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What Is A Cummins QSB Engine?

Written By, Mike McGlothlin


With Cummins’ reach extending into so many different industries, from land to sea, dirt to pavement, and in applications that serve duty around the world, it only makes sense for a global engine manufacturer to produce various versions of the same power plant. For on-highway, light-duty applications, many are familiar with the B-series and ISB Cummins. But what exactly is a QSB series Cummins? On the surface, a QSB Cummins—the “Q” representing Quantum—is essentially just an off-road version of its on-road cousin, the ISB. However, once technicalities enter the equation, several important differences emerge between the QSB and ISB.

As with all OEM engine programs, emissions standards dictate the build sheet. However, when powering industrial, marine, and off-road applications, QSB engines aren’t subjected to complying to the same stringent emission regulations ISB versions must adhere to. This often means more horsepower and torque is on tap (especially in marine vessels), and also that long-term reliability—at least from an emissions system perspective—is typically more favorable. This time, we’re highlighting the different QSB models Cummins produces (and has produced in the past), as well as spelling out the key differences and interchangeable parts between the QSB and ISB platforms.


General Differences Between ISB And QSB Engines

Cummins’ QSB line of engines is the off-road sibling of the wildly popular B-series family.QSB6.7 Cummins vs Ram 6.7L Cummins Being that the QSB power plants were often designed for some of the toughest industrial, agricultural and construction environments imaginable, versatility and utmost durability is built into each QSB Cummins produces. Many QSB’s can be equipped with either a rear geartrain or front geartrain and, while each version is very similar to its on-highway counterpart, QSB’s often benefit from the kind of subtle yet meaningful hard-part changes that lend themselves to exceptionally long service lives. Hint: the ISB acronym in on-highway Cummins engines stands for Interact System B, while QSB is short for Quantum System B.


Current QSB Engines Offered

The QSB4.5

Small, light and powerful given its size is the QSB4.5’s claim to fame—and it delivers. YouBig Bear Engine QSB4.5 Cummins Surplus Dieselcan find this 275 ci inline-four Cummins humming away in everything from agriculture and construction equipment to well servicing, rail engines, and even mining engines. Much like the modern QSB6.7 Cummins, the current version of the 4.5L QSB is equipped with high-pressure common rail injection, full electronic control, a variable geometry turbocharger, and meets Tier 4 Final/Stage IV emissions standards. The compact QSB4.5 measures 32.2-inches x 28.1-inches x 34.5-inches (L x W x H), weighs 818 pounds wet, produces 110 to 173 hp depending on the application, and can turn out as much as 520 lb-ft of torque (at 1,500 rpm).


The QSB6.7

QSB6.7 Cummins Common Rail Diesel EngineThis is perhaps the most relatable configuration for the on-highway crowd: the inline-six 6.7L QSB. Like the engine offered in Ram heavy duty trucks, it boasts 408 cubic inches, high-pressure common-rail injection and integrates Cummins’ quick-reacting VGT turbocharger with one-piece sliding nozzle technology. For use in construction, agriculture, mining, logging, rail, and well servicing equipment, the QSB6.7 is offered in ratings ranging from 146 hp to 310 hp. However, in commercial and light-duty marine propulsion, the QSB6.7 is rated as high as 550 hp and 1,250 lb-ft of torque. The QSB6.7 measures 41.7-inches x 28.6-inches x 37.8-inches (L x W x H) and tips the scales at 1,047 pounds.



The QSB3.3

Many don’t even know it, but there is another four-cylinder QSB option from Cummins,Cummins QSB3.3 Industrial Diesel Engine and it checks in in the form of a 199 cubic inch (3.3L) I-4. It fits where the QSB4.5 won’t, primarily serving time in oil and gas engines, and well servicing. The QSB3.3’s dimensions measure 27.7-inches x 24.3-inches x 29.9-inches (L x W x H) and with a 585-pound dry weight, it’s about as lightweight as an industrial-use diesel engine comes. Like its bigger brother, the QSB3.3 is graced with high-pressure common-rail injection, Cummins’ Quantum System electronic controls, and benefits from a charge air cooler. Designed to run anywhere in the world, the QSB3.3 operates seamlessly on high sulfur content or ultra low sulfur diesel (ULSD).


Older Versions Of The QSB

The QSB5.9

Very similar to the 5.9L Cummins offered in the 2003 Dodge Ram 2500 and 3500’s, theMarine Vessel Cummins QSB5.9 Diesel Engine QSB5.9 featured high-pressure common-rail injection, a 24-valve cylinder head, and a stronger cast-iron block than previous versions of the 5.9L had employed. In agricultural configurations, the 359 ci inline-six could produce anywhere from 130 to 275 hp and up to 730 lb-ft of torque. In marine applications, the QSB5.9 really shined, capable of producing as much as 480 hp. Like all engines powering watercraft, abundant cooling is always at hand, making it possible for marine engines to live at higher horsepower and higher loads. At 480 hp, the QSB5.9 produced nearly 40 psi of boost right out of the box yet the engine only saw 1,300 degrees F at the exhaust manifold.


Pre-Common-Rail QSB4.5

Cummins ISB4.5 Long Block Big Bear Engine Company

Electronic controls have always been present on 4.5L QSB engines, but high-pressure common rail injection hasn’t been. Granted, most Cummins QSB4.5 power plants feature the latter fuel system, but some versions came with an electronic-over-mechanical style arrangement similar to what was employed on the VP44-equipped Cummins ISB 24-valve 5.9L engine. These early QSB4.5’s were fitted with a rotary-style Bosch VP30 which, like the VP44 employed on the 5.9L I-6, is controlled via an integrated ECM affixed to the top of the pump. The VP30 feeds pressurized diesel to mechanical pop-off style injectors.



The QSB3.9

Older still is the QSB3.9, an engine based off of the tried-and-true 4BT architectureCommon Rail Cummins QSB3.9 Industrial Diesel Engine (forged-steel crankshaft, camshaft and rods, and with the same 4.02-inch bore and 4.72-inch stroke). Similar to the ISB170, a common-rail version of the 4BT (up to 170 hp) you could find in Fed Ex trucks once upon a time, the QSB3.9 was also graced with high-pressure common-rail injection. It’s typically found overseas, plugging away in construction equipment and small farm or forestry equipment. In these applications, power ratings often check in between 115 and 130 hp, with a rated maximum engine speed of 2,900 rpm.


Key Differences


As previously mentioned, emissions requirements control an engine’s hard part makeup.Cummins QSB Vs ISB Diesel Pistons Beyond changing the fuel injection pressure, timing, or injector nozzle spray angle, piston selection is a key ingredient in meeting emission standards—and QSB pistons vary considerably from their ISB counterparts. For example, in an industrial QSB5.9 application, the piston bowl is notably more shallow (i.e. open), while the bowl in an ISB piston is steeper (and deeper). Additionally, QSB5.9 pistons employed in 440 to 480 hp marine engines undergo double heat treatment and receive a radius’d fuel bowl lip in an effort to reduce stress and ensure no cracking occurs at high horsepower, constant load, and high heat).


Early QSB5.9 Vs. Late QSB5.9 Pistons

5.9L QSB Cummins Piston vs. ISB PistonOn top of the differences between QSB and ISB pistons, there are marked distinctions between the versions employed in early and late QSB5.9 engines. For ’03 and ’04 model year QSB5.9’s, a reentrant style fuel bowl similar to what was used in the ISB5.9 was utilized (although the ISB rendition is generally believed to be more reliable and also heavier duty). However, beginning in ’04.5 a non-reentrant style fuel bowl was implemented and ran through ’07 (the end of 5.9L production). The ’04.5-’07 piston design was conducive to making more horsepower while meeting tighter emission standards, but it did prove slightly less durable than the early piston. It’s worth noting here that QSB pistons can be interchanged with ISB units, which enthusiasts in search of higher horsepower tend to do in performance 5.9L and 6.7L Cummins builds.


Past, Present And Future Emissions Compliance

As United States off-road engine emissions standards began to tighten up around the turnMarine Diesel Cummins QSB6.7 Engine of the century, Cummins’ QSB line was poised and ready to meet them. Early QSB models easily met EPA federal Tier 1, Tier 2, and even Tier 3 standards. After that, stringent Tier 4 and Tier 4 Final standards were met with additional particulate matter and NOx pollution strategies and equipment in place. In the years ahead, and as the EPA finalizes Tier 5 requirements, we expect Cummins to lead the way once again in producing some of the cleanest, most reliable, and powerful off-road diesel engines in the world.


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