Written By, Mike McGlothlin
Highlights:
- Governor Springs: Higher rpm springs increase the usable rpm curve of the engine
- Timing: Advancing the start of injection (BTDC) improves mid-range and top-end horsepower
- Delivery Valves: Provide a quick, efficient end to the injection event, but can be restrictive
- Turning the Barrels: Increases horsepower by delaying lift to the plunger spill port opening
- Quick Rate Cams: Offer quicker fill time, higher lift, and improve high-rpm performance
- Larger Plungers and Barrels: Increase both the rate and volume of fuel that is injected
- Proper Fuel Supply and Injectors: Required to maximize both P-pump performance and durability
It’s widely known that significant power can be unlocked from the 6BT Cummins platform with a few simple hand tools. This is especially true on 5.9L engines equipped with the mechanical Bosch P7100 pump, where more than 100 additional horsepower can be unleashed on the cheap, and for free. However, beyond the normal tweaks such as backing out the pre-boost screw, turning the star wheel, and moving (or removing) the fuel plate, a whole different world of Cummins 6BT P-pump performance exists. It’s not a free or cheap world to play in, but it’s one where exotic internal parts—combined with the proper surrounding modifications—can support gargantuan flow numbers, extreme rpm, and thousands of horsepower. Welcome to the extreme side of the P7100.
First Things First—The Test Bench
Whether you’ll be performing run-of-the-mill mods for a street pump or going all-out with
a competition unit, your P7100 needs to visit the test bench. It takes 12 hp to spin the P7100, so a 15 hp or (even better) 20 hp electric test bench is required in order to properly calibrate (flow) one. The P7100’s volume output is measured in cubic centimeters (cc’s) of fuel per 1,000 strokes. Beginning with the P-pumps found on 5.9L-equipped ’94-’98 Dodge Rams, the 160 hp and 175 hp versions (’94-‘95) flow the same at roughly 135 to 140 cc’s, depending on the flow bench. The 180 hp unit (’96-‘98) moves 170 cc’s, and the often-coveted 215 hp pump (’96-‘98) flows the most at 180 cc’s. In stark contrast, some extreme P7100-based pumps can flow more than 1,500 cc’s.
Timing Is Everything
It’ll take more than a few simple hand tools to advance the timing on a P7100 but doing so
can provide significant power gains at higher rpm. From the factory, the P-pump was set very conservatively at 12 to 14 degrees BTDC. Bumping overall timing to 17-21 degrees is a great way to wake up a street-driven application while still maintaining sufficient low-end torque, good drivability, and tailpipe cleanliness—not to mention that, at a minimum, 30 more horsepower usually enters the picture. Increase timing much further and cold starts can become an issue, and the life of the head gasket can also be cut short. Many competition-only P7100’s, some of which are timed at an incredible 40 to 50 degrees, are equipped with an adjustable gear for quick, easy timing adjustments.
Delivery Valves
The delivery valves inside a P7100 provide for a quick, efficient end to the injection event as well as serve to isolate the high-pressure injection lines from the plunger and barrel assemblies. From pump to pump, all factory P7100 delivery valves are interchangeable but, depending on your specific combination of modifications some may yield better results than others. What’s known as the collar of an OEM delivery valve can be machined to provide a lengthier injection period, but this is often done at the expense of increased smoke and exhaust gas temperature (EGT) levels. In higher horsepower arrangements, the delivery valves can become a considerable point of restriction between the pump and injectors. P-pump builders get around this by replacing them with application-specific, higher flowing versions, often referred to as full-cut or laser cut delivery valves.
Governor Springs
A 12-valve Cummins in a ’94-’98 Dodge Ram 2500 or 3500 can begin to defuel as early as
2,500 rpm due to the factory governor springs—highly prohibitive in making more horsepower. This is precisely why aftermarket governor spring kits (such as the 3,000 and 4,000-rpm kit shown from Dynomite Diesel Products) are so popular, as they allow fueling to continue at higher engine speeds. In the more exotic P-pumps on the market, you’ll find governor spring assemblies—some of which even boast billet governor weights—that enable fueling all the way up to 7,000 rpm. In these high-end performance P7100’s, what’s known as an RSV (or “Ag”) governor assembly is extremely common, and it’s an arrangement that often comes with a single, external fuel adjustment screw for making quick, easy tweaks.
Turning the Barrels
Turning (or racking) the barrels in the P7100 is an age-old trick that works. However,
while it delays lift to the plunger’s spill port opening which in turn adds horsepower, the primary reason for turning the barrels is to balance the pump’s flow. For this reason, the process should be carried out on a test bench by a P-pump expert. Most reputable injection shops will rotate the barrels when the owner asks for more horsepower, but the difference between the average Joe doing it in his or her garage vs. a veteran pump builder will be a finished product that yields smooth idle and operation throughout the rpm curve. Most P-pump gurus will also ensure the pump is seeing full (or optimum) rack travel when they rotate the barrels.
Rotating The Barrels + Aggressive Fuel Plate = Big Fuel Increase
While turning the barrels will add that little something extra to any P-pump, combining it with an aftermarket fuel plate—especially one set at the full-forward position for more rack travel—will bring big fuel volume into the equation. For a real-world example, and as we stated earlier, an untouched 180 hp P7100 from a ’96-’98 5.9L Cummins powered Dodge Ram equipped with an automatic transmission flows 170 cc’s of fuel. With the same 180 hp pump’s barrels turned and the factory fuel plate in place, flow increases to 190 cc’s. Add in the popular #100 fuel plate and leave it in the factory position and output jumps to 326 cc’s. Move the #100 plate fully forward and flow leaps to 445 cc’s. As you can see, with very minimal work the P7100 can be made to flow roughly 300 to 450 cc’s.
Quick Rate Cams
You may already know that the “903” camshaft employed in the 180 hp and 215 hp
versions of the P7100 is one of the driving reasons behind their high desirability. Its advantageous return ramp provides for a much quicker fill time and promotes great high-rpm performance. In the world of competition, P7100-based pumps, radical proprietary camshaft designs—complete with lightning quick fill rates and higher lift—are employed. Scheid Diesel is renowned for its high-end 13mm, 14mm, and 16mm P7100’s, and the company developed its own in-house camshaft profile for the extreme horsepower applications these pumps support. In addition to the performance they provide, its cams are DLC coated for reduced friction and long-term durability.
Larger Plungers and Barrels
Big horsepower calls for copious amounts of fuel, and larger plungers (and their
corresponding barrels) are vital to increasing both the rate and volume of fuel that is injected. This is where the aforementioned 13mm and larger P7100’s come in. The 13mm denotes the overall diameter of the plungers (and we’ll note that a factory P7100 utilizes 12mm plungers), and they can support huge horsepower—anywhere from 1,200 hp to north of 2,500 hp. Installing larger plungers and barrels calls for boring out the pump’s case in order to accommodate them. Like the high-performance camshafts we covered earlier, larger plunger and barrels are usually DLC coated for optimum durability given the excessive speeds they see (engines turning 5,500 to 7,000 rpm).
Properly Matched Injectors
Of course, to maximize the performance of any benched P7100, a well-matched set of fuel injectors is paramount. This means an injector that’s suited to handle the added volume a worked-over P-pump will send its way—along with being set to the proper pop-off pressure. Injector bodies with dual-feed and even triple-feed ports are common in the top tiers of diesel truck pulling and drag racing, as are high-flow injector nozzles. Performance nozzles primarily retain the factory, 5-hole style, but their passages can be downright massive (five holes, each measuring .035-inch or larger, or 5x35 in injector speak). Last but not least, the injector nozzles have to match the piston bowl beneath them in the form of having the proper spray pattern.
Adequate Fuel Supply
Ample low-pressure fuel supply is vital in both achieving peak performance and keeping a
P-pump alive. Not only does adequate feed increase the chances of optimizing power, but it also ensures the plungers and other internals receive adequate lubrication. Too little supply pressure and you risk damaging the P7100, or sticking the fuel rack (think of a runaway scenario). For modified P-pumps, and especially those of the 13mm or larger variety, either a gear-driven or electric fuel supply pump (i.e. lift pump) that’s up to the task of delivering no less than 50-psi must be part of the equation. In fact, a pump that flows 400-gph or better and that holds 60-psi or so is ideal for performance and durability, as is a set of correctly sized injection lines and a second supply line feeding the P7100.
