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.
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<span class="anchor" id="3.9"></span>
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 .
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 115 hp @ 2,500 rpm, a peak torque output of 270 lb-ft @ 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.
6.7 L<span class="anchor" id="6.7B"></span>
B6.7
{| class="wikitable"
! B6.7L Cummins
|-
|Production: 2007 (2017 as B6.7) – present (diesel variant production ended 2026)<br>Displacement: <br>Bore x stroke: <br>Max power: at 2600 rpm<br>Torque: at ~1600 rpm<br>Compression ratio: 17.3:1<br>Emissions control: EGR and DPF<br>Turbocharger: Holset Engineering (variable)<br>Fuel system: Common rail High pressure direct injection with 7-hole injectors<br>Camshaft drive: Gear driven <br>Crankshaft: Forged steel, 7 main bearings<br>Oil 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 not does 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.
7.2 L<span class="anchor" id="7.2B"></span>
B7.2L
{| class="wikitable"
! B7.2L Cummins
|-
|Production: 2026-present<br>Displacement: <br>Bore x stroke: <br>Max power: at 2400 rpm<br>Torque: at ~1600 rpm<br>Compression ratio: N/A<br>Emissions control: EGR and DPF<br>Turbocharger: Holset Engineering (variable)<br>Fuel system: Common rail High pressure direct injection with 7-hole injectors<br>Camshaft drive: Gear driven <br>Crankshaft: Forged steel, 7 main bearings<br>Oil System capacity with filter:
|}
The Cummins B7.2L was announced in March of 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.
ISB engine<span class="anchor" id="ISB"></span>
4.5 L ISB<span class="anchor" id="4.5"></span>
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.
5.9 L ISB <span class="anchor" id="5.9ISB"></span>
{| class="wikitable"
! 5.9 L ISB Cummins
|-
|Production: 1998.5–2007<br>Camshaft drive: Gear driven <br>Crankshaft: Forged steel, 7 main bearings<br>System capacity with filter: <br>Fuel system: Common rail High pressure direct injection, symmetrical combustion chamber<br>Management: Bosch Mechanical with electronic advance<br>Power output: at 2900 rpm<br>Torque: at 1600 rpm<br>Compression ratio: 17.2:1<br>Emissions 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. The 5.9l ISB CR was the last diesel engine put in Dodge Ram Trucks that did not require emissions technology.
6.7 ISB<span class="anchor" id="6.7"></span>
thumb|6.7 Demo
{| class="wikitable"
! 6.7 L ISB Cummins
|-
|Production: 2007 – 2017 (to 2026 as B6.7)<br>Displacement: <br>Bore x stroke: <br>Max power: at 2800 rpm<br>Torque: at ~1600 rpm<br>Compression ratio: 17.3:1<br>Emissions control: EGR and DPF<br>Turbocharger: Holset Engineering (variable)<br>Fuel system: Common rail High pressure direct injection with 7-hole injectors<br>Camshaft drive: Gear driven <br>Crankshaft: Forged steel, 7 main bearings<br>Oil System capacity with filter:
|}
The 6.7 L ISB is 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 .
For 2019 this engine was updated to produce 400 hp and 1000 ft-lb 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
There are many changes over the previous 5.9 L ISB for the Dodge truck, the most obvious being the larger displacement. The 6.7 ISB had an increase of cylinder bore and piston stroke to , respectively, thereby giving a displacement of .
With the 6.7 L ISB came the introduction of the Variable Geometry Turbocharger (VGT). The VG Turbocharger was introduced to reduce turbo lag by adjusting the vanes by sliding a steel ring in the exhaust housing dependent on engine RPM creating more or less pressure inside the exhaust housing and controlling the speed of the turbocharger. It also works as an integrated exhaust brake system and is all controlled by an electronic actuator on the turbocharger. This VGT system has been an extremely common issue with the 6.7 L ISB and is typically diagnosed by the loss of the vehicle's exhaust brake.
QSB engine <span class="anchor" id="QSB"></span>
5.9 QSB <span class="anchor" id="5.9QSB"></span>
The 5.9 L QSB (Quantum System B) is an off-road, heavy duty version of the ISB. Typically used in marine, agricultural, and construction applications, these engines share many of the same parts as the ISB and utilize the same Bosch fuel system.
Cummins "HELM"
Next gen engines
The newest of the cummins B-series engines are part of cummins' "HELM" line up, including the B7.2L, and the B6.7 OCTANE. The HELM line up is not exclusive to the B-series, it also includes part of the X-series (X15, X15N, X10). The "HELM" line up "allows customers to choose the fuel type that best suits their needs".
Fuel system
Mechanical injection with mechanical timing
In the earlier models of the Cummins B-Series Engine, it was almost entirely mechanical including its fuel system. The fuel pump used in these engines was the Bosch P7100 injection pump, this pump is driven off the camshaft gear and drives its own internal camshaft to inject fuel to the individual injectors. This pump itself was one of the most popular options for fueling for the B-Series Engines because of this simplistic design and how reliable it was. The P7100 injection pump also allows for large amounts of fuel to be delivered into the system with simple tweaking to the system to allow for larger injection events.
Mechanical injection with electronic timing
In the later models of B-Series Engine, the fuel system was switched from mechanical injection and timing to mechanical injection with electronic timing. This was all thanks to Bosch's new VP44 radial distributor injection pump. The VP44 injection pump is driven at half the camshaft speed and produces an injection pressure of up to .
Mechanical injection with electronic timing and common rail pressurization
The most recent method of fuel injection used in Cummins B Series engines is electronically controlled common-rail injection. It has been used in Cummins B Series engines and the B-Series based NEF engine since 2003.