How to Get More Power From Your P-Pump Cummins
Any time we write about the valve L Cummins, we can always count on plenty of feedback from readers. The very mention of this legendary mill seems to reinvigorate the never-ending mechanical vs. common-rail debate, reminding us where diesel performance stemmed from or (it never fails) kick off a brand loyalty brawl. But, this isn’t another holy grail piece where we worship the almighty valve. Instead, we’re highlighting the biggest reason for this engine’s celebrity: the Bosch P.
That’s right, the injection pump hanging off the side of the renowned and revered ’’98 L Cummins is more important than the engine itself. The Bosch P is arguably the single most recognizable symbol of diesel performance. It represents easy horsepower for those handy with a wrench, huge horsepower for anyone willing to spend some money and — like the Cummins engine it was bolted to — million-mile durability.
In this article we’ll explore all the components within the P that can be modified to improve its performance. We’ll begin with the mods that won’t cost you a dime, but that can add more than hp. Then we’ll explore the internal upgrades that can take this pump from mild to wild.
Your free P-pump tutorial starts here.
Back in the mid ‘90s, long before common-rail injection had debuted and horsepower wasn’t quite as easy to come by, this was the four-barrel carburetor of the diesel world. If you wanted your truck to make horsepower (a big number back then), it wasn’t going to happen with what the competition was offering (the HEUI-fired L Power Stroke or GM’s indirect injection L, respectively).
Contrary to what some would have you believe, the P’s robustness, performance and reliability don't make it simple. This marvel of mechanical engineering is fairly complex. Think of the P-pump as a mini inline-six cylinder engine bolted to the side of your inline-six cylinder engine. There are many moving parts inside.
Air Fuel Control
At the back of the P, you’ll find the air fuel control (AFC) assembly. In addition to dictating when the governor arm hits the AFC arm, the AFC is in charge of controlling the pump’s fuel rate at low boost. By sliding the AFC housing forward (toward the front of the pump) and backing out the pre-boost screw (on the backside of the AFC housing), fueling can be brought in at a much lower rpm, the governor linkage is altered and the rack is adjusted forward. And when combined with a star wheel adjustment (located within the AFC housing), as much as lb-ft of torque can be added, along with a peak horsepower increase of 50 to 60 horsepower.
Turning the Star Wheel
As for the aforementioned star wheel adjustment, turning it toward the passenger side of the truck opens the fuel rack more. The fuel rack controls the amount of fuel that’s allowed to enter the pump’s plungers and barrels. The star wheel is accessed via the top of the AFC housing, and adjustments should be made in very small increments. If you go too far, heavy smoke will result. Go too little, and you won’t see the power gains you’re after. It’s up to you to find the right balance here.
Increased Rack Travel
While adjusting the star wheel increases rack travel, the factory rack plug will only allow so much fuel into the plungers and barrels. The popular “Mack rack plug” shown above increases rack travel from 19 mm (stock) to 21 mm. While 2 mm might not sound like much, the Mack rack plug adds roughly 70 cc’s worth of fuel to the mix and can be good for anywhere between 10 to 35 additional horsepower depending on your pump’s overall combination (specific P model, delivery valves, fuel plate setup and injectors).
While not a “free” mod, this one will only set you back $10 to $ It’s important to note that the best way to install the Mack rack plug is with the pump off the engine. If the pump stays in place, some grinding of the timing cover will be in order. In addition, the Mack rack plug isn’t typically recommended for hp pumps (the P used on ’’98 trucks with five-speed manual transmissions).
Removing the Fuel Plate
The fuel plate controls the maximum fuel output of the P, and the stock unit is conservative to say the least. While there is no shortage of aftermarket fuel plate options (along with plenty of custom-made, one-off versions), removing the fuel plate completely will yield more rack travel and considerable power gains (typically 35 to 40 hp).
On a ’’98 Dodge sporting the factory injectors, a full-forward AFC housing, turned star wheel, no fuel plate and an untouched stock turbocharger, 85 to hp can be gained. Disable the wastegate on the turbo (increasing boost production by to psi) and you could find another 10 to 20 hp. As far as performance investments go, picking up to hp for free is a no-brainer!
Delivery Valve Holders
Going beyond the freebie mods, the next logical step is to take a look at the pump’s delivery valves. The delivery valves are positioned inside delivery valve holders, both of which are positioned between the plunger and barrel assemblies and the injection lines that feed fuel to the injectors. Aftermarket delivery valve holders with a larger internal orifice (shown above) allow more fuel to travel through them.
Delivery Valves (DVs)
Delivery valves isolate the plunger and barrels from the injection lines (making sure reverse flow doesn’t occur) and also allow for a pressure drop in order to facilitate the precise closure of the injector nozzle during the injection event. Aftermarket delivery valves offer higher flow (up to cc’s), with some being application specific (i.e., some are good for towing, some good for daily driving, while others are reserved solely for performance). Common delivery valves used in the P are ’s, ’s, ’s, ’s and full cut units, with the full cuts typically being reserved for dedicated sled pull, drag race or dyno applications.
Governor Spring Kits
Because the factory P on ’’98 Dodge trucks governs the engine to a grandpa-like 2, rpm (with the factory governor actually starting to defuel around 2, rpm), an aftermarket governor spring kit should be one of the first items you add. Popular kits from Dynomite Diesel Performance, PacBrake, BD Diesel and Industrial Injection provide full fueling up to 4, rpm, with even higher governor springs being available for competition-ready pumps that have been properly set up on a test stand.
The ability to fuel at high rpm is what opens the door to all kinds of horsepower potential with the valve engine. It’s important to note, however, that stiffer valve springs need to be installed in the head if you plan to spin the engine beyond 3, rpm or run big boost. For all-out competition pumps, companies like Scheid Diesel and Columbus Diesel Supply offer governor springs that provide full fueling up to 7, rpm!
Larger Plungers and Barrels
Just as an engine can have its cylinders bored to add displacement, the P can be bored to accept larger plungers and barrels — and that’s exactly what happens in the extreme segment of the diesel aftermarket. Increasing the diameter of the plungers and barrels from stock (12 mm) to 13 mm adds immense fueling potential.
For example, a 12 mm pump is capable of flowing a maximum of cc’s of fuel (possibly cc’s on a good day or a friendly test bench), while a properly setup 13 mm unit can flow cc’s or more. Shops like Scheid Diesel, Columbus Diesel Supply, Northeast Diesel Service and Hart’s Diesel are highly reputable in the 13 mm (or larger) P-pump world.
Also similar to a miniature engine, the P utilizes a camshaft. The cam features one profile for each individual plunger and as the cam rotates the plungers are driven up and down within their respective barrels. In extreme horsepower applications, the cam inside the P is swapped out in favor of a unit with a more radical profile. With a more aggressive cam, more fuel is injected — and it’s done at a quicker rate. The camshaft pictured above comes from Hamilton Cams; when it’s used in a 13 mm P, it can inject 11 percent more fuel volume than a stock cam. It’s also good for as much as 80 additional horsepower.
Timing Is Everything
To be sure, injection pump timing has a lot to do with overall power output. A P set at the factory timing mark will only mildly benefit from the modifications explained above. However, a pump that’s set at 18 to 20 degrees of timing advancement will see impressive gains. The 18 to degree mark is the unofficial sweet spot for timing on a P-pumped Cummins in that it allows for good all-around power, drivability and cold-start performance. While considerably more power might be made with 25 or 26 degrees of timing, drivability will trend downward — not to mention that getting the engine to fire up in cold weather can become a chore.
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Don’t forget that the automatic transmissions offered by Dodge during the ’‘98 era were easily overmatched by the Cummins in front of them. And while the five-speed NV is very strong, the clutch in front of it will waste no time slipping once it’s tasked with harnessing even more low-end torque. If you plan to tinker with your P-pumped Cummins, know that you will likely be performing some type of upgrade to the transmission in order to enjoy the added horsepower and torque.
L P Dragon Fly 12MM Pump (Hp)*
Built with stock 12mm pumps capable of cc fuel delivery. Higher RPM setting available up to RPM.. Higher timing adjustments are made upon request. Modifications are made to the aneroid for valet driving conditions. Modified torque curves. Core charge: $
- Horsepower Rating: HP
- Warranty Period: 1 Year(s)
- Increase Torque
- Increase Horsepower
- Modified torque curves
- Capable of cc fuel delivery
- Great for a performance street truck, racing, or sled pulling
- Fully rebuilt and calibrated to ensure complete satisfaction
- Modifications are made to the aneroid for valet driving conditions
- Kit Includes:
- Injection Pump
- Hardware Installation Kit
- L Cummins /
CORE CHARGE-WE CHARGE $ AND YOU WILL GET THAT MONEY BACK IF YOU SEND US THE EXACT PUMP NUMBER YOU ARE PURCHASING. WE HAVE TO HAVE LIKE FOR LIKE CORES RETURNED. IF NOT, YOU WILL NOT GET ANY CREDIT FOR THE CORE YOU RETURN.
|Dimensions||24 × 12 × 13 in|
A Core Charge of $ applies to this part. For more information, CLICK HERE
This is a performance part
WARNING: Cancer and Reproductive Harm – www.P65Warnings.ca.gov
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Dodge L L Cummins - VE, P, VP44 and CP3 Pumps- Chevy GMC Duramax L - CP3 Pumps Supersedes old part # IISPPItem #: Industrial InjectionVE6 Injection Pumps - Dodge L|VP44 Injection Pumps - Dodge 24V|Performance P Injection Pumps - Dodge L|Stock P Injection Pumps - Dodge L|Industrial Injection
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- Dodge L 12V Cummins Manual Transmission This item requires a Core Return or Core Charge Please see link below for mItem #: Bosch Diesel PartsStock P Injection Pumps - Dodge L
- Dodge L 12V Cummins California Emissions Version This item requires a Core Return or Core Charge Please see link below fItem #: Bosch Diesel PartsStock P Injection Pumps - Dodge L
- Dodge L Cummins with P PumpItem #: Performance Diesel PartsPerformance P Injection Pumps - Dodge L|Stock P Injection Pumps - Dodge L|Performance Diesel Parts
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14mm Sealing Washer VP44 and P Fuel Injection Pump Fuel Inlet and Outlet FittingsItem #: Interstate-McBeeVP44 Injection Pumps - Dodge 24V|Performance P Injection Pumps - Dodge L|Stock P Injection Pumps - Dodge L
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L Dodge Cummins Reman P Pump
Reman P Fuel Pump for Dodge L
Pump is built to order If ordered RED or BLUE Freight they will be shipped in approximately days.
$ Core Charge
*Never pay a Core Charge if returned within 30 days*
Please read our Core Policy
Please specify Automatic or Manual in the notes when placing your order, We need to know this in order to ensure you receive the correct pump.
All pumps are disassembled, cleaned & inspected, all worn or damaged parts are replaced with genuine original replacement parts. The pump is then reassembled and tested to meet or exceed factory specifications. Pump is built to stock specifications
*Performance Pumps & other Upgrades are available* Please Call
|Dimensions||5 × 5 × 5 in|
Industrial Injection Remanufactured P Injection Pump
The part/product that you are purchasing is an obsolete part. Obsolete parts are items that are no longer available from the manufacturer and require a core part to rebuild to your specifications. The core must be the part number you are requesting to build. Any other core sent in may not have the required parameters needed to build to the specifications requested. You must send your core in to be repaired and returned to you. This requires an RGA number from Industrial Injection.
The Industrial Injection P Fuel Injection Pump for your 12 Valve Dodge Cummins has been built with brand new Factory OEM parts and calibrated to OEM specs using our in house Bosch test equipment. All of our technicians are Bosch trained and certified. This is a direct replacement stock pump and comes with a 1 year Unlimited Mileage Warranty.
Rebuilt With New OEM Parts
Fully Calibrated & Tested On Bosch Machines
1 Year Unlimited Mileage Warranty
|Dimensions||24 × 12 × 12 in|
WARNING: Cancer and Reproductive Harm – www.P65Warnings.ca.gov
A Core Charge of $ applies to this part. For more information, CLICK HERE
HP (Manual Trans), HP (Auto Trans), HP (Auto Trans), HP California Emissions, HP (Manual Trans)
|Repair & Return|
I understand that this product must be shipped to Industrial Injection, repaired, then returned to me and that I must obtain an RGA number from Industrial Injection.
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