Engine Cummins 4BT 3.9L

Engine Cummins 4BT 3.9L

The 4BT engine is a water-cooled, in-line 4-cylinder turbodiesel engine from the Cummins B series. The engine has a cast iron cylinder block with holes drilled directly into the cylinder block. Valve mechanism of the engine – OHV. The camshaft is mounted in a block and is driven by the crankshaft through gears at the front of the engine (as well as the oil pump).

The camshaft opens and closes the valves, pushing the rocker arms with tappets and solid tappets. The cylinder head is also made of cast iron. The inlet and outlet pipes are on opposite sides (cross flow cylinder head). There are 8 valves, 2 per cylinder. 4BT is equipped with a mechanical direct injection system. Early versions were equipped with a mechanical P7100 injection pump, driven through a camshaft gear.

The 4BT engine has a turbocharger but no intercooling. The intercooled version (named as 4BTA, where the letter A stands for “aftercooling”) has a lower compression ratio of 16.5: 1. Low compression and cold air provide higher boost. The 4BTA engine provides more power and torque and is more suitable for installation in a small truck or pickup, but keep in mind that the 4BTA is an industrial engine. Air-cooled car engines are numbered differently depending on model and year. A naturally aspirated industrial version called 4B with a compression ratio of 18.5: 1 is also available.

The 4BT is inherently not as balanced as its 6-cylinder sibling, so it produces a lot of vibration and is not as smooth as it gets. The 3.9 liter Cummins engine has been a popular choice for many commercial cargo vans, used in industrial equipment such as powertrains, drilling machines, large water pumps, wood chippers, etc.


Manufacturer Cummins
Production years 1984-1998
Cylinder block alloy cast iron
Cylinder head material cast iron
Fuel type diesel
Fuel system Direct injection, mechanical injection pump
Configuration inline
Number of cylinders 4
Valves per cylinder 2
Valve train layout OHV
Cylinder bore, mm 102
Piston stroke, mm 119
Displacement 3.9 liters
Configuration Four-stroke, turbocharged
Compression ratio 17,5:1 (4BT)
16.5:1 (4BTA)
18.5:1 (4B)
Displacement 80-130 hp at 2300-2600 rpm
Torque output 330-530 Nm at 1600-1700 rpm
Weight (dry) 388 kg
Firing order 1-3-4-2
Oil volume 9,5 л (with filter)
Oil change interval 10000 km or 6 months
Installed industrial equipment, commercial vehicles (vans, trucks), ships and boats

Frequent problems and reliability 4BT Cummins

Cummins 4BT 3.9L

B Series motors require very little electronics, are cheap and easy to rebuild or repair. P-pump diesels are easier to modify and more reliable than newer diesels with modern VP electric pumps. But the 4BT is actually an old massive diesel engine. It’s noisy, wobbly, smoky like an old diesel. The 4BT is quite difficult to install under the hood of a small vehicle due to its large dimensions – it is tall and weighs a lot, and it also needs space for an intercooler and pipes. This power is not enough for a full-size truck or SUV, while its weight increases the load on the front suspension and negatively affects the handling of a small 4-wheel drive vehicle.

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