncommon to find P7100-based pumps running 35 degrees of timing or more. As you can imagine, this extremely early start of injection is ideal for getting as much fuel as possible into the cylinder—and it’s reserved solely for competition-only purposes.
While it’s true that turning the star wheel increases rack travel, you can only gain so much with the factory rack plug in place. The “Mack rack plug” is a well-known mod that replaces the OEM unit and gains you 2mm worth of travel (19mm of travel stock versus 21mm of travel with the Mack plug), which comes out to roughly 70 cc’s more fuel. It’s not free, but you’ll only have to fork over $10 to $15 for one, depending on where you source it. The Mack rack plug (PN 2095) can gain you anywhere from 10 to 35 hp.
One key area where the P-pump really shines is at high engine rpm (3,000-plus). Unfortunately, stock ’94-’98 Rams are governed to 2,700 rpm and begin to defuel as soon as 2,400 rpm. If you want to increase your usable power window, a 3,000 or 4,000rpm governor spring kit should be at the top of your mods list. Kits are fairly inexpensive (ranging from $95 to $160) and come with everything needed to swap out the factory hardware (inner springs, spring seats, and shims).
Another area of improvement within the P7100 exists in its delivery valves, as well as the delivery valve holders. Located between the plunger and barrel assemblies and the injection lines that span from the pump to the engine, the delivery valve holders’ internal passageways can be opened up to allow more fuel to be sent to the injectors. The high-flow holders shown here have been opened up to 0.093 inches.
The delivery valves that sit inside the aforementioned delivery valve holders open based on pressure present in the plunger cavity, and they close due to the force of the spring positioned above them. Modified and aftermarket delivery valves (such as 191, 022, and 024s, to name a few) are typically reserved for applications where larger injectors have been installed. However, the added fueling they provide does improve low-rpm torque and mid-range power when used in engines with the stock injectors still in the mix. Longer delivery valve springs can also yield performance gains, but they work best when matched to your injectors’ specific pop-off pressure.
This product isn’t free, but it’s revolutionary when you think about what it does for P-pump Cummins performance. Instead of breaking out the wrenches to make tuning and fueling adjustments, AFC Live from Power Driven Diesel allows you to precisely increase or decrease rack travel (total fuel delivery) and the rate of the fuel increase (fuel curve and smoke control) on the fly. Installation requires removal and modification of the AFC assembly (namely to install a new AFC spring) and running a few lines through the firewall, but it will likely be the last time you ever have to dig into the AFC. Once in place, AFC Live allows you to take your truck from all-out performance to street-friendly or tow-ready with the turn of a knob.
Gauges will also force you to break open your wallet, but with added fueling and the stock turbo still in the mix, things can get hot in a hurry. Keep an eye on EGT with a pyrometer and make sure you don’t overspeed the turbo with a boost gauge. As a general rule of thumb, don’t spend a lot of time above 1,300 degrees on the pyro or let the turbo produce more than 35 to 40 psi of boost pressure. Exceeding those maximums for short bursts (5 to 10 seconds) is fine, just don’t let the truck live in that range.
Unfortunately, the transmission options offered in Dodge’s 3/4-ton and 1-ton Rams have never been known for their ability to handle the added power (and especially the torque) that a 5.9L Cummins can dish out. Once you’ve performed all the tweaks listed in this article, don’t be surprised if the factory torque converter begins to slip. While the freebie P-pump mods are relatively easy to perform and yield great results, installing an upgraded torque converter and performing the appropriate valvebody work should also be on your to-do list if you want the 47RH or 47RE to live. If you’re working with the NV4500 five-speed manual, the factory clutch will need to be swapped out for an aftermarket version with added holding power. For more on that check out the products offered by South Bend Clutch ( or Valair Performance Diesel Clutches (
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Bosch Auto Parts
Farmington Hills, MI 48331
Garmon's Diesel Performance
Griffin, GA 30223
Power Driven Diesel

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