In the off-road community engine swaps can be a touchy subject. Most of the time torque is what you’re looking for as that’s what will allow you to turn big tires at low RPMs. Because of that, Chevrolet LS engines are pretty common, especially the truck version of the LS family. But, diesel engines, specifically the 4BT Cummins, are becoming increasingly popular. It’s not just off-roaders who are using this engine. It can also be found in everything from 240SX drift cars to Mustang drag cars.
Before I tell you everything you need to know about this engine, I must first inform you of the absolute the basics of the 4BT. You can find even more 4BT information on Wikipedia.
4BT Cummins: Engine Basics
In the world of “light-duty” trucks, the 6BT Cummins, also known as the 12-valve Cummins, is legendary. The large size of the 6BT meant that it couldn’t fit in vehicles or equipment where there wasn’t a massive amount of space which is where the 4BT Cummins comes into play. The 4BT shares most of its components with the 6BT, but with two cylinders missing. The pistons, injectors, connecting rods, and valvetrain design are straight off of the 12v Cummins. Even though the 4BT is significantly shorter than the 6BT, it’s still a large engine, especially for a four-cylinder.
- Displacement: 3.9L – 292ci
- Bore: 4.02″
- Stroke: 4.72″
- Configuration: Inline Four-Cylinder
- Deck: Closed Deck
- Weight: 750+ lbs
- Cylinder Head Material: Cast Iron
- Engine Block Material: Cast Iron
- Compression Ratio: 17.5:1
- Valvetrain: OHV – 2 Valves per Cylinder
- Horsepower: 105hp (4BT) – 170hp (4BTA)
- Torque: 265lb-ft (4BT) – 420lb-ft (4BTA)
Out of the units listed above, weight is the most surprising. While most four cylinder come in around 400-600 lbs, the 4BT weighs a hefty 750+ lbs (the exact number depends on engine dressings and fluid level.) Horsepower may seem relatively low, especially for a 3.9L engine, but you must remember that the 4BT was designed for low-end torque and superb reliability. The post-1998 version, 4BTA, had a significant power bump thanks to its jump from eight to sixteen valves.
4BT Cummins: Real World Applications
Thanks to its size, power output, and mechanical fuel injection system, the 4BT can be found in a plethora of different applications. It’s not uncommon to see this engine in industrial equipment. The most common place you’ll find a 4BT is in bread trucks. These are large trucks which may have a relatively heavy payload, but have a tiny engine bay, making the 4BT the obvious choice. There is not any official list of 4BT applications, but bread trucks are definitely the most common application. Modern applications have since moved on to newer and more efficient diesel engines including the R2.8 Cummins.
4BT Cummins: Tuning Potential
The 4BT is by no means a performance engine. As we said above it was used in applications such as bread trucks and the numbers show it. Outputting a measly 105 horsepower @ 2,300 RPM and 265 lb-ft @ 1,600 RPM, it’s far from a screamer. Of course, the 6BT is also very weak straight from the factory, but with some modifications to the fuel system and fuel timing, big power can easily be achieved. The common 6BT fuel modifications apply to the 4BT, as they share the same fuel pump and injectors.
If you want big horsepower, a turbocharger upgrade is a great place to start. An HX35, HE341, or HE351 are great budget options which will flow significantly more air than the stock 4BT turbocharger at the cost of throttle response. All 4BT applications lack an intercooler which is great for packaging constraints but terrible for power. An intercooler is a great way to pick up horsepower and improve power consistency by keeping charge air temperatures lower.
As far as fueling is concerned, upgraded injectors are an excellent place to start, but are not necessary for small power increases. The P7100 fuel injection pump is the same one found in the 6BT. Sliding the Air Fuel Control assembly forward will result in power gains by adding fuel at low RPM. You can also ground down or remove the fuel plate which is what controls the maximum fueling. By sliding the AFC assembly forward and grounding down the fuel plate, you can see gains up to 100hp and 200+ ft-lbs. Fuel timing is crucial on the 4BT, so be sure to set it right, or you may have a blown engine on your hands.
Past the basic fueling mods, aftermarket governor springs are a great way to pick up power. The OEM governor springs of the P7100 pump limit the engine to 2,700rpm with de-fueling starting at 2,400rpm. Aftermarket governor springs will allow full fueling up to 3,000rpm or even 4,000rpm depending on what spring kit you purchase. It should be noted that four-cylinder engines are not as inherently balanced as six-cylinder engines, so don’t expect to rev a modified 4BT engine as high as a modified 6BT engine.
Tuning potential depends entirely upon budget, but it’s not too hard to get a 4BT up to 300 horsepower and 700lb-ft. It should be noted that the fueling modifications listed above apply only to 4BT engines with the Bosch P7100 pump. Models with the later VE-pump require different modifications. Check out DrivingLine’s P-Pump Cummins fuel modifications for more detailed information.
4BT Cummins: Why Swap?
So from everything, I’ve told you so far, why would you want to swap a 4BT into your vehicle? If you’re able to fit a 6BT 12 valve into your vehicle, then that is probably the way better option. It’ll make more power, last just as long, and might be cheaper to buy.
The real reason you might want to swap a 4BT is for fuel mileage, and size constants. A 4BT barely fits into an XJ Cherokee with a 6″+ lift, so a 6BT is definitely out of the question for many Jeep owners. If fuel mileage is your legitimate reason for swapping, then the 1.9 TDI from VW is a far better option.
So what have we learned so far? The 4BT came in heavy duty applications such as bread trucks, it’s nearly identical to the 12-valve, it responds to modifications just like a 12-valve, and it’s incredibly reliable. Many 4BT engines have gone way past 300,000 miles.
Jeep people love the 4BT because it is pretty much the best diesel that can fit inside of a Jeep other than the much less common 1.9 TDI swap. Let me know what you think of the 4BT in the comments below! Also check out our 6BT Cummins: Everything You Need to Know article!
One of the most common Diesel engines used on diesel conversions is the 3.9L 4BT Inline 4-cylinder Cummins engine. It’s extreme durable like its 6BT relative but it takes up significantly less space. We see a ton of 4BT engines swapped into Jeeps, SUVs, and even cars. In addition to their small size, the 3.9L 4BT Cummins diesel engine offers other advantages as well, such as impressive performance specs and the ability to last hundreds of thousands of miles. They’re also fairly easy to locate!
If you are looking for the perfect Diesel engine to swap into your project vehicle, the 4BT Cummins engine might be the right choice for you. To help our readers decide, we create this post, our comprehensive 3.9L 4BT Cummins Guide. In it, we cover 4BT Cummins engine specs, performance attributes, and where these engines can be found for sale. Let’s get into it!
3.9L 4BT Cummins Engine Design
The 3.9L 4BT Cummins is a diesel turbocharged engine that utilizes an inline 4-cylinder design. The acronym, 4BT, means four-cylinder, “B” series turbocharged. It shares many of the same components as the 6BT Cummins, the 12-Valve 5.9L Cummins engine found in the 1st Generation Cummins. In fact, if you take away two cylinders you almost have the same engine.
The 4BT engine uses the same pistons, connecting rods, injectors, injection pump and valvetrain as the famous 12V Cummins. Both the engine block and the cylinder head are made of cast iron. This results in a durable foundation capable of withstanding serious abuse. When the 4BT Cummins was released in 1983, it featured 2 valves per cylinder, for a grand total of 8 valves. After 1998, these engines used a four valve per cylinder design, for a total of 16 valves. This is one of the updates that helped second generation 4BTA Cummins engines produce greater performance specs.
The 4BT engine is ideal for engine swaps in smaller vehicles because of its relatively small size. It’s smaller than many V-8 engines commonly used for swaps and offers significantly better fuel efficiency. The 4BT measures in at 30.6″ in length, 24.6″ in width, and 37.7″ in height.
Just like the 6BT 12-Valve 5.9L Cummins Engine, the 4BT is rock solid. It is overbuilt, especially for its size. They also last an extremely long time. It’s also essentially service free thanks to the fact that it is gear-driven. Its camshaft, oil pump, injection pump, and accessory drive systems are all gear-driven, meaning there is no timing belt or chain. Early 4BT engines (Pre-1998) also lack electronics, adding to their reliability and simplicity.
4BT Cummins Performance
The 4BT Cummins Engine was originally designed for agricultural, industrial, and light commercial applications. First generation 3.9L 4BT Cummins Diesel engines produce 105 horsepower and 265 lb-ft of torque. Second generation engines, known as 4BTA engines, use 4 valves per cylinder, and produce higher performance numbers. The most common iterations of the 4BTA Cummins Diesel engine produce 170 horsepower and 420 lb-ft of torque.
The 4BT Cummins Engine was way ahead of its time when it was released in the ’80s. It uses direct fuel injection, turbocharging, and an inline-4 design to provide impressive performance specs and fuel efficiency in a tiny package. The only down side of the 4BT is that it runs relatively rough when compared to 6BT engines. This is because the 4BT isn’t balanced, meaning it’s prone to a lot of vibration.
While these numbers are a fraction of what the 12V or 24V Cummins engines produce from the factory, these numbers can go up quick! This is especially the case on 4BT engines that utilize a Bosch P7100, or P-Pump fuel injection pump. By modifying the p-pump injection pump, you can get extra horsepower/torque very easily.
You can improve 4BT Cummins performance specs easily because they share many of the same internals as 12V 5.9L Cummins engines. 4BT Cummins engines are frequently used in swaps in which a Diesel engine is added to a vehicle like a Jeep Wrangler or a light-duty pickup truck. In these cases, the engines are often modified for greater than stock performance. There are a ton of 4BT Cummins performance upgrades out there like modified injectors, turbochargers, and others. Aftermarket intercoolers are a must-have as these engines don’t come with one from the factory. Many people also upgrade them for greater durability by using ARP head studs.
Cummins diesel engines are known to be fuel efficient by most diesel truck enthusiasts. This engine holds that stereo-type to be true. The combination of this engine’s design, turbocharging, and direct fuel injection make the 4BT very fuel efficient. This is a major reason in why this engine is commonly used in Jeep Wranglers or even old classic cars. It can offer mpg in the high-teens or 20s in many applications.
3.9L 4BT Cummins Engine Specs
1st Gen 4BT
2nd Gen 4 BTA
|Engine Design:||Inline 4-Cylinder Turbo-Diesel|
|Transmissions:||Built with a variety of Flywheel Options to Accommodate a wide variety of applications|
|Displacement:||3.9 Liters or 239 cubic inches (Ci)|
|Engine Weight:||750-800 lbs|
|Bore:||4.02 inches or 102 mm|
|Stroke:||4.72 inches or 119 mm|
|Cylinder Head:||Cast Iron|
|Engine Block:||Cast Iron|
|Firing Order:|| 1-3-4-2|
|Fuel Injection: |
|Originally supplied was a Bosch |
Mechanical Direct Injection system,
using an in-line rotary injection pump. Some engines
feature a P7100 Injection Pump, also known
as the p-pump. Second Generations of the
4BT use an electronically controlled fuel
|Aspiration:||Turbocharged, No Intercooler|
|Valvetrain:||OHV 2 Valves per cylinder (First Gen 4BT)|
OHV 4 Valves per Cylinder (Second Gen 4BTA)
|Engine Oil Capacity:||10 Quarts|
|Governed Speed:||Varies by Application|
|Horsepower (Varies):||First Gen: 105 Horsepower @ 2,300 Rpm|
Second Gen (4BTA): 170 Horsepower
|Torque (Varies):||First Gen: 265 lb-ft @ 1,600 rpm|
Second Gen (4BTA): 420 lb-ft
3.9L 4BT Cummins Engines For Sale
Where is the best Place to find a 4BT Cummins Engine for sale? This is a frequently asked because so many people are after these engines to swap into their project vehicles. Thankfully, because these engines were so widely used there are a ton out there. Unfortunately while there are plenty that exist, many of these engines are old, beaten up, and need some love to get back to their former glory days. Even used engines are likely to cost you a pretty-penny though! Many people have realized the value and potential of 4BT Cummins engines so even abused engines are still expensive.
If you are looking for a good deal on a 4BT engine, check out our list of the 4 best places to find a 3.9L 4BT Cummins Engine for sale.
If you’re looking to find a 4BT Cummins Engine for a good price, you should go to Craigslist first. Simply search “3.9L 4BT Cummins Engine” in the search box on Craigslist.com for your area and click the search button. You’ll be given results in your local area for these motors. Be cautious though, as you can often find motors in need of serious work. This is the best option for very mechanically inclined people who plan to build their vehicle on their own.
You can often also find completed project vehicles that already have a 4BT Cummins installed. This is an easier route that will likely save you tons of time and headaches when compared to building a project vehicle from the ground-up. Check out this jeep we found.
2. Diesel Forums
Another good way to find a 4BT Cummins for sale is by posting and searching in diesel truck forums. Try focusing on ones specifically for Cummins-equipped vehicles. A lot of diesel enthusiasts have one or more projects laying around, and sometimes the one doesn’t even get finished. This is a good way to land one that might even have been modified already!
If you’re looking for a 4BT engine and you’ve had little success so far, I strongly recommend giving this company a shout. Big Bear Engine Company specializes in 4BT and 6BT Cummins engines and offer overhaul kits, used engines, and even high-quality basically new 4BT engines. They even offer a ton of knowledge on these engines for free on their blog! If you have the money to spend, this would probably be the preferred route that I recommend the most. It will also just likely be the most expensive.
I’m not a huge believer in Ebay myself, but they do have a large selection of 4BT engines available for sale. This would be my last resort. Just be very cautious about buying engines here, I have not done it before my self. It’s much better to find the engine locally that way you can properly inspect it before forking over a large chunk of change.
Can’t Find a 4BT Cummins Engine for Sale?
If you can’t find a quality 4BT Cummins Engine near you, another alternative is to consider using the R2.8L Cummins Engine. While it does come with some basic emissions devices, a DOC and EGR system, it offers significantly more power than a stock 4BT with a smaller displacement.
4BT Cummins Engine FAQ
The 4BT Cummins Engine has been used in a large variety of commercial, industrial, and agricultural applications. These include bread trucks, skid steers, backhoes, and more. The 4BT is also used in marine applications. People now often swap 4BT Cummins engines into light-duty trucks, jeeps, rat rods, and classic cars.
You will want to tailor your modifications to what you will use this engine in and what it needs to do. You can increase performance by the addition of upgraded injectors, a better turbo, and other typical diesel mods.
The cost of a 4BT Cummins engines varies according to its condition. Before they were well known you could cheaply find them by purchasing old vehicles or equipment they were used in. Now people realize their value and popularity. Costs of a 4BT Cummins in bad shape can range from $1500 to around $7000-$9000 for a practically new engine.
You can purchase 4BT engines on Craigslist, Ebay, Re-manufacturers, or 4BTengines.com. Be careful as condition greatly differs engine-to-engine.
Because there are many different variations of the 4BT actual weight can vary. Most of these engines weigh between 750 and 800 lbs.
4BT Cummins engines produce 105 horsepower at 2,300 rpm and 265 lb-ft of torque at only 1,600 rpm. This is thanks to direct fuel injection and turbocharging. 4BT Cummins performance specs can be improved easily, especially in models equipped with the P-pump or P7100 injection pump.
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Founder of Diesel Resource and a complete diesel head. Has a little bit of problem buying too many trucks. Learn more about him by checking out his truck.
Cummins 4BT 101 | BASICS AND PERFORMANCE
4BT BASICS AND PERFORMANCE
THE POPULARITY OF diesel-powered vehicles here in the U.S. has jumped dramatically in recent years. More and more manufacturers are offering new diesel power plants in ½-ton trucks, cars and midsize SUVs. Maybe the reasons for the boost are better highway fuel efficiency and the giant increase in torque offered by a diesel engine, but it’s probably an attempt to meet everincreasing CAFE (Corporate Average Fuel Economy) numbers. Either way, these smaller V-6 and four-cylinder engines will be sticking around. But back in the day, it was Cummins that developed its own small diesel engine market with the 3.9L 4BT.
The 4BT—which stands for “fourcylinder B series turbocharged”— was used mostly in midsize box trucks, agricultural equipment and small industrial vehicles, and is basically a smaller version of the popular 5.9L 12V Cummins found in 1989-98 Dodge trucks. The 4BT shares virtually all of its parts with its big brother, the 6BT (e.g. pistons, connecting rods, injectors and valve-train design).
The 3.9L 4BT Cummins diesel engine has become an extremely popular platform for conversion projects throughout the past couple of years, and for good reason. Sharing virtually every internal and external piece with its big brother, the 5.9L 12V Cummins found in the 1989-98 Dodge Ram pickups, and coming as regular equipment in hundreds of commercial and industrial applications, parts availability, power potential, fuel mileage and the lack of electronics make it an easy go-to engine when it’s time to squeeze a diesel platform into the frame rails of just about anything.
The major internal difference comes from the lack of two cylinders, which means two fewer journals on the crankshaft, a shorter camshaft, two fewer pistons in the injection pump, etc. But even these pieces are the same as the 6BT, just in an inline four-cylinder version. Because of these similarities, the 4BT Cummins has an extremely strong aftermarket, since many 5.9L 12V performance parts can be used in smaller 3.9L engines. This, paired with the somewhat compact size of the 4BT, makes it a great candidate as a transplant engine for just about any vehicle. We’ve seen 4BTs in everything from Jeeps to old-school rat rods. The simplicity of the mechanical injection system also helps make it an easy swap because there’s little to no wiring or electronics required.
The 4BT engine has been used in commercial utility trucks and industrial construction equipment since the late-’80s, and because of its extremely basic functionality, it works well as a transplant engine when space is tight. With the cylinder head off the 4BT engine, someone with 12V Cummins experience should feel right at home. The 4BT is virtually the same, just two pistons shy.
Since the engine was originally developed for industrial applications, big horsepower wasn’t a major concern, and the 239-cid motor most commonly came from the factory at around 105 hp at 2,300 rpm and 265 lb-ft of torque at 1,600 rpm. But these numbers could vary as much as 150 hp depending on the application.
The 4BT engine uses a simple two-valve-percylinder system, one for intake and one for exhaust. While this design became somewhat outdated with the introduction of the fourvalve- per-cylinder design found in the 1998.5- plus 6BT engines, it’s still a sufficient and robust design, especially on the smaller cubic inch, low 130-180-hp applications for which most of the 4BT engines were designed.
The 4BT engine weighs 750- 780 pounds, which is heavier than most of the gas engines it would replace in a conversion project, but much lighter than the sixcylinder 6BT version. The inlinefour also offers a fairly compact size at just 30.6 inches long and 37.7 inches tall, which is useful with space constraints. The engines came with a few different injection pump systems, but the P7100 is, by far, the most popular, since it’s capable of producing larger amounts of fuel and more easily modified by the aftermarket to increase power outputs.
ACD Engines of Salt Lake City is a strong Cummins engine and parts dealer with years of experience in both the 6BT and 4BT platforms. It’s gained a reputation for its 4BT conversion projects and has developed new engine internals to take the average industrial 130-hp four-cylinder diesel to impressive performance heights. Starting with a one-of-a-kind connecting rod and piston design, ACD will soon offer complete Stroker Kits to pump up the power of 4BT platforms. The new H-beam 4340 billet connecting rod (left) was engineered specifically for the 4BT engine. These rods not only help increase engine stroke, they also provide unmatched durability to withstand extreme cylinder pressures from running massive power and torque through these engines. The new H-beam rod (top right) is pictured with a factory 4BT rod (bottom right). An untrained eye may not notice much, but the design and material of the ACD connecting rods is far superior to the standard.
As previously mentioned, the 4BT can easily be modified to produce higher than stock power levels when outfitted with the P7100 (P-pump). Fueling mods can be taken to the same extremes as with the 5.9L 12V Cummins: 12mm and 13mm pumps with laser-cut delivery valves, high-rev governor springs, full-travel rack plugs and modified injectors can all be used in the 4BT platform. To go along with major fuel upgrades, common cylinder head and turbocharger upgrades are required, but the aftermarket already has most of that covered as well with head studs, better valve springs, larger valves and even performance exhaust manifolds and camshafts built for four-cylinder diesels. While these upgrades can take the 4BT to all-new levels, we have to mention that because of its lack of cylinders and nearly identical operating rpm range, each piston is going to undergo a combustion cycle much sooner than that in a 6BT Cummins, so things like camshaft profiles and pump timing become critical for maximum efficiency at higher horsepower levels.
A close-up of the two connecting rods’ piston ends show where some of the additional piston stroke comes from. Notice the much smaller diameter used to encase the piston’s wrist pin. This new connecting rod design runs exclusively with ACD’s custom-forged pistons using a tool-steel wrist pin. The combination of rod and piston raises the 4BT’s piston stroke from a factory 4.72 inches to a massive 5.430 inches. This alone will increase the engine’s cubic inches from an OEM 239 to just over 275, turning the average 3.9L 4BT into a 4.5L engine.
Knowing that customers interested in a 4BT Stroker Kit would want to make as much horsepower as possible, ACD decided robust forged pistons would be the best option to ensure maximum strength and durability. The combination of the forged piston and tool-steel wrist pin, with special coating to reduce wear and friction, should create a near indestructible product. The piston bowls have also been worked over compared to a stock piston to promote a better swirl effect for a cleaner, more efficient burn in the combustion chamber.
ACD Stroker Kit
Like any engine platform, it’s a well-known fact that more cubic inches generally means more power, and there’s only so much a 239-cid motor can produce, at least safely and efficiently. With this in mind, ACD Engines of Salt Lake City has developed its Stroker Kit to take the 4BT to heretofore unheard of performance heights.
LIKE ANY ENGINE PLATFORM, IT’S A WELL-KNOWN FACT THAT MORE CUBIC INCHES GENERALLY MEANS MORE POWER, AND THERE’S ONLY SO MUCH A 239-CID MOTOR CAN PRODUCE, AT LEAST SAFELY AND EFFICIENTLY.”
Also known locally as “All Cummins Engines,” ACD has been a full-line Cummins dealer for more than 20 years, specializing in midrange and industrial engines. It offers virtually everything Cummins: new or remanufactured engines, new genuine Cummins parts, used engines and even salvage parts. Since it deals in nothing but Cummins, ACD stocks many hard-to-find parts, and if it’s not in stock, the staff knows where to get what you need. Through their years of experience, the staff has become extremely wellversed in the 4BT platform and has the conversion process down to a science, inserting the small Cummins engine into Jeeps, small SUVs, pickups and even a mid- ’50s ambulance.
Sitting side by side on the bench, the stock piston on the left looks quite a bit different from the ACD version. The shorter skirt and much higher wrist pin location were used to assist piston stroke, allowing more displacement and performance potential. More cubic inches means more air and fuel can be drawn into the cylinders to create more power.
In the search for more power, owner Robby Pederson began development of a Stroker Kit that would increase the length of stroke and add some additional cubic inches to the inline-four diesel. In the gas world, Stroker engines are nothing new; the GM small-block 383 Stroker engines have been around for nearly 35 years. The 383 is built using a standard GM 350- cid engine block with a modified 400-cid crankshaft, which changes how far the piston travels.
For the new forged piston design, ACD opted for complete gapless piston ring sets to limit cylinder pressure blow-by and keep the combustion power inside the cylinder where it belongs. This ring design is also durable, but block machining is critical, especially with the use of forged pistons. Where a stock cast piston would only need .005-inch clearance between it and the cylinder wall, a forged piston will swell more under heat and stress, so piston-to-wall clearance needs to be increased to keep the rings from sticking and potentially ruining the piston and engine block.
ACD has plans to install this first monster Stroker 4BT engine into the shop’s H1 Hummer that rests on custom-built axles and 40-inch tires. The shop’s aim is to prove the power potential of the little 4BT and its extreme versatility as the perfect diesel conversion engine for an older vehicle with mild power upgrades (like ACD’s camshaft and injection pump). The goal is to increase a stock 4BT from its underwhelming 130 hp to 200 or even 250 hp to a full-on competition 700-plus-hp engine, like this Stroker should become. It all starts with a clean, bare engine block machined to precise cylinder and crank measurements.
New Rods and Pistons
Because the factory 4BT crankshaft is a robust piece, Pederson didn’t want to do much crankshaft work, so he opted for an all-new piston and connecting rod design to gain the additional stroke he was after. A factory 4BT engine runs a 4.02-inch bore with a 4.72-inch stroke; this is how we come to a 239-ci motor. The new parts from ACD will allow the overall piston bore to expand to 4.402 inches with a much longer 5.430-inch stroke, effectively taking engine output to 333 ci, or 5.46L, almost that of the 5.9L 12V Cummins.
While the 5.9L 6BT Cummins can run into block flex and main cap issues at high power levels, the shorter 4BT won’t be as susceptible to those issues. However, ACD still prefers to use a bottomend Gorilla girdle to tie everything together in the lower part of the engine. ARP main studs are used to ensure proper clamping force is torqued onto the girdle and crankshaft’s main caps.
Knowing that the clientele for such a kit would be after extreme power levels, only the best materials were used to ensure the ACD Stroker Kit would stand up to high boost and high cylinder pressures. Rather than use standard-cast pistons, like the stock units, ACD went with a much stronger forged piston design that offers a different bowl design to improve the air/fuel swirl effect, helping create a more efficient burn in the combustion chamber. The piston design also has a much shorter overall height, and the wrist pin location was moved closer to the deck. These modifications account for most of the additional stroke.
On the cylinder head, upgrades to the ACD 4BT are much the same as those found in performance 12V Cummins builds: high-rev valve springs, titanium keepers, chrome-moly pushrods, etc. For this particular application, ACD will also run fully ported and polished intake and exhaust ports along with larger valves to increase the air volume fed to and expelled from the engine. To help those larger valves perform, ACD spent countless hours developing different camshaft profiles that maximize lift and duration in 4BT engines. Since the injection events are so close together with the inline-four design, the right camshaft profile is critical to how these engines perform. ACD can recommend the correct cam for any build, whether for power or fuel efficiency.
In high-revving and high-horsepower engine applications, the factory press-in freeze plug design is a common failure point. To prevent failure, ACD tapped the cylinder head water ports to accept threaded plugs.
To match the new piston, connecting rods were developed and made from billet 4340, like those being used in all of the high-horsepower diesel engines. The new H-beam rod design is not only stronger than a factory connecting rod, but it also has a much smaller wrist pin journal, which will only work with the ACD piston. While the pistons and rods are the true heart of the Stroker Kit, ACD has also developed a host of 4BT parts to complement them and ensure true peak performance is achieved. Specific camshaft profiles were designed, custom cylinder head work was done and 4BT-specific adjustable injection pump timing gears were installed. The short time between injection events on the four-cylinder engine required a slight engineering change for these parts, and ACD thinks its developed the perfect pieces to turn your run-of-the-mill 105-hp 4BT into a tire-shredding 800-hp monster.
Because this engine will run higher boost pressures, the cylinder head will be machined to use a custom fi re-ringed head gasket kit and ARP head studs. The factory rocker pedestals must be machined for clearance to accommodate the stronger ARP hardware.
The Bosch P7100 injection pump has come a long way since its start in 1994-98 Ram trucks. Originally developed to support the mid 200-hp range, the 4BT platform with just four cylinders moved even less fuel because it only needed to support 130-150 hp in most applications. Using knowledge from the massive 12V performance market, those same techniques and modifications were used in the ACD injection pump. This 13mm pump uses laser-cut delivery valves, max-rack travel plug, stronger hold-downs and a custom cam design specific to the 4BT pump to maximize fuel injection under the quicker injection events to which the four-cylinder engine is subject. This particular pump is capable of moving up to 800ccs of fuel, more than enough to move the big H1 Hummer down the street—doing long third-gear burnouts the whole way.
The fuel injectors for the 4BT engine were also reworked to maximize the amount of fuel that could be effi ciently injected and burned in the cylinders. The dual-feed injectors and customhoned nozzles should be more than enough to support ACD’s 800-hp goal. The custom highfl ow injection lines will also be used to eliminate any restrictions between the injection pump delivery valves and the injectors.
ACD Engines understands that the complete Stroker Kit might be more than most 4BT project vehicles need, so it has an array of 4BT-specific products that can do everything from increasing power to improving fuel mileage and engine efficiency. It also carries a full line of custom brackets and conversion pieces to make your conversion or transplant project go a little easier.
ACD will use the OEM Cummins valve covers on this high-horsepower build, which offers dual port ventilation. While the gapless piston rings should cut down on most of the engine’s blow-by, the high-flow breathers in these valve covers will be much more efficient than the individual factory cylinder covers found on most 4BT and 6BT engines.
Since the cylinder head intake and exhaust sides were ported and polished to maximize airflow through the head, it’s only natural to upgrade the exhaust manifold. While there are different OEM-style manifolds for the 4BT that suit different installation needs, they don’t necessarily fl ow what a high-horsepower build like this one would need. ACD turned to Steed Speed for help. It offers these customfabricated manifolds with both T3 and T4 turbo flanges along with center- and top-mount exit locations. These different turbo mount locations should be more than enough to fit a 4BT into the chassis of just about anything.
• ACD Engines
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 horsepower. Exactly how much remains to be seen, as specific parts combinations vary from application to application. However, we can unequivocally say that for embarking on 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 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 just fine 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 being specifically built to handle its own unique task, never skip a beat—whether they’re at the track, blasting through the mud, or hauling hay.
Compound Turbocharged 4BT Square Body
When your love for square body Chevy’s runs deep but 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, and a sizeable second-generation Dodge intercooler help ensure 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
In case you were wondering why compound turbocharging is so common in the world of high horsepower 4BT’s, it boils down to its lack of displacement. While big brother 6BT can bring a larger single turbo to life in a reasonable manner, its 359 cubic inches plays a big part in it. The 4BT, with its much smaller, 239 ci displacement, lacks the same off-idle torque that helps the 6BT spool a larger turbo. Mild head porting and larger valves can greatly improve spool up on a large 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 low-rpm drivability.
Maxed-Out Yet Street-Friendly P7100
Surprise, surprise, a Bosch P7100 fuels the square body Chevy mentioned above, but it’s not your ordinary 4BT P-pump. Built and benched by Northeast Diesel Service, it’s about as radical as a streetable P7100 gets and 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 the 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
This is one aftermarket component that works wonders for the P-pumped 12-valve 6BT 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, which means you can control smoke output and custom-tailor your fuel curve to match your turbocharger(s) performance. The fuel control knob (left) is a great tool for controlling EGT.
Head Gasket Insurance: Head Studs
Six head bolts per cylinder is nice, but once elevated boost and cylinder pressure are part of the equation 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: South Bend’s Mud Buggy
One 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 configuration, it definitely held its own. Rumor has it that the 4BT belted out a little over 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 encountered every weekend. The company has since retired its mud-blasting buggy, but it remains one of the most popular higher horsepower 4BT Cummins applications ever pieced together.
The P7100 that fueled the ’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 beyond 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 will typically only find on a 1,500hp 6BT.
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 by externally wastegating each turbocharger.
800 Horsepower 4BT Ranger
Years ago, when diesel drag racing was 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 cam from Scheid Diesel, 9/16-inch head studs, and a cylinder head benefitting from oversized valves. The original BorgWarner S200 over S300 compound turbo arrangement yielded 707 horsepower on the dyno, but later on an S300/S400 upsizing uncorked the 4BT to the tune of 824 horsepower (along with 1,267 lb-ft of torque).
The Mini Monster
In order to support north of 800 horsepower, Robbins had Shiver Diesel set up 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 shifts—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, but rather a mandatory addition.
Back-halved, lowered, and graced with a 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 under triple-digit boost pressure. Robbins’ Ranger remains one of the best examples of just how far the 4BT platform can be pushed.
Cummins power 4bt
4BT Cummins Diesel
The 4BT is dimensionally similar to the 6BT, save for a much shorter length. The pistons, injectors, connecting rods, and valvetrain design are straight off of the 12v Cummins (6BT). Because inline 4 cylinders are not inherently balanced like inline 6 cylinder engines, the engine is noticeably "harsher" than its larger brother and does not build power as "smoothly". The engines can be found in applications with up to a 16,000 lb GVWR, which should serve as a limit for anyone whom plans to use the engine in a custom application.
4BT Cummins Specs
Inline 4 cylinder
3.9 liters, 239 cubic inches
Bore x Stroke:
4.02" x 4.72"
Cylinder Head Material:
Engine Block Material:
17.5 : 1
Direct injection, mechanical injection pump
OHV, 2 valves per cylinder
745 - 782 lbs w/ oil
Varies with application
105 hp @ 2,300 rpm*
265 lb-ft @ 1,600 rpm*
* Common rating, output may vary considerably with application.
Cummins 4BT 101
4BT Basics And Performance Options
The popularity of diesel-powered vehicles here in the U.S. has really jumped in a positive direction in recent years. More and more manufacturers are starting to offer all-new diesel powerplants in half-ton trucks, cars and midsize SUVs—the introduction of a 3.0L diesel in the new Ram 1500 trucks, an EcoDiesel in the latest Chevrolet Cruze passenger car and there’s even been talk of a 4.5L Duramax coming down the pipeline. With the big car builders starting to offer diesel upgrades from the standard gas powerplants, there must be legitimate reasoning.
Maybe it’s better highway fuel efficiency and the giant increase in torque offered by a diesel engine. In reality, it’s probably an attempt at meeting ever-increasing CAFÉ (corporate average fuel economy) numbers. Either way, these smaller V-6 and four-cylinder engines will be sticking around. But back in the day, it was Cummins that developed their own small diesel engine market with the 3.9L 4BT.
The 4BT—which stands for four-cylinder, B-series, Turbocharged—was used mostly in midsize box trucks, agricultural equipment and small industrial vehicles, and is basically a smaller version of the popular 5.9L 12V Cummins found in the 1989-98 Dodge trucks. The 4BT shares virtually all its parts with its big brother the 6BT; i.e., pistons, connecting rods, injectors and valve train design.
Obviously, the major internal difference comes from the lack of two cylinders, which means two less journals on the crankshaft, a shorter camshaft, two less pistons in the injection pump, etc. But even these pieces are the same as the 6BT, just in an inline four-cylinder version. Because of these similarities, the 4BT Cummins has an extremely strong aftermarket, since many 5.9L 12V performance parts can be used in the smaller 3.9L engine. This, paired with the somewhat compact size of the 4BT, makes it a great candidate as a transplant engine for just about any vehicle. We’ve seen 4BTs in everything from Jeeps to old school rat rods. The simplicity of the mechanical injection system also helps make it an easy swap, as there’s little to no wiring or electronics required.
Since the engine was originally developed for industrial applications, big horsepower wasn’t a major concern and the 239-cid motor most commonly came from the factory at around 105 hp @ 2,300 rpm and 265 lb/ft of torque at 1,600 rpm. But these numbers can vary up to as much as 150 hp depending on the application it was installed in.
The 4BT engine weighs in at around 750-780 lbs, which is heavier than most of the gas engines it would be replacing in a conversion project, but much lighter than the six-cylinder 6BTversion. The inline-four also offers a fairly compact size at just 30.6 inches long and 37.7 inches tall, helping with space constraints. The engines came with a few different injection pump systems, but the P7100 is, by far, the most popular, since it’s capable of producing larger amounts of fuel and more easily modified by the aftermarket to help increase power outputs.
As previously mentioned, the 4BT can easily be modified to produce higher than stock power levels when outfitted with the P7100 (P-pump). Fueling mods can be taken to the same extremes as with the 5.9L 12V Cummins; 12mm and 13mm pumps with laser-cut delivery valves, hi-rev governor springs, full-travel rack plugs along with modified injectors can all be used in the 4BT platform. Obviously, to go along with major fuel upgrades, common cylinder head and turbocharger upgrades would be required. But the aftermarket already has most of that covered as well with head studs, better valve springs, larger valves, and even performance exhaust manifolds and camshafts that are being built for the four-cylinder diesel. While these upgrades can take the 4BT to all-new levels, one thing that needs to be mentioned is that because of its lack of cylinders and nearly identical operating rpm range, each piston is going to undergo a combustion cycle much sooner than that of a 6BT Cummins, so things like camshaft profiles and pump timing become critical for maximum efficiency at higher horsepower levels.
ACD Stroker Kit
Like any engine platform, it’s a known fact that more cubic inches generally means more power and there’s only so much a 239-cid motor will be capable of, at least safely and efficiently. With this in mind, ACD Engines of Salt Lake City, Utah, has developed what they refer to as a “Stroker Kit” that will take the 4BT to all-new performance heights.
Also known locally as “All Cummins Engines,” ACD has been a full-line Cummins dealer for more than 20 years, specializing in midrange and industrial engines. They offer virtually everything Cummins—new or remanufactured engines, new genuine Cummins parts, used engines and even salvage parts. Since they deal in nothing but Cummins, anyone looking for that hard-to-find part can most likely call ACD and find they’ll have it in stock or at least know where to get one. Through their years of experience, they’ve become extremely well versed on the 4BT platform and have the conversion process down to a fine science, inserting the small Cummins engine into Jeeps, small SUVs, pickups and even a mid-’50s ambulance.
In the search for more power, owner Robby Pederson began development of a Stroker Kit that would increase the length of stroke and add some additional cubic inches to the inline-four diesel. In the gas world, Stroker engines are nothing new, as the GM small-block 383 Stroker engines have been around for nearly 35 years. The 383 is built by using a standard GM 350-cid engine block with a modified 400-cid crankshaft, which changes how far the piston travels.
New Rods and Pistons
Not looking to do so much crankshaft work, as the factory 4BT crankshaft is a robust piece, Pederson opted for an all-new piston and connecting rod design to gain that additional stroke he was after. A factory 4BT engine runs a 4.02-inch bore with a 4.72-inch stroke; this is how we come to a 239-cubic-inch motor. The new parts from ACD will allow the overall piston bore to expand to a 4.402-inch and a much longer 5.430-inch stroke is used, which will effectively take engine output to 333 cubic inches, or 5.46-liters, almost that of the 5.9L 12V Cummins.
Knowing that the clientele for such a kit would be after extreme power levels, only the best materials were used to be sure the ACD Stroker Kit would stand up to high-boost and high-cylinder pressures. Rather than use standard-cast pistons, like the stock units, ACD went with a much stronger forged piston design that offers a different bowl design to improve the air/fuel swirl effect, helping create a more efficient burn in the combustion chamber. The piston design also has a much shorter overall height, along with the wrist pin location being moved closer to the deck; this is where most of the additional stroke comes from.
To match the new piston, connecting rods had to be developed, which were made from billet 4340, like those being used in all the high-horsepower diesel engines. The new H-beam rod design is not only stronger than a factory connecting rod, but it also has a much smaller wrist pin journal, which will only work with the ACD piston. While the pistons and rods are the true heart to the Stroker Kit, ACD has also developed a host of 4BT parts to go along with them to ensure the true peak performance can be met. Specific camshaft profiles have been designed, along with custom cylinder head work, and even 4BT-specific adjustable injection pump timing gears. The short time between injection events on the four-cylinder engine require a little different engineering of these parts than that of a six-cylinder Cummins, and ACD thinks they’ve developed the perfect pieces to help turn your run-of-the-mill 105-hp 4BT into a tire-shredding 800-hp monster.
ACD will soon be installing this completely built 4BT engine into their shop vehicle, a H1 Hummer Army body sitting on the chassis from a Dodge Ram 2500. The Stroker 4BT will help push fully built axles, custom suspension and drivetrain sitting on 40-inch tires down the road. We’ll be sure to do a complete write-up on the project when it’s up and running in another issue of Diesel World.
ACD Engines understands that the complete Stroker Kit may be more than most 4BT project vehicles will need, so they have an array of 4BT-specific products that can do everything from help increase power to just improving fuel mileage and engine efficiency. They also carry a full line of custom brackets and conversion pieces that may help your conversion or transplant project go a little easier. DW
4BT Land Cruiser Conversion
The team at ACD Engines has made 4BT conversions their specialty and had this 1992 Toyota Land Cruiser up on the rack while we were shooting photos of their new 4BT Stroker Kit. The owner of the Land Cruiser wanted more pep than the factory gas engine could offer, but didn’t want to sacrifice fuel mileage, making it the perfect candidate for a mildly upgraded 3.9L Cummins.
Knowing that major horsepower and torque would be needed and looking to keep things budget friendly, they opted for the less common and slightly cheaper VE injection pump series motor, versus the more popular and easily modified P7100 injection pump model. The VE pump working with custom ACD injectors and camshaft should bring power output up from the original 105 hp to a more user-friendly 200 hp. While this may not sound like much, the low-end torque of the 4BT backed by the NV4500 five-speed manual transmission (from the 1994-2001 Dodge Ram truck) will make for a great daily driver and off-road combination. On this particular build, ACD will also be able to retain the factory A/C and heating system, and were even able to add an intercooler system behind the grille. ACD and the vehicle’s owner are hoping this package nets a consistent 25-30 mpg.
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3.9L Cummins Engine: 4BT Diesel Pros & Cons
At the recreational level of the off-road hobby, engine swaps are a mixed bag. Gas engines – namely the LS variety – are the most dominant, but diesel engines are steadily gaining market share.
One diesel engine in particular that pops up more than most is the 3.9L Cummins. The 3.9L (4BT) Cummins inline-four diesel engine creates 105hp and 265 lb-ft of torque from its relatively small size of 30.6 in long, 24.6 in wide and 37.7 in high. This is what makes it such a popular candidate for engine swaps. Weighing in at nearly 800 pounds, the robust inline four-cylinder engine is the little brother of the cherished 5.9L 12-valve Cummins diesel engine. The fact that both the 3.9L and 5.9L share much of the same DNA gives diesel enthusiasts confidence the engines can go the distance.
Cheap and Simple
Hitting the market in 1983, the first 3.9L Cummins (known as the 4BT) has been used in everything from delivery trucks to wood chippers. This means they can be found anywhere, and for really cheap. In addition, since the engines require very few electronics to operate, it simplifies the conversion process.
The Bigger the Size, the Greater the Power
Diesel engines are notoriously large, and the family of B-series Cummins diesels are some of the longest and tallest. Both of these facts make them difficult to package. The 4BT measures out (in inches) with an overall length of 30.6, height of 37.7 and width of 24.6. While not exactly the smallest four-cylinder out there, we’ve seen them squeezed under the hood of everything from a Ford Mustang to a Jeep Wrangler.
The pre-1998 4BT engines have eight valves and received power ratings of 105hp and 265 lb-ft of torque. Post 1998 units (known as the 4BTA) went to a 16-valve configuration. With this valvetrain adjustment came a base horsepower bump to 170 and a more impressive 420lb-ft of torque. Depending on the application, the power figures vary.
Available and Reliable
The earlier 4BT engines are still the easiest to come by – the most desirable equipped with the P7100 mechanical injection pump, often referred to as the P-pump engine. These injection pumps are easier to modify and are thought to be more reliable over the later-style electric VP pumps. A P-pump engine won’t significantly change the asking price of the engine.
Power? Or Fuel Economy? Choose One.
One commonality between the 4BT and 4BTA is that both are non-intercooled, turbo-diesel engines. From a packaging front, this is helpful as intercooler ducting, as the intercooler itself can be challenging to fit under the hood. The trouble for those chasing more power is that an intercooler of some sort will need to be added. With the weight of the engine being a third of Jeep Wrangler, the added mass up front can have a negative effect on the handling.
The added weight also puts more strain on the vehicle's suspension and axles. For a full size truck or SUV, the weight won’t be as much of an issue. Weight is often relative when it comes to a heavily modified 4x4.
Some will argue that the fuel savings will justify the conversion. This can depend greatly on what powerplant you are moving from. If your rig currently averages 15 miles-per-gallon, it’s less likely you’ll make your money back from the fuel savings of the conversion. Most real-world numbers we’ve seen have ranged between 17-21 mpg. Again, this is dependent on application.
Simply put, don’t let fuel economy be your only reason for swapping in the 3.9L.
A Bit Old and Kind of Rough
The biggest drawback of the 4BT is that it’s not a modern diesel. In fact, it’s a bit antiquated and unrefined. While that’s not always all bad, for some, the rattle, shake and noise of this diesel will be too much. Also, you can’t simply hook a programmer to a 4BT and voila! you have 500hp.
Can you turn up the power? Absolutely. However, it’s going to take real work and a serious investment of funds to get the numbers into the bragging realm.
So what’s the takeaway?
We can’t deny that the 3.9L Cummins diesel is an engine of merit. The mass availability, simplicity and slew of manual and automatic transmission options make it look great on paper.
There are ultimately better diesel options, but not many are cheaper.
If you have a fullsize application, the 4BT isn’t a bad option, especially if you are looking to build a reliable overland-type rig.
For something as small and nimble as a Wrangler, there are a better options.
No matter what you decide, when it comes to engine swaps, the 3.9L Cummins is definitely worth bringing to the table.