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3.9L 4BT Cummins Engine Specs

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cummins logoThe 3.9L Cummins 4BT engine is from the B series of Cummins engines which included the popular 5.9L 6BT 12v and 24v engines. However, the primary difference was that the 4BT Cummins was only an inline 4-cylinder turbocharged diesel instead of a 6-cylinder engine.

The Cummins 4BT was unique because it was the first time Cummins produced an engine that was not for a heavy-duty application. Their new 4-cylinder engine was used in light commercial and industrial applications such as delivery trucks, excavators and military vehicles.

Since the engine was used in so many delivery trucks, the 4BT has often been nicknamed the “Bread Truck.”


The 4BT Cummins engine was produced in its original form from 1984 until 1998. All of the B-series engines used in North American applications, including the 4BT, were manufactured at the Cummins Rocky Mount engine plant in North Carolina.

Diesel enthusiasts love the 4BT because it was an extremely simple engine that could be used in a wide variety of applications. This was primarily due to the fact that the engine relies on no electronic controls. After the engine was installed, the only wiring required was for the starter and the fuel shut-off solenoid.

Due to its simplicity the 4BT is an easy engine to work on and troubleshoot, that is if you ever have a problem.

While the original 4BT production ended in 1998, Cummins still produces an inline 4-cylinder diesel engines similar to the 4BT. However, the main difference is that new Cummins engines rely heavily on electronic controls to meet tougher emissions standards.

4BT Performance Specs

The 3.9L Cummins engine can be thought of as the little engine that could. Thanks to the help of a Holset turbocharger, this engine produced 105 horsepower with as much as 265 lb-ft of torque.

As the name suggests, this engine has a displacement of 3.9 liters or 239 cubic inches. It has a bore of 4.02 inches and a stroke of 4.72 inches. The firing order is 1-3-4-2 and has a compression ratio of 17.5:1.

As is typical of diesel engines produced in the 1980’s and 90’s the engine block and head was made of Iron. From 1984 to 1998 the 4BT engine had 8 valves, 2 per cylinder. After 1998 the 4BT was redesigned to accommodate 16 valves, with 4 per cylinder.

Another reason the 4BT continues to be popular with truck and offroad enthusiasts is that the engine shares many of the same parts as the 5.9L 6BT. For instance, the 4BT uses the the same pistons, connecting rods, valvetrain and injectors as the 5.9L 12v Cummins.


For gearheads that want a compact diesel engine for their engine swap, the 4BT Cummins is an easy choice. After all, it has no electronic controls except for the starter and fuel shut-off solenoid.

Unlike the diesels engines of today with complex emissions systems, the 3.9L Cummins engine is a breath of fresh air for engine installers that want to avoid complexity.