Jeep Wrangler driving off-road with a 4BT Cummins engine
Call us

Cummins 4BT Engine Jeep Swaps

Cummins 4BT Engine History

Cummins 4BT Engine Inside a Jeep/CarThe Cummins 4BT engine (3.9L) was originally developed for industrial applications in conjuction with Consolidated Diesel Corporation (i.e. Case). This small displacement diesel made its debut in 1983 as  part of the popular B Series line of Cummins Engines. Many of you know the 4BT was as the clattering workhorse engine installed primarily in Bread Trucks and Step Vans, hauling freight around cities. However, the Cummins 4BT found a happy home in a multitude of applications, both on and off the highway. Power generation, construction equipment, forestry, and even marine applications utilized 4BT power—and in each and every configuration the engine enjoyed high durability and minimal maintenance costs.

A Simplistic (Yet Robust) Design

Built similarly to the 6BT (its bigger, 5.9L brother), the Cummins 4BT sports a deep-skirt, cast-iron parent bore block, a cast-iron, cross-flow cylinder head with two valves and six head bolts per cylinder, and benefits from the crankshaft, camshaft, oil pump and accessory drive system all being gear driven. Also like the 5.9L Cummins, the 4BT’s crankshaft is made of forged-steel, with its fillets and journals treated to induction hardening for increased wear resistance. Large diameter, 14mm main cap bolts also serve to secure the crankshaft in place. The same exact connecting rods (albeit two fewer than what you’ll find in the 6BT) are present in the 4BT, too, with their I-beam design and forged-steel construction built specifically to handle the kind of immense cylinder pressure produced with big low-rpm torque.

Old Reliable

On paper, the Cummins 4BT’s B50 life expectancy before overhaul is roughly 400,000 miles. However, out in the real world—and when properly maintained—it’s common for a 4BT to last significantly longer than that before the factory seal is broken. Some owners even swear by the 4BT’s ability to offer a million miles of service. Other key reasons for the 3.9L Cummins’ long-term durability are its use of zero electronics (outside of the starter and fuel shut-off solenoid), a fixed geometry turbocharger, and a fully mechanical injection system, from tank to engine. The standard Cummins 4BT was produced from 1983 to 1998. After 1998, the engine gained a 16-valve cylinder head, a rotary injection pump, an ECM, and changed names to the ISB 4BTA.

Unsurprisingly, the off-roading community began swapping Cummins 4BT Engines into Jeeps as far back as the 1980s, and the little Cummins continues to appeal to them for four primary reasons:

  • It provides a significant torque ratio improvement over comparable gas engines
  • It's the biggest diesel engine that will fit in most Jeep applications
  • It offers vastly improved fuel economy
  • It enjoys a long life expectancy, and room for performance gains with simple upgrades

Engine Specs and Fuel Economy

Two Jeeps Off-Roading on a Rocky MountainAlthough the Cummins 4BT isn't known for its raw horsepower, only producing 105 horsepower at 2,300 rpm in standard form, its torque output is 265 lb-ft of torque at a low 1,600 rpm. Sprinkle in a few common performance upgrades and this engine can produce 750 lb-ft of torque (more on the subject of added performance below). When it comes to fuel economy, the 4BT’s abundance of off-idle torque enables effortless acceleration while minimizing fuel consumption. A standard output 4BT swap (105 hp and 265 lb-ft) will yield 18-20 mpg in the city and 22-25 mph highway. Manual transmission applications, higher (lower numeric) gearing, and the right performance mods can stretch fuel economy into 30-mpg territory. The combination of low-end torque and great fuel efficiency make the 4BT a natural choice for both low-speed maneuvering and driving to the trail head.

Rotary Pump Performance

The rotary style (also known as distributor style) injection pump (namely the Bosch VE, although a VP-14 pump was also available) is known to offer a slight edge in fuel economy over the inline P7100 variant (i.e. the P-pump). This is because the rotary pump’s circular cam plate design allows for a constant backpressure of fuel flow, which results in better fuel economy. On the other hand, the VE pump doesn’t have as much potential in the horsepower department. That said, there are several “go-to” modifications that can unlock a substantial amount of extra power—and the first one is free! By gaining access to and turning the VE pump’s maximum fuel screw in, more than 30 hp can be unlocked. From there, a higher rpm governor spring will allow the added horsepower from the increased fueling to carry the power band out further. Another inexpensive mod is to modify or purchase an aftermarket fuel pin (or fuel cone), which helps the VE fuel harder as rpm rises. After that, larger injectors and a higher-flowing turbo enter the picture, but require a much bigger budget.

Inline (P-pump) Performance

Cummins 4BT Engine Diesel Pump

On Cummins 4BT engines equipped with an inline, plunger pump, the P7100, P3000 and A pump were all offered. Of course, not unlike its six-cylinder sibling (the 6BT), the most coveted of pumps is the Bosch P7100. This pump is the king of free and easy horsepower. With simple hand tools and minimal effort, the pump’s air fuel control housing can be moved forward and its pre-boost screw backed off, which can result in gains of as much as 130 lb-ft of torque. The cheap or free mods continue with an aftermarket rack plug (which increases rack travel, thereby allowing more fuel to enter the plungers and barrels) and removal of the fuel plate (for even more rack travel). Beyond that, delivery valves, delivery valve holders, and higher rpm governor spring kits provide great bang-for-the-buck, not to mention the fact that an advancement in the pump’s timing will wake up a 4BT engine immensely.

Common 4BT Swap Vehicles

The Cummins 4BT Engine can be swapped into almost any 4 x 4 vehicle. Common swap canidates include: Jeep Wranglers, Jeep Grand Cherokees, Land Rovers, Toyota FJ Cruisers, Toyota 4Runners, Nissan Xterras, Mercedes-Benz G500s, Ford F-150s, Ford Broncos , Ford Rangers, and even Chevy Colorados. The engine is roughly a third of the weight of a Jeep Wrangler, so heavy-duty suspension is must when installing this engine in that application. The axles must also be upsized or upgraded. Half-ton truck axles are a bare minimum for the 800 pound engine. Dana 44s or Dana 60s are common axel choices in the4BT swap community. For bigger trucks or SUVs, the added weight shouldn't be a huge issue. However, it is always a wise decision to A Red SUV in a Desertmake sure the chassis is balanced and can properly hold the additional weight (think stress on springs and shock absorbers, as well as oil pan to axel clearance). By far, the vehicle most commonly converted to 4BT power is a Jeep. Check out some of the most popular OHV Trails in Colorado once you get your 4BT swap up and running!

Common Jeep Wrangler 4BT Swap Models Include:

  • CJ 1976 – 1986
  • YJ 1987 – 1995
  • TJ 1997 – 2006
  • XJ/MJ 1984 – 2001
  • KJ/KK 2002 – 2013
  • WJ/WK/WK2/ZJ 1993 – 2015
  • JK 2007 – Present

Cummins 4BT Engines For Sale

Cummins 4BT Engine Dozer Side ViewAt Big Bear Engine Compny, we offer both long blocks and extended long blocks for sale. As a general guide an extended long block is roughly 75% finished. Extended long blocks are what is known in the engine industry as 'component builds', assembled to specific engine serial numbers and CPLs (critical parts list). A typical component build contains 90% OEM parts, 10% aftermarket parts. Our bread and butter here at Big Bear is replacement industrial assemblies, however we recognize that the enthusiast market enjoys a special relationship with the 4BT. That being said, our extended long blocks are intended for existing assembly replacement for off-road use only. It is up to you to know your local and state regulations with respect to permitting and emissions testing. Each state (and even county) is different; it's best to do your homework before embarking on a vehicle swap. In addition to component builds, we also sell used and remanufactured engines.

  • 4B Long blocks
  • 4BT 105 hp
  • 4BA 130 hp
  • 4BAA 140 hp

Click Here to learn more about what our long block and extended long blocks come with.

A Number of Cummins 4BT Engine and Big Wooden Boxes Take a look at some of our previous builds:

Please note that, while we do specialize in 4BT swaps, we are diesel engine manufacturers and machinists, not performance mechanics. If you are a shop specializing in 4BT swaps let us know and we can ship one of our new component built, used or remanufactured 4BT engines to your facility. Check out some of our installation partners for info on mods and engine installs.

Cummins 4BT Specs

Engine Data

ConfigurationInline-four, four-stroke
Displacement3.9L, 239 ci
Bore x Stroke4.02 inches x 4.72 inches
Cylinder HeadCast Iron, cross-flow design with 2 valves per cylinder
Engine BlockCast Iron, deep skirt design
Firing Order1-3-4-2
Compression Ratio17.5 : 1
InjectionDirect Injection, Mechanical Injection Pump
AspirationTurbocharged, non-intercooled
Valve TrainOHV, 2 valves per cylinder
Weight745-782 lbs with oil
Oil Capacity10 quarts
Governor SpeedVaries with application
Horsepower105 hp @ 2,300 rpm
Torque265 lb-ft @ 1,600 rpm


Give us a call today at 844-340-4114 for immediate sales and support for the Cummins 4BT, Cummins 6BT or CAT 3306. If you have any questions, check out our FAQ Page. We are always ready to help or just listen to your crazy off-roading adventures!

Request A Quote

Cummins 4BT Engines Gallery

4BT 140 HP Build 1
4BT 140 HP Build 3
140 HP 4BT Engine Unit Arrival 2
140 HP 4BT Side Mounted Turbo
140 HP Units Arrival 9
Cummins 4BT 105 HP BBEC 1
Cummins 4BT 105 HP BBEC 2
Cummins 4BT 130 HP BBEC 1
Cummins 4BT 130 HP BBEC 3
Cummins 4BT 130 HP BBEC 4
Cummins 4BT 140 HP BBEC 1
Cummins 4BT 140 HP BBEC 2

Request a Quote

Please fill out the form below with your information and request. A member of our staff will be with you shortly.


Need Help Researching an Engine Swap?

Get our FREE 10-Page Buyer's Guide

    I am looking for an