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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.

4BT Reliability

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.

Upgrading Performance

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.

Fuel Economy

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

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.

1. Craigslist

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!

3. 4BTEngines.com

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.

4. Ebay

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