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Cummins 4BT 3.9L Engine (Design, Reliability & Tuning)

Updated: Feb 2

✔ This article has been fact checked.

The Cummins 4BT is a water-cooled inline four-cylinder turbocharged diesel engine from the Cummins B series, it produces around 105 hp and 360 nm of torque.

It was initially intended for use in industrial, commercial, and agricultural applications.

  • First generation engines produce 105 hp and 265 lb-ft of torque.

  • Second generation 4BTA engines have four valves per cylinder and deliver 170 hp and 420 lb-ft of torque.

  • 4BT engines are also fairly fuel efficient for their age, delivering around 15-20 mpg.

The most common Cummins B series engines are the 3.9L inline-four and 5.9L inline-six models, four-cylinder engines of 3.3L and 4.5L are also available.

Table of Contents:

Cummins 4BT 3.9L Engine

Cummins 4BT Engine Design

The engine is made of cast iron, with cylinder bores bored straight into the block. All 4BT engine blocks are similar, with the exception of the gearbox adapter plate, which varies depending on the application. The engine has an OHV valvetrain.

A camshaft is located in the block and is powered by the crankshaft via gears at the front end of the engine, as well as the oil pump. Rocker arms, pushrods, and solid tappets are used by the camshaft to enable valves to open and close.

The cylinder head is constructed of cast iron. The intake and exhaust ports are situated on the opposing sides (crossflow cylinder head). There are eight valves in total, two for every cylinder. Mechanical direct injection technology is used.

The P7100 mechanical injection pump, which is controlled by the camshaft's gear, is used in the early models of the engine. 

Since the 4BT is not inherently balanced like the six-cylinder 6BT engine, it vibrates more and runs less smoothly. Numerous commercial uses, such as vans and other commercial vehicles used this engine. 

  • The standard 4BT engine dos not have an intercooler and has a compression ratio of 17.5:1.

  • The 4BTA ("A" standing for "after-cooled") is a version of the engine which incorporates an intercooler, it has a compression ratio of 16.5:1

  • Another version called the 4B engine is naturally aspirated and has a compression ratio of 18.5:1.

4BTA Engine

The 4BTA engine is better suited for use in a light truck or pickup since it has greater power and torque, but keep in mind that it is an industrial engine. Depending on the kind and year of manufacture, several numbers are assigned to automotive charge air-cooled engines. 

4B Engine

There is also the 4B, an industrial variety with naturally aspirated combustion and a compression ratio of 18.5:1. The 4BT and the 6BT share several characteristics. Since many of the components are replaceable, it is constructed similarly to the original.

Industrial machinery such as power units, drilling apparatus, enormous water pumps, and wood chippers all utilised 4B engines. In the modern world, engine swaps on Jeeps, Dodge pickup trucks, and smaller vehicles/SUVs are common.

Engine Specs



Production Years

1983-1997 (1st Gen 4BT), 1998+ (2nd Gen 4BTA)

Block Material

Cast Iron

Cylinder Head Material

Cast Iron

Fuel Type


Fuel System

Mechanical injection pump, some use P7100 injection pump (p pump) (1st Gen 4BT), Electronically controlled fuel injection (2nd Gen 4BTA)


239 cubic inches (3.9 litres)


​4.02 inches (102 mm)


4.72 inches (119 mm)


Inline four-cylinder


OHV 2 valves per cylinder (1st Gen 4BT), OHV 4 valves per Cylinder (2nd Gen 4BTA)


Four-stroke, turbocharged (naturally aspirated on 4B engines)

Compression Ratio

17.5:1 (4BT), 16.5:1 (4BTA), 18.5:1 (4B)

Firing Order


Engine Weight

~855 lbs (388 kg) (dry weight)


~105 hp @ 2,300 Rpm (1st Gen 4BT), 170 hp (2nd Gen 4BTA)


~265 lb-ft (359 nm) @ 1,600 rpm (1st Gen 4BT), 420 lb-ft (569 nm) (2nd Gen 4BTA)

Engine Oil Capacity

10 qts (9.5 liters) with filter

Oil Change Interval

6,000 miles (10,000 km) / 6 months


Industrial equipment, vans and trucks, marine vehicles and boats, sometimes used in smaller vehicles when engine swapped

Problems & Reliability

The 3.9-liter engine is readily available and reasonably priced due to its broad use in a variety of applications. Because there are few electrical components in the B series engines, they are inexpensive and simple to replace or fix.

Diesels with P-pump are more reliable and easier to repair than modern diesels with complicated electric VP pumps. The 4BT is a big, old diesel engine. It sounds and feels shaky, smokey, and noisy.

Due to the 4BT's huge size it is challenging to install in a smaller vehicle. It doesn't have the power to drive a large SUV or larger vehicles, and its weight puts additional stress on the front axle, which affects how a small 4x4 handles.

They also have a long lifespan. There is no timing belt or chain because the drive for the camshaft, oil pump, injection pump, and accessories are all gear-driven. Electronics are also absent from pre-1998 4BT engines, which increases their reliability and simplicity.

Overall the 4BT engine doesn't seem to have a large amount of issues and often they are easy and inexpensive to repair if they go wrong.

Tuning the 4BT Engine

4BT engines can be tuned for more performance, this is particularly true for 4BT engines that make use of the Bosch P7100, often known as the P-Pump fuel injection pump.

The p-pump injection pump can be modified to provide more horsepower and torque.

Because 4BT Cummins engines and 5.9L Cummins engines share many internal components, it is fairly easy to increase their performance. Those who engine swap a 4BT into their vehicle often upgrade it to produce more power.

There are several performance improvements available, including upgraded turbochargers and injectors. Aftermarket intercoolers are also installed. Many people also modify them with ARP head studs for more durability.


Below are images of the Cummins 4BT engine.

Cummins 4BT 3.9L Engine

Cummins 4BT 3.9L Engine



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