Here’s Why The Mighty Cummins 4BT Is The LS Swap Of The Diesel World

The General Motors LS V8 engine is a legend in countless car enthusiast circles and, for many, the default choice for an engine swap thanks to the LS’ highly attractive mix of high power, low cost, and ease of integration into countless cars. But what if you’re a diesel nut? The diesel world has an equivalent, and it’s the Cummins 4BT. This glorious inline-four diesel comes from the same family as the iconic Cummins 5.9 and fits into a surprising number of engine bays. That means a shocking number of vehicles that never had modern diesel performance can now get great pulling power and fuel economy.

Back in August 2024, I wrote about how Ford Ranger owners have figured out a way to create the perfect small pickup truck that Ford didn’t. Those enthusiasts lifted compact four-cylinder turbodiesel engines out of Volkswagen cars and bolted them into their trucks. A Ford Ranger “TDI” sounds like the kind of truck everyone says they want. Depending on the Volkswagen TDI engine chosen and exact tuning, you can get a small truck that gets over 30 mpg when unloaded, but makes the same power, if not better, than the old gasoline-chugging lumps Ford equipped those trucks from the factory with.

After that article, several readers noted that there’s an even more popular diesel swap out there: the Cummins 4BT. Enthusiasts are dropping the 4BT into basically anything that the engine can squeeze into, including Ford Rangers, Jeep Wranglers, Ford Crown Victorias, Nissan 240SXs, Chevrolet Camaros, and, shockingly, even a Toyota Prius. Just like with the GM LS, if some mad wrencher thinks they can fit a Cummins 4BT into something, they will probably get it to work. For example: the Ford Police Interceptor below has a Cummins 4BT from a bread truck and allegedly averages 33 mpg.

The amazing thing is that there really isn’t one single reason to put a 4BT into a vehicle. Some may do it for the low-end power of a diesel. Some may do it because the diesel their truck had from the factory sucked to begin with. Or some may do it just to say they did it. If you’re handy, and don’t mind operating without the safety net of a full set of instructions, you can probably pull off one of these swaps by yourself.

Why The Cummins 4BT Is A Diesel Legend

I think we should first discuss why people select the 4BT in the first place. The world is full of diesel engines. Why are enthusiasts so obsessed with this one?

In 1979, the Cummins team realized they had a blind spot in the engine range. There were engines for big trucks and equipment, but the diesel giant wasn’t maximizing its potential in the medium-duty midrange space. The answer came with the iconic B Series, and Cummins’ legendary 5.9 straight-six led the charge.

Weirdly, while the B Series is incredibly important in the history of Cummins, the engine family gets only a short mention in the company’s history. Thankfully, David Tracy got the scoop years ago. From David:

To help tell the tale of the “five nine,” I reached out to Cummins’ marketing director David Goggins, who told me that the engine’s durability is rooted in its design for grueling industrial applications, saying:

A lot of the reason that engine is as durable as it is is because we designed it to be a heavy duty, commercial kind of engine.

Leading the development of the B series was Philip E. Jones, a British engineer known for his work at Perkins. Over at Cummins, Jones started with a clean sheet and designed a four-cylinder and six-cylinder architecture that shared core components between engines. Jones also designed the engine around metric measurements, resulting in notes in technical papers like how the B Series had a displacement of 0.98 liters per cylinder.

David continues:

He went on to say that the 5.9-liter engine actually originally started as a joint venture between the Indiana-based diesel engine company and Case Corporation, which builds tractors and construction equipment. And indeed, starting in 1984 (well before the 5.9-liter engine ever found itself powering a Dodge Ram), Cummins offered three different variants of the 5.9-liter called the 6B, 6BT (turbocharged) and 6BTA (turbocharged, aftercooled), which served duty in tractors, combines, excavators, road graders, pavement rollers, boats, field sprayers and even school buses.

These are all seriously heavy duty applications. Add that to the fact that these engines were available in dozens of different markets with different climates and work environments (like Russia, China and India), and you can imagine how relatively understressed that Cummins Turbodiesel was once it finally made its way into a Ram pickup in 1989.

[…]

But perhaps even more impressive than its factory torque numbers is the engine’s reputation for longevity. Once you start looking at the mechanical bits, you begin to see just how overbuilt the B-Series engine really is. The block and head are cast iron, the crankshaft and connecting rods are forged, the seven main bearings are massive, and like many heavy-duty diesel trucks, the crank and cam are connected by a steel timing gear—not a chain or belt like you’d find in normal cars and trucks. The Holset turbos are also known to last until the end of time.

Cummins says that the B Series saved the day, specifically the 5.9-liter that diesel nuts love so much. It’s a solid engine, even today. The B5.9 features a straight-six design with two valves per cylinder, and reportedly had 40 percent fewer working parts than a competitive engine. Cummins used a 4.02-inch bore and a long 4.72-inch stroke, which was great for low-end torque. Add a turbo from Cummins’ Holset subsidiary, and the Cummins 5.9 made a healthy 160 horsepower and 400 lb-ft of torque while proving itself to be able to go hundreds of thousands of miles between overhauls.

The Cummins 4BT joined the B3.9 family in 1983 and offered a powerful option for smaller equipment. Like the B5.9 series, the B3.9 was available in a line of models. The base 4B didn’t even have a turbocharger, and a common output configuration was good for 80 HP. These engines are great for small tractors, wood chippers, trenchers, and other pieces of equipment that will be expected to work for decades, but don’t need a whole heaping of power.

The 4BT added a turbocharger (that’s the ‘T’ in the name) and produced 105 HP and 265 lb-ft of torque in its initial configuration. In 4BTA spec, the valve count was upped to sixteen and an aftercooler was added to the turbo-diesel recipe. This boosted horsepower to 170 and increased torque to 420 lb-ft. And finally, there’s the 4BTAA, which featured an air-to-air intercooler and was known for making about 130 HP and 327 lb-ft of torque when new. These turbocharged variants of the 4B have found homes in smaller commercial vehicles like stepvans and delivery trucks.

It’s noted that the B3.9 and its variants have a lot of similarities to the B5.9 that everyone loves. Early examples still have two valves per cylinder, like the B5.9 and similar overbuilt internals.

Not The Simplest Swap

The Cummins 4BT helped start a bit of a diesel engine swap revolution. Diesel swaps aren’t anything new by any means. People have been loading full-size trucks with Cummins 6BT conversions forever and other diesel swaps have existed before that. But the 4BT offers a special mix of attributes that you cannot find in every diesel engine.

A Cummins 4BT measures at 30.6 inches long, 24.6 inches wide, and 37.7 inches high. It’s not exactly the smallest four-cylinder diesel out there, but it’s roughly similar in length and width to a domestic small block V8, though taller than gas mills. Those dimensions make the 4BT small enough to fit into everything from pony cars and small pickup trucks to smaller off-road SUVs. Some people even drop these engines into their full-size trucks, too.

Its size alone is a huge deal, but the 4BT would not be as popular without two other factors. These engines are simple, and you can find them all over the place. Remember how I said that the 4BT was put into everything from stationary power equipment to all sorts of small commercial vehicles? Well, the sheer number of 4BT engines out there is simply absurd. Seriously, go onto your local Facebook and hammer in “Cummins 4BT.” You’ll find dozens of these things within 500 miles of you and maybe thousands around America as a whole.

You’ll find that many of these engines do not run. However, if you dig hard enough, you might be able to score an original running example for $2,000 or $3,000. If you want one that’s nicer, it looks like you’ll be ponying up around $6,000, maybe more. Some enthusiasts say that these engines used to be far cheaper, which is a shame if you’re only getting into the idea of a 4BT swap now. You’ll sometimes find old stepvans like the one below with 4BTs in them.

Securing the engine itself can be a wild task all in itself. As DrivingLine notes, part of the reason why the Cummins B3.9 was offered in so many different applications was because Cummins was able to make lots of small changes to the engine to fit specific uses. Because of this, you may want to check an engine’s data tag before you buy it. On the data tag will be a Control Parts List number, or CPL, that will help you determine what emissions controls, injectors, injection pump, and turbo that you might be dealing with.

For example, DrivingLine notes that engines with CPL codes of 1839, 2001, 2195, and 2304 indicate that the 4BT you’re looking at was meant for an on-road application. Engines with CPL 767 and 858 will have Bosch VE rotary injection pumps and are rated at 105 HP and 265 lb-ft of torque. Meanwhile, engines with CPL 857 and 986 had the same pump, but had intercooling and an output of 120 HP and 300 lb-ft of torque.

Some recycling facilities just cut off the front end of a truck and sell it whole. This Cummins 4BTA comes with an Eaton Fuller FS-4205 transmission, radiator, intercooler, and more for $3,750.

You can refer to DrivingLine‘s list for even more CPLs. The thing that you need to know most is that not every 4BT is the same, nor will every 4BT even make the same stock output. As DrivingLine notes, the good news is is the parts between these engines are interchangeable. You can buy a 4BT meant for industrial use, make a few alterations for passenger car duty (for example, changing the injection pump’s governor to allow for higher RPM), and have a diesel engine that’s ready to hit the road. 

There’s one way to integrate a 4BT into the vehicle of your choice, so no singular guide will get you to the diesel-swap finish line. Instead, you’ll be depending on various forums, YouTube videos, and your own ingenuity. This means a 4BT swap isn’t as easy as putting a Ford Fusion engine into your Mazda Miata.

However, there is a known starting point. An old 4BT doesn’t need any electronics and only limited wiring to run. First, you’ll need to wire in the starter. Chances are, you can just use the existing starter wiring from your vehicle to make that work. Next, you’ll need to wire in the fuel shut-off solenoid, and that needs just one wire. Ideally, that single wire will come from a source that is either charged with 12V power with a manual switch or is energized when you turn your ignition switch to the on position. In theory, you could just run a line from the battery, to a switch, and then to the solenoid.

You’ll also need to wire in the alternator, and once again, chances are you’ll be able to make your vehicle’s existing alternator work. That’s more or less the bare minimum that you need to get the engine to run. However, a proper conversion is going to require more out of you.

Let’s say you have a fifth-generation Ford Bronco and you want to put a Cummins 4BT in that. Sweet, that can be done! You’ll need a special adaptor plate to hook the engine up to the E4OD. You’ll also want to get a custom flex plate. Oh, the Cummins also isn’t going to use the same engine mounts as whatever gasser was in the truck before, so now you have to get custom mounts. Many shops recommend hydraulic mounts because the 4BT is notorious for transmitting tons of vibrations.

If you want the instruments to work correctly, you’ll also need sensor adaptors and then you’ll want line couplers to hook up the power steering system. If your truck has an E4OD automatic transmission, you’ll then need a custom TCM. Thankfully, there are aftermarket companies that provide all of these parts.

Cummins 4BT swaps are also somewhat popular to do in Jeep Wranglers, and Big Bear Engine Company mentions some of the hurdles you’ll have to overcome. The list includes reusing the factory radiator, but cobbling together your own air intake. You also might need to have a custom suspension made because the 4BT weighs a whopping 800 pounds. Further complicating things is potentially needing to have a custom wiring harness made to make sure your gauges still work. The list is huge, so I recommend clicking here to read deeper.

Car publication DrivingLine also notes that you should probably avoid the naturally-aspirated 4B engine if you desire turbocharging. Reportedly, this variation of B3.9 was not meant to be turbocharged, and doing so can threaten the head gasket and engine internals.

Likewise, you should be realistic with your expectations. These engines are very old-school. They’re loud, they’re harsh, and even later 4BTs have limited electronics. You’re not going to hook up a 4BT to a computer and crank up the taps with a few clicks. This isn’t the engine you want if you’re looking for the silky smoothness of a modern common-rail engine.

Things do get weird if you try to convert a vehicle that doesn’t have strong aftermarket support. If you’re thinking about putting a 4BT into your Honda Ridgeline, you’ll probably going to have to figure it out yourself. In that case, the Big Bear Engine Company, a provider of 4BT engines, mentions that New Venture Gear 4500 and Aisin AX15 transmissions are popular choices for people doing custom builds. However, you can buy adaptor plates for transmissions like the E4OD, the TH400, the SM465, and the T19.

Hot Rod magazine has a helpful article detailing even more things to consider, such as if you’ll need to cut up your ride to fit the engine, or whether your frame will even be able to take the 4BT’s torque.

4BT Swap The World

So, swapping a 4BT into a vehicle is not for the faint of heart. I know if I attempted a 4BT swap, I’d probably just have an 800-pound brick gathering dust. But if you pull it through, you’ll join a community of Cummins-obsessed people who seem to want to put 4BTs in everything that moves.

What is the craziest 4BT swap? Wisconsin wrencher Ben Schulz cut apart a second-generation Toyota Prius, mounted a 4BT under its hood, then gave it a truck transmission and a rear differential, turning the formerly gas-sipping hybrid into a rolling middle finger. That project required an incredible amount of custom fabrication, including welding new panels into the Pri’s unibody to fit the huge transmission. I will warn you, this video below has a lot of coal-rolling:

Leading this with a Prius is interesting, and a little hilarious because the swap gave the car worse fuel economy. The folks of Big Bear Engine Company say they’ve seen up to “mid-20s” in some applications, while the folks of DrivingLine claim around 17 mpg to 21 mpg. Fuel economy results are noted to be heavily dependent on exact application. For example, Big Bear Engine Company claims that a 4BT diesel swap might be able to double the fuel economy of a truck that had a Chevy small block V8 prior to the swap. However, a Jeep will likely see milder gains.

In other words, I probably wouldn’t do this for fuel economy reasons unless the vehicle you’re starting with is already getting awful fuel economy. Your specific swap may not exactly result in perfection, either.

Here’s one person talking about the results of their 4BT swap in a Jeep CJ7:

As promised here is the dirty nasty engine bay shot. Here is why I would never do it again.

#1 The engine is super expensive for what it is. I bought it in 2007 for 1500. Now they cost 3k for a crappy one.

#2 They are not the powerhouse everyone thinks they are. I have mine turned up and I can’t tow cause my EGTs are borderline. I turned it up to make it funner to drive. I put a fuel pin in/advanced timing/smaller exhaust side/gov springs/added fuel. I know I could add intercooler to help and that is planned.

#3 this engine is heavy. 850lbs all on front end isn’t fun. I would have to change everything about the setup to move it back 6 inches.

#4 gear splits in the nv4500 suck to daily drive. Start in second and build 28lbs of boost and shift to end up bogging third. Rinse and repeat until you are tired of shifting with one eye one EGT.

#5 You have to totally redesign fuel system. If you suck air in the tank even for a split second it’ game over.

Here is why I am keeping it in.

#1 Trail tractor. This thing refuses to stall on the trail. I let my nephew drive it and he locked up the front brakes in 1st gear low and broke the front end. Hence the dana 44 front. I mostly use it to pull logs around my property.

#2 Good on fuel. I get 24 HONEST mpg. I would get more if I drove more highway but a cj7 on the freeway is like being on the space shuttle challenger on re entry. I also drive like a teenage boy.

Of course, someone had to put a 4BT into a Miata!

Reading into enthusiast forums and places like Reddit, it also doesn’t appear that 4BT swaps would be the best choice for a modern full-size pickup truck. Remember that even healthier stock 4BTs make a touch less power than a diesel pickup truck engine made over two decades ago.

But lots of folks have put 4BTs into classic trucks with success. These engines can be tuned for more power, but you should be realistic there, too. Combing through the 4BTSwaps forum, it’s suggested that if you want a reliable street truck with a single turbo, you’re looking at closer to 250 HP. If you want 300 HP or more, now you’re looking at compound turbos and potentially losing some reliability. But hey, even 250 HP is a lot better than what a lot of trucks shipped with back in the 1960s and 1970s. I’m not surprised to see that some folks have put these in old GM ‘Squarebody’ trucks.

Combing through the forums some more, there are also various reports of towing performance. Some folks report good towing results so long as you aren’t expecting to go fast. Others noted exhaust gas temperatures that were way too high, which can damage the engine. Some others recommend going with a 6BT build if you’re looking for a tow beast. So, again, you’ll want to temper your expectations. It seems most enthusiasts say that a 4BT will tow, but a lot of these builds aren’t specifically for towing.

In other words, you’re not going to make a 4BT-swapped truck into the equivalent of a B6.7 HO. However, you should be able to achieve something really desirable. For example, the best engine in a 2000 Ford Ranger was a 4.0-liter V6 that made 160 HP. You can beat that with a 4BT and have a really cool truck.

From my research, the benefits of a 4BT swap will vary. If you’re looking for a Jeep that gets better fuel economy and has tons of low-end, stump-pulling power? The 4BT can do that. If you want to build a crazy diesel Chevy Camaro? Sure, it’ll do that. If you’re looking to take a compact or mid-size truck that was never offered with a diesel and give it that forbidden engine? Yep, the 4BT is good for that. The 4BT also seems to be a decent choice for replacing older, more tired diesels in classic trucks.

Much like the GM LS V8, it really does seem that if a vehicle exists, someone will inevitably put a Cummins 4BT into it. If you don’t believe me, check out the 4BTSwaps forum for a list of the vehicles members have converted, and it’s frankly insane. People have put 4BTs into Dodge Dakotas, a Plymouth Satellite, Ford Explorers, a giant Ford F-800, plenty of Ford Rangers, all kinds of Chevy C/Ks, and yes, even more than one Nissan 240SX. Those threads sometimes also have details about how the swap was completed, too, which can be helpful.

That’s what’s great about 4BT swaps. Like the fabled LS swap, people have done 4BT swaps for any number of practical reasons, but also just for the fun of it. I mean, I won’t lie, the thought of a Cummins-powered Smart makes me smile. So, if you have some old car lying around and you want to do something that’s not an LS swap, maybe consider the diesel version of the LS swap.

Top graphic images: Craigslist via Engine Swap Depot; Cummins; depositphotos.com