Cummins B3.9 – The 4BT

Cummins B3.9

Introduced in 1983, the first Cummins B3.9 diesel quickly became a workhorse industrial diesel engine all around the world, powering everything from wood chippers and excavators to military vehicles and delivery trucks. As Cummins technology advanced, the line expanded with the turbocharged B3.9T and the aftercooled B3.9TA.

Commonly known by enthusiasts simply as the 4BT, this family of engines was built to offer long-lasting and trouble-free diesel power, and it delivered.

The 4BT marked an important milestone in Cummins history, expanding Cummins power well beyond the heavy-duty engine market. The engine gained such a reputation for reliability that it retains a loyal following well after production ended.

So, what makes it stand out?


Heavyweight Fan Favorite

Though the 4BT saw duty across a variety of applications, it was most commonly used in step vans and other light commercial vehicles. While this has led many Cummins enthusiasts to refer to the 4BT as the Bread Truck or Bakery Truck, the name simply indicates a 4-cylinder, B series diesel with a turbocharger.

The simplicity and power of the 3.9L B make it a favorite conversion engine for diesel enthusiasts, built into everything from muscle cars and Jeeps to full-size pickups. Since it relies on no electronic controls – just starter wiring and three wires for the fuel shut-off solenoid – and pairs with a variety of powertrain options, the 4BT can easily be repurposed into a variety of vehicles. The 4BT and the aftercooled 4BTA deliver between 40 and 170 base horsepower and up to an impressive 420 pound-feet of torque.

Beyond the B Series

In 1998, Cummins introduced the ISB, upgrading the original line of B series engines to include a leading-edge full authority electronic controls and 24 valves (for 6-cylinder products). The 4-valve-per-cylinder ISB engines gave Cummins customers a distinct advantage, continuing the B series reputation for power and durability.

Since then, Cummins has continued to innovate. With its long life, the 4BT can still be found in many online marketplaces and from corporate fleets retiring their trucks. However, we’ve introduced a handful of engines that carry the 4BT’s torch of performance and reliability for industry and recreation.

ISF, QSF and QSB diesels deliver unmatched compact, lightweight power for personal and commercial vehicles, mining equipment, and more. With modern innovations like Cummins Common Rail fuel systems and near-zero emissions, these engine lines combine the dependability loved by 4BT fans with the advancements of the world’s diesel technology leader.

While the ISF line is designed specifically for on-highway and light commercial vehicles, the QSF brings a number of advantages to the compact industrial and agricultural markets. With advanced technologies and a compact design, it is better in every dimension.

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