Jeep Wrangler driving off-road with a 4BT Cummins engine
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P7100 Fuel Pump Modifications for the 8 Valve 4BT

4BT Engine History and Background

4BT 140 HP Build 2

4BT (3.9 L) 140 HP Engine

The Cummins 4BT has mostly been utilized in applications such as box trucks, small industrial vehicles and agricultural equipment. The 4BT is pretty much just a downsized version of the famous 5.9L 12V Cummins as these engines share almost all of their parts such as pistons, connecting rods, injectors and valve train design.

Clearly, what separates them is the difference of two cylinders, two less journals on the crank shaft, a shorter camshaft, two less pistons in the injection pump, and so on...However, these components are also the same as the 6BT, but just designed to fit an inline four cylinder. Due to the commonalities between the two engines there is a huge aftermarket available for the 4BT as many of the 5.9L 12V performance mods can be used in it. One of these transferable modifications is the P7100 pump which is capable of producing over 800- 1000 hp in a full supped up 4BT Build. It is important to curb your expectations before embarking on a P-Pump upgrade. The pump alone is only a small aspect of a performance HP build which also includes lower compression pistons, camshaft improvements, head studs, ported head, aftercoolers, cold air intakes, twin turbos etc...

Design of the P7100

4BT P7100 Inline Fuel Pump

4BT P7100 Inline Fuel Pump

The P7100 pump is designed to mount the engine and work along side the crankshaft via the gear train. Using a time reduction gearset system the gear train is then able to power the pump. Connected to the internal injection pump camshaft is the drive plate, which is the main element in the gear train. The cam then starts triggering numerous systems like the critical timing components which are located in the injection pump cam box. The cam box is the lower portion of the pump which encompasses the camshaft, tappets and central oil sump. Each lifter is accountable for delivering power to a pump component which is guided by a plunger and barrel application. The barrel is immobile (unlike other systems) and is bored with holes in the upper segment of which are exposed to the fuel storage gallery.

Fuel Distribution

When the fuel gallery is primed with fuel, that fuel is then ready to be pumped into the injectors. The deliverance of fuel starts when the plunger is pushed upward by the camshaft and seals the spill port. Now the spill port is closed, pressure in the fuel chamber builds which opens a delivery valve within the pump. Once that valve is opened the fuel shoots through the injector fuel line transmitting fuel at high pressures to the injector nozzle which then pulse pushes the fuel to the cylinder. All this happens in milliseconds of when the plunger exposes the helical recess to the spill overflow port. Then, the fuel pressure drops instantly once the camshaft shifts position and isolates the port.

The pump must be synced with the engine before it can begin to operate. The pump is usually synced with the #1 cylinder. The pumps port closure must be timed to a precise number of degrees on the cylinder and cannot be even a single degree off the crankshaft.

Performance Modifications

The 4BT engine can be enhanced with ease to achieve greater levels of power from its original stock design. This is made possible when installing the inline P7100 pump. The 5.9L Cummins engine is known to be modified with installations such as high-rev governor springs, full-travel rack plugs and modified injectors. But, the 4BT is capable of being modified to this extent also. However, if your 4BT is to undergo these enhancements it is imperative to accommodate the fuel upgrades with other components such as cylinder head and turbocharger upgrades. Luckily, aftermarket workings such as head studs, higher quality valve springs, larger valves, performance exhaust manifolds and camshafts built for four-cylinder diesels are widely available.

Regardless of the fact these modifications can drastically enhance the 4BT, its lack of cylinders and operating rpm range means the pistons will undergo a combustion cycle sooner than a 6BT engine. To that end, camshaft profiles and pump timing is imperative if you want the 4BT to be running at a better efficiency while at higher horsepower levels.
If you are wanting to enhance your 4BT then the P-pump would definitely be the way to go for a performance build. If you are torn between the P-pump and VE pump you must note that the VE pump is a single piston pump, therefore, is limited on what you can get out of it. The P7100 on the other hand, has a piston for every cylinder with a cam setup similar to the valve train on the majority of engines. The P7100 is capable of handling 800-1000 HP to an already robust performance build whereas with the VE/VP Pumps you'll be able to get 250-500 HP with 3200 RPM governor spring modifications. As we said before, the fuel pump is only part of a fantasy 800 HP build and will not put out 800-1000 HP all by itself. With a performance built you'll get 100+ HP here, another 80 HP there with each component upgrade you work on.

How to get the full potential from your P-Pump

So, now you know the ins and outs of the P-pump, you’ll now learn how to get the best out of it and roughly what it will benefit you. The best part is, some of these modifications won’t cost you a single penny.... (but then again, some will).

P7100 AFC Housing

P7100 AFC Housing

Air Fuel Control – Located at the rear of the P7100 is the (AFC) air fuel control assembly. As well as telling the governor arm when to hit the AFC arm, the AFC is also in control of the pumps fuel rate and a low boost. If you slide the AFC housing toward the front of the pump and backing out the pre boost screw, fueling can be pushed in at a considerably lower rpm, the governor linkage is altered and the rack is moved forward. When these alterations are done with a star wheel adjustment you can get as much as 200 lb-ft of torque along with a peak horsepower addition of around 50-60 horsepower. This may leave you eager to discover how to turn the star wheel...

P7100 AFC Housing

P7100 AFC Housing Parts

Turning the star wheelThe star wheel is located in the AFC housing; you should turn it toward the passenger side of the truck to open the fuel rack more. This fuel rack dictates the amount of fuel that’s permitted to enter the plungers and barrels. To access this, you must go through the top of the AFC housing and adjust it in small increments or run the risk of over doing it and causing heavy smoke...or, not doing it enough and not seeing any power gains.

Increasing rack travel – Yes turning the star wheel increases rack travel, however, the factory rack plug will only let certain amounts of fuel in the plungers and barrels. You can replace this with a Mack Rack Plug for about $15 - $20 which will increase the rack travel to 21mm. I know you're thinking that 2mm isn’t exactly a drastic difference, but the Mac Rack Plug will add around 70cc’s to the mix giving between 10 to 35 horse power dependent on your combination of delivery valves, fuel plate set up and injectors etc.

4BT P7100 Fuel Plate

4BT P7100 Fuel Plate

Removing fuel plate – The job of the fuel plate is to control the max fuel output; the OEM one that comes with the P7100, will deprive your engine of some well needed horse power. You can either purchase some aftermarket ones or remove it completely to gain around 35-40 hp due to the increased rack travel. You can read step by step how to remove the fuel plate here.

Governor Spring Kits – Aftermarket governor spring kits should spring to mind when your thinking how to modify your P-Pump, some of the most popular kits can provide fueling up to around 4,000 rpm and even higher springs are available for competition ready pumps that have been professionally set up on a test stand. Fueling at high rpm’s can make a lot of horse power potential achievable. However, do remember that stiffer valve springs must be installed in the head if your wanting to spin the engine over 3,500 rpm.

Plungers and barrels – Engines and P-Pumps alike, they can both have their cylinders bored to add displacement. The P-7100 can be bored to fit larger plungers and barrels which is the technique used by diesel aftermarket extremists! If you increase the diameter of the plungers and barrels by just 1mm it can add extreme fueling potential. To give you an idea, a 12mm stock pump can allow a max of 550cc’s of fuel usually, but a properly setup 13mm unit can bring that up to 850cc’s and beyond.

Cutaway P7100 Pump Governor Spring

Cutaway P7100 Pump Governor Spring

Enhanced camshafts – As you learned in the first sections, the 4BT P7100 utilizes an internal camshaft. The cam has one profile for each plunger and as the cam rotates the plungers and forced up and down within their barrels. If you’re desperately wanting more power, you can swap the stock cam out for a unit with a more aggressive cam, which ultimately means more fuel is injected and at a quicker rate. This can bring injection up by around 11% and also add up to another 80 horse power.

Timing – Injection pump timing can really dictate the overall power output. The P7100 set to the factory timing mark will only benefit a little from the other mods written above. But there is hope, a pump that’s set at 18 to 20 degrees is the “unofficial” sweet spot for timing on a P-Pumped 4BT by which it allows for decent all round power. Driveability and cold start performance. But go to far to around the 24th or 25th degree, the drivability will decline.

More power is certainly a top reason why enthusiasts decide to put a Cummins 4BT in an off-road vehicle... who doesn't love to go fast? However, keep in mind that low end torque is the name of the game when it comes to rock crawlers equipped with a diesel engine. Also note that it is extremely easy to mess up any performance modification and grenade your 4BT. If you go down this road take one performance mod at a time and do so under the guise of a performance mechanic or fuel rebuild shop.

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