Jeep Wrangler driving off-road with a 4BT Cummins engine
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Outer Limits: How Much Horsepower Can You Make With The 4BT Cummins?

By Featured Writer: Mike McGlothlin

Thanks to sharing much of its mechanical makeup (and even some of the exact same hard-parts) with its big brother, the 6BT, the 4BT Cummins can support a substantial amount of additional horsepower. Exactly how much remains to be seen, as specific parts combinations vary from application to application. However, we can unequivocally say that when it comes to pursuing an all-out horsepower effort with the 4BT, everything starts with the injection pump—and there is no debate as to which pump you need. It has to be the Bosch P7100. No, not the single plunger VE rotary and no, not even the A-series inline pump. Of course, you’ll need more than simply the right injection pump. But like the 6BT, everything revolves around the “P-pump.”

In our experience, the 4BT’s stock rotating assembly—which was only subjected to 105 hp in factory form—can handle up to 400 horsepower and 800 lb-ft of torque without breaking a sweat. In more serious builds, we’ve seen balanced and blueprinted 4BT’s with lowered compression, head work, head studs, properly spec’d turbochargers, and the right fueling modifications live at more than 700 hp. For two examples of the latter and one of the former, we’re bringing three high horsepower 4BT builds to life below. These real-world applications, each one specifically built to handle its own unique task, never skip a beat—whether they’re at the drag strip, blasting through the mud in a Jeep, or hauling hay in a pickup truck.


Compound Turbocharged 4BT Square Body

Brown Cummins 4BT Compound Turbochargers

When your love for square body GM’s runs deep and you’ve got a lot of work to do on the family farm, a 4BT swap makes perfect sense. After this Missouri-based ‘85 Bow Tie spent some time at LinCo Diesel Performance, the compound turbocharged, 4BT-equipped farmhand makes more than 400 horsepower. It also performs its towing and hauling duties with ease thanks to being specifically built to run cool under load. The combination of a Holset HX25 (the factory 4BT turbo) over a BorgWarner-based S363 from Stainless Diesel—in conjunction with a sizeable, second-generation Dodge intercooler—help ensure that EGT never crests 1,100 degrees F. Of course, registering 70-psi on the boost gauge is always possible between chores…


Two Turbos—The Preferred Path to Power

Brown-Colored Holset HX25 4BT Cummins TurbochargerEver wondered why compound turbocharging is so common in the world of high horsepower 4BT’s? It boils down to the 3.9L engine’s lack of displacement. While the 6BT can bring a large single turbo to life in a reasonable manner, its 359 cubic inches play a big part in it. The 4BT, with its much smaller, 239 ci displacement, lacks the same low rpm torque that helps the 6BT spool a larger turbo. Mild head porting and larger valves can greatly improve spool up on a single turbo 4BT, but if that work hasn’t been performed and the owner is pursuing big horsepower then compound turbocharging is the only productive way to achieve sufficient drivability at low rpm.


Maxed-Out Yet Street-Friendly P7100

Blue Bosch P7100 Cummins 4BTSurprise, surprise, a Bosch P7100 fuels the square body Chevy mentioned above, but it’s not your ordinary P-pump. Built and benched by Northeast Diesel Service, it’s about as radical as a streetable P7100 for a 4BT Cummins gets. It flows 425cc’s of fuel—which is a lot when you consider the 6BT’s factory P7100 flowed just 135 to 180cc’s, depending on model. The worked over 12mm pump features fresh plungers and barrels, a high-rpm governor spring kit, a few degrees of timing advancement, and has been paired with a set of larger, 5-hole nozzle injectors.


Like 6BT, Like 4BT

Live Cummins Fuel Control of a 4BT CumminsThis is one aftermarket component that works wonders for the P-pumped 12-valve 6BT Cummins and that can be directly applied to 4BT applications. It’s called AFC Live and it offers in-cab fueling control over the P7100. Specifically, the fuel rate knob (on the right, next to the fuel pressure gauge) affords you the ability to change the pump’s fuel rate (via rack travel) on-the-fly. This means you can control smoke output and custom-tailor your fuel curve to match your turbocharger(s) performance. The fuel control knob on the left is a great tool for controlling EGT.


Head Gasket Insurance: Head Studs

Black Head Studs of a 4BT CumminsSix head bolts per cylinder is nice, but once elevated boost and cylinder pressure are part of the equation in a 4BT Cummins, further reinforcement is necessary to guard against a blown head gasket. Here, you can see that the head bolts on this 4BT have been replaced with studs, in particular the studs, nuts, and washers offered by ARP (PN 247-4206). The compound turbo setup on the aforementioned square body Chevy makes 70-psi of boost, which means the engine is borderline in-need of fire-rings or O-rings to help ensure combustion is always contained.


The 4BT Ford Victoria Mud Buggy

White Off-Road Ford Victoria Cummins 4BTOne of the more extreme 4BT Cummins builds could be found in the mud pit. South Bend Clutch assembled this ’32 Ford Victoria with the goal of going head-to-head with the big-block gasoline competition. Thanks to its balanced and blueprinted, low-compression engine, heavy fueling, and compound turbocharger arrangement, the 4BT definitely held its own. Rumor has it that the little Cummins belted out more than 700 horsepower on fuel, and much more than that on nitrous—which must’ve been accurate for the buggy to run neck-and-neck with the kind of fierce V-8 competition it was up against every weekend. South Bend has since retired its mud-blasting buggy, but it remains one of the most popular higher horsepower 4BT Cummins applications ever pieced together.


13mm P7100

Black P7100 P Pump 4BT CumminsThe P7100 that fueled South Bend’s ’32 Victoria represented the quintessential hot-rod pump. Spending time at Northeast Diesel Service, the 12mm P7100 was transformed into a competition-ready P-pump, complete with 13mm plungers, high-rev governor springs, and the ability to fuel higher than 5,000 rpm. Oversized injection lines supplied excess fuel volume to the high-pressure injectors positioned above each piston. The injectors themselves were equipped with 5 x 0.025-inch nozzles, a nozzle size you typically only find on a 1,500hp 6BT Cummins.


Garrett And Holset Turbo Combo

Black K31 Garrett Turbocharger Cummins

Boost production for the mud buggy began with a Garrett K31, a turbo which left the assembly line with a 71mm inducer compressor wheel. The second stage of compression took place by way of a Holset HX40 (a 60mm turbo originally found on the 8.3L 6CT Cummins) bolted to the exhaust manifold. To keep boost from sky-rocketing, especially with a load of nitrous oxide in the mix, peak boost was limited through the use of externally mounted wastegates.


800 Horsepower 4BT Ranger

Gray Compound Turbo 4BT Cummins Ranger

Years ago, when diesel drag racing was just beginning to pick up steam, Wayne Robbins decided to campaign something unique in a world dominated by the 5.9L Cummins. His lightweight Ford Ranger was powered by a 4BT and he swung for the fences. Built for utmost strength and longevity at TFS Performance, the engine’s main bearing saddles and caps were line-bored, the crankshaft was balanced, and the block was bored with a torque plate installed. The rest of the no-frills 4BT build entailed 11:1 compression Arias pistons, a custom roller camshaft from Scheid Diesel, 9/16-inch head studs, and a cylinder head that benefitted from oversized valves. The original BorgWarner S200 over S300 compound turbo system yielded 707 hp on the dyno, but later on an S300/S400 turbo upsizing uncorked the 4BT to the tune of 824 hp (along with 1,267 lb-ft of torque).


The Mini Monster

Gray 4BT P7100 Cummins of a Ford RangerIn order to support north of 800 hp, Robbins had Shiver Diesel build and bench the P7100. Coined the “mini monster,” its maximum flow checked in at a whopping 750cc’s and—as was evident from the truck’s 5,300-rpm shift points—full fueling was available well past 5,000 rpm. Competition injectors with dual-feed bodies took care of in-cylinder fuel distribution. Also notice the steel bar linking the intake elbow to the cold-side intercooler pipe. This is called an intercooler strap and it’s designed to ensure the intake elbow and charge pipe never separate. Trust us, with 100-psi of boost on tap (on fuel alone) and 140-psi of boost on nitrous, an intercooler strap wasn’t a suggestion, it was mandatory.


9-Second Quarter-Miles

Black Smoke Coming Out of a Ford Ranger 4BT Cummins Diesel EngineBack-halved, lowered, and graced with a rear Moser 9-inch and slicks, Robbins’ 4BT-powered ’98 Ranger dipped into the 9’s in the quarter-mile. While a few knick-knack issues surfaced with the truck, none of them were engine-related. The 4BT proved durable—even while regularly seeing triple-digit boost pressure. Robbins’ Ranger remains one of the best examples of just how far the 4BT platform can be pushed.


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