The Cummins B Series is a family of diesel engines produced by American manufacturer Cummins. In production since 1984, the B series engine family is intended for multiple applications on and off-highway, light-duty, and medium-duty. In the automotive industry, it is best known for its use in school buses, public service buses (most commonly the Dennis Dart and the Alexander Dennis Enviro400) in the United Kingdom, and Dodge/Ram pickup trucks.
Since its introduction, three generations of the B series engine have been produced, offered in both inline-four and inline-six configurations in multiple displacements.
General engine features The B-series features engine bores machined directly into the block (rather than the wet liners used on earlier Cummins engines). It is also set apart by the use of a shallow one-piece head, requiring closer tolerances than in other Cummins products. Unlike earlier diesel engines, the B-series Cummins used direct injection and did not need glow plugs for cold starting. The engine was first manufactured in Rocky Mount, North Carolina, and other plants were later added in Mexico, Brazil, Turkey, and Darlington, UK.Kennett, p. 57
It uses a gear-drive camshaft for extra reliability. Also specified is a deep-skirt engine block and extra-strong connecting rods. A Holset turbocharger is used. The original B Series was updated with 24 valves and an electronic engine management system to become the ISB in 1998.
B engine 3.9 L The 3.9-liter 4B/4BT/4BTA Cummins is categorized under the B Engine family alongside the 5.9-liter 6B/6BT/6BTA Cummins diesel engines. The 3.9 is an inline four-cylinder, either naturally aspirated (4B) or turbodiesel (4BT/4BTA), which was popular for many step van applications including bread vans and other commercial vehicles. Additionally it has seen broad usage in agricultural equipment. It has also gained popularity as an engine swap into smaller trucks and SUVs.
4B The lowest powered, naturally aspirated 3.9-liter Cummins, the 4B, produces . This variant is most commonly found in equipment such as generators and wood chippers.
4BT The 4BT is one of two turbocharged variants of the 3.9L B-Series engine. It has two valves per cylinder for a total of eight. The most common output of this variant is 105 horsepower.
4BTA The 4BTA is turbocharged, and aftercooled with four valves per cylinder for a total of sixteen. The most common output of this variant is .
5.9 L
5.9 L B-Series CumminsProduction: 1984–1998Camshaft drive: Gear driven Crankshaft: Forged steel, 7 main bearingsSystem capacity with filter: Cooling system: coolantSystem capacity incl heater: coolantPower output: at 2500 rpmTorque: 400 - at 1600 rpmCompression ratio: 17.5:1 The 5.9 L Cummins, also known as the "12-Valve" Cummins, was the first member of the Cummins B-Series to be used in a light truck vehicle. The 6BT used Bosch fuel systems, injector, and VE rotary pump and P7100 inline injection pumps. Some early 6BTs were supplied with CAV rotary pumps instead, before the Bosch system became the sole standard. This engine started life in 1984 designed as an agricultural engine, for use in Case agricultural equipment.Cummins 5.9-liter and 6.7-liter inline six-cylinder diesel engines. Allpar.com. Retrieved on 2011-08-04. After 1989, the 6BT engine was used in light duty, medium duty and select heavy duty trucks and buses. The 6BT engine has recently become very popular for use in repowering various vehicles.
6B The 6B is the naturally aspirated version of the 6BT with a power output of at 2,500 rpm, a peak torque output of at 1,200 rpm, and a compression ratio of 19:1. There were no OEM automotive applications for 6B engines.
6BT Appearing in the 1989–1998 Dodge Ram pickup truck, it became a popular alternative to the large gasoline V8 engines normally used in full-size pickup trucks, since it produced torque at low engine RPM, and achieved significantly better fuel mileage. Due to the direct fuel injection, the 6BT has no glowplugs.
6BTA The 6BTA is Turbocharged, and aftercooled, with an air to water after cooler mounted on the intake. Has a compression ratio of 16.5:1.
6.7 L B6.7 B6.7L Cummins Production: 2007 (2017 as B6.7) – present (diesel variant production ended 2026)Displacement: Bore x stroke: Max power: at 2600 rpmCummins; Defense Overview Brochure , Accessed 5 August 2026Torque: at ~1600 rpmCompression ratio: 17.3:1Emissions control: EGR and DPFTurbocharger: Holset Engineering (variable)Fuel system: Common rail High pressure direct injection with 7-hole injectorsCamshaft drive: Gear driven Crankshaft: Forged steel, 7 main bearingsOil System capacity with filter: The B6.7L is just a re-named version of the ISB 6.7L. Cummins renamed it to B6.7 instead of ISB6.7 most likely sometime around 2018 or 2019. The only real difference from the initial ISB engine is the change of emissions equipment over the years and the valve cover does not have that hump, do note some slight differences in power ratings. The B6.7 ended production in 2026, being replaced by the Cummins B7.2L. The B6.7N and B6.7 octane variants will still be produced.
B6.7N The B6.7N is a CNG variant of the B6.7L/ISB6.7L.
B6.7 "Octane" The B6.7L "Octane" is a version of the B6.7L diesel that can use Propane, or 87 Octane Gasoline as fuel. It is marketed to "have the power of diesel and the simplicity of gas". Engine production starts with model year 2027. The B6.7 Octane is Cummins’ first gasoline-powered engine built on the B-series platform, delivering up to 300 horsepower and 660 lb-ft of torque while maintaining the durability typically associated with diesel engines. It uses 87-octane gasoline, the B6.7 Octane provides diesel-like torque.
7.2 L B7.2 B7.2L Cummins Production: 2026–presentDisplacement: Bore x stroke: Max power: at 2400 rpmTorque: at ~1600 rpm Cylinder Configuration : Straight-sixCompression ratio: N/AEmissions control: EGR and DPFTurbocharger: Holset Engineering (variable)Fuel system: Common rail High pressure direct injection with 7-hole injectorsCamshaft drive: Gear driven Crankshaft: Forged steel, 7 main bearingsOil System capacity with filter: The Cummins B7.2L was announced in March 2025 for production in 2026 or 2027. It is the evolution of the B6.7L, thus becoming the latest B-series engine. Cummins claims that the higher displacement will allow for more torque options and better emissions. The B7.2 has 240-340 horsepower, its applications are Medium duty trucks, school busses, transit busses and recreational vehicles. It will be manufactured at the Rocky Mount Engine Plant in Rocky Mount, North Carolina
ISB engine 4.5 L ISB
The 4.5 L ISB is essentially a four-cylinder, two-thirds version of the 6.7 L ISB rated at , used in the New Routemaster, a series hybrid diesel-electric doubledecker bus in London.Wrights Hybrid Wrightbus
5.9 L ISB 5.9 L ISB CumminsProduction: 1998.5–2007Camshaft drive: Gear driven Crankshaft: Forged steel, 7 main bearingsSystem capacity with filter: Fuel system: Common rail High pressure direct injection, symmetrical combustion chamberManagement: Bosch Mechanical with electronic advancePower output: at 2900 rpmTorque: at 1600 rpmCompression ratio: 17.2:1Emissions control: Electric fuel control The ISB (Interact System B) is one of the largest straight-six engines used for light truck vehicles and school buses, and the improved high output 600 version was on the Ward's 10 Best Engines list for 2004.
One unusual feature of the ISB is that it is a multi-valve pushrod engine design, with four valves per cylinder (popularly referred to as the "24-Valve" Cummins). The engine displaces , with a cylinder bore and piston stroke. A turbocharger is used to increase the output in the high-compression (17.2:1 in recent versions) diesel. It is an all-iron engine with forged steel connecting rods, an assembled camshaft, and a cast aluminum intake manifold. The engine is produced in Columbus, Indiana.
The ISB uses electronically controlled Bosch fuel systems, unlike the 6BT systems which were mechanical. Early ISB engines utilize Bosch injectors and a Bosch VP44 high pressure pump. Later ISB designs have common rail fuel injection, Bosch injectors, and a Bosch CP3 high pressure pump.
Dodge Ram ISB Midway through model year 1998, the Dodge Ram switched from the 6BT to the ISB to meet updated emissions requirements. Like other ISB's, these engines started out using the Bosch VP44 rotary injection pump. The VP44 setup meant that timing and fuel could be precisely controlled, which led to cleaner emissions. However, VP44 failure rates were higher than the older P7100 injection pump. The compression ratio in these engines was 17.2:1. The 1998–2000 ISB was rated at and when equipped with the 47RE automatic transmission. The 1998–2000 ISB was rated at and when equipped with a manual transmission. For the 2001–2002 years, a standard output and a high output ISB Cummins engine were offered. The standard output, which was the same as the previous engines was rated to and when equipped with either a manual transmission or automatic. The high output ISB was rated at and , with only a NV5600 six-speed manual transmission available. The high output engine was different in a few ways from the standard output engine; it had higher compression (17.3:1), powdered metal valve seat inserts, a larger flywheel, the Bosch fuel system was reworked to allow higher fuel flows, and fuel-injection timing was altered.
Dodge Ram ISB CR
For the 2003 model year, the Cummins was introduced with Bosch high pressure common rail fuel injection, again increasing power output. On automatic equipped vehicles, the 47RE was upgraded internally to increase durability and torque capacity, now known as the 48RE. The 2003 rating for the Dodge truck was released at and . Midway through the 2004 model year, the Cummins 600 was introduced, producing at 2,900 rpm and at 1,600 rpm. This engine was noticeably quieter than the previous engines. The 5.9l ISB CR was the last diesel engine put in Dodge Ram Trucks that did not require emissions technology.
6.7 ISB
6.7 L ISB Cummins Production: 2007 – 2017 (to 2026 as B6.7)Displacement: Bore x stroke: Max power: at 2800 rpmTorque: at ~1600 rpmCompression ratio: 17.3:1Emissions control: EGR and DPFTurbocharger: Holset Engineering (variable)Fuel system: Common rail High pressure direct injection with 7-hole injectorsCamshaft drive: Gear driven Crankshaft: Forged steel, 7 main bearingsOil System capacity with filter:
The 6.7 L ISB was the latest version of the B Series from 2007 to 2018. It was the largest straight-six engine produced for a light duty truck or school bus. It produces and in the 2007.5 and newer Dodge 2500/3500 pickup trucks with the Chrysler-built six-speed 68RFE automatic transmission built at the Kokomo Transmission plant in Kokomo, Indiana. Engine torque is slightly reduced with the Mercedes G56 6-speed manual transmission at and . The 2007 and newer 3500 Cab & Chassis trucks only get the and version of the B6.7, whether it has the Aisin AS68RC or the Mercedes G56 6-speed manual transmission. As for the 2008 4500/5500 medium duty Chassis Cabs or the Sterling Bullet Trucks, they receive the and version of the B6.7, whether it has the Aisin AS68RC or the Mercedes G56 6-speed manual transmission. Late model 2011 Ram trucks produce and , with the exhaust brake rating boosted from to .2011 Ram 2500 and 3500 – Capability – Powertrain . Ramtrucks.com. Retrieved on 2011-08-04.
For 2019 this engine was updated to produce and torque.
It was also used in almost every new school bus in the US, being a default engine option with most manufacturers from 2010/2015-2026, and BMC Procity city buses
Changes over the 5.9 L ISB