What is a 5.9-liter Cummins diesel engine? Well, for hardcore enthusiasts, it's arguably one of the most popular oil-burners ever produced. Cummins engines were dominant in agriculture long before their debut in heavy-duty trucks. The 12-valve 5.9-liter Cummins I-6 engine was developed and installed in Dodge pickups starting in 1989, creating one of the most legendary partnerships in automobile history.

Although Cummins' 5.9-liter (B-Series) engine was produced in many different configurations, for this report we're starting at the beginning. The powerplant was originally designed for medium-duty commercial trucks, but in 1989, Dodge's truck division began installing the first version of this engine, which is more commonly referred to as the "12-valve," in on-highway pickup trucks.

If you were buying a new Dodge Ram truck in 1989 (aka First Generation), you had options. You could choose a worthy 360ci gas-powered V-8 engine, or a powerplant that had the performance characteristics of a big-rig's engine: a 5.9-liter Cummins diesel. With the engines installed in Dodge Ram rigs, sales went through the roof. Coupled with a beefy five-speed manual transmission or a heavier-duty version of the A727 three-speed automatic, the 5.9-liter Cummins was an engine everyone wanted.

Unlike other diesel engines during this time, the 5.9-liter Cummins was the only one that featured direct injection. It didn't need glow plugs, nor was it naturally aspirated. This hoss was turbocharged (Holset H1C), and consumers loved the powerplant because of its perceived reliability, huge torque output (for the time), and excellent fuel economy. If you can believe it, their 160-horsepower and 400 lb-ft of torque rating was class-leading performance.

The Bosch-built fuel-injection systems found hanging on 5.9-liter Cummins diesel engines have changed several times throughout the I-6 powerplant's 18-year run. The original 12-valve was equipped with a camshaft-driven, rotary-style injection pump called a "VE" pump. The unit has proven its reliability over the years, but it has the least power potential.

Although there are many ways to increase the horsepower and torque of a VE-equipped 5.9-liter Cummins, it isn't the most popular pump for performance. With only a few hand tools, fueling can be increased enough to enable the engine to make 70 more horsepower than its stock output, but this injection pump is stingy when it comes to fuel quantity.

Even with the pump cranked up, more conservative fuel injectors present a bigger roadblock to better performance. Luckily, though, there is plenty of aftermarket support for getting more power and torque: upgraded turbos, injectors, as well as internal pump parts like governor springs, fuel pins, and more. Although the VE may leave a lot on the table in terms of performance, VE-equipped trucks with modifications can certainly be made more fun to drive.

In 1994, the 5.9-liter Cummins was similar in many ways to the 1989-1993 engine. It had the same displacement, and reused the first-generation's block, connecting rods, and head. However, the diesel experienced a few changes that made it even more popular, and capable of significant power output. The biggest and most exciting change was the introduction of Bosch's P7100 inline injection pump (P-pump), which replaced the VE pump.

While today it's all but impossible to associate a P-pump with anything emissions-related, this injection pump was introduced for 5.9-liter Cummins engines to help the powerplants meet emission standards in 1994. Unlike the VE, the P7100 offered higher injection pressures that slashed particulate matter and promoted better combustion.

Today, this is a sought-after platform when it comes to cranking out horsepower and torque. The P7100 offers almost limitless fueling capabilities, which, when paired with the correct airflow, results in big-time power. As a result of this, a few additional new components were necessary. These included a new style of fuel injector, a revised set of pistons, a larger intercooler, and a wastegated turbocharger (Holset HX35W or HX35).

Because of the improvements made to the 5.9-liter in 1994, the output increased. Depending on the transmission being used, a P7100-equipped 12-valve can churn out 175 horsepower and 420 lb-ft of torque. When you think about diesel performance, P-pumped 5.9-liter Cummins diesel engines are the first to be thought of.

This injection pump is one of the most preferred models for racing and performance-centered competition. In fact, 1,000 horsepower can be achieved because of the almost endless fuel supply. With tuning and upgrades, this version of the second-generation 5.9-liter can also withstand hundreds of thousands of miles.

Halfway through 1998, Dodge Ram trucks were forever changed. That is when the first ever (and least popular) electronically controlled fuel-injection system for the 5.9-liter Cummins engine debuted. The setup featured a high-pressure, Bosch-branded VP44 injection pump, also known as the "VP." The change was the result of more strict emissions standards with which the Cummins/Dodge Ram partnership was forced to comply.

The VP wasn't the only change that year, however. This was also when first versions of the 24-valve, 5.9-liter engines were introduced. The new four-valves-per-cylinder design improved the engine's efficiency and allowed it to crank out even more horsepower and torque than its predecessor. The initial 24-valve version of the 5.9-liter produced 235 horsepower and 460 lb-ft of torque with a manual transmission. With an automatic, outputs were lower (215 horsepower and 420 lb-ft of torque).

The improved 5.9-liter offered performance and drivability improvements and allowed for better coolant flow than its 12-valve predecessor. However, this came with issues of its own. The 24-valve's VP injection pump offered the least reliability, and although these engines were workhorses, at any time the injection pumps could fail. Failed lift pumps would inevitably starve the injection pump of fuel, causing it to die as well. This became a common repair in the late 1990s and early 2000s.

Another big change was the engine block. Some of the 5.9-liter castings are stamped with the number "53," and their thinner walls are susceptible to cracking and can often develop coolant leaks. Despite the advancements over the 12-valve, when 53-stamped engines are introduced to higher cylinder pressures, they are known to be nightmares—so much so that some were repaired under warranty back in the day.

As each of the big-three truck manufacturers' diesels drew closer to making 500 lb-ft of torque, the incentive to win grew. In 2001 (and for 2002 models), Dodge offered a high-output version of the 24-valve which allowed them to eclipse 500 lb-ft with the help of a new Holset HY35W turbocharger. With an output of 245 horsepower and 505 lb-ft of torque, the 24-valve was here to stay. The high-output, VP-equipped 5.9-liter Cummins was backed with a tough six-speed NV5600 transmission.

Although it seemed Cummins was headed in the right direction for on-road performance, when it came to diesel competitions (dyno, drag racing, sled pulling), the VP44's shortcomings left a lot on the table. Luckily, like the other versions before it, VP44-equipped trucks now have plenty of aftermarket support to improve on stock performance: plug-and-play tuners, upgraded turbos, larger injectors, and more.

Despite its issues, the VP44-equipped 24-valve edition of the 5.9-liter was highly desirable. There are a lot of late-2000s Cummins purists out there, but their favorite version of the engine only lasted until 2003, when, once again, the 5.9-liter diesel powerplant experienced yet another set of changes, bringing back that nostalgic reliability. It is also when a new body style was introduced, and the term "Third Generation" was born. Not only is this 100-percent electronically fueled engine one of the most popular versions of the 5.9-liter Cummins, the truck it powers is revered by Dodge Ram fans.

High-pressure, common-rail fuel injection was introduced in 2003, and its improved efficiency over previous generations offered a huge jump in power and torque output (305 horsepower and 555 lb-ft of torque).

Third Generation 5.9-liter engines utilized a Bosch-built CP3 injection pump, which over time has proven to be one of the most reliable and efficient fueling platforms. An electric lift pump sends diesel to the CP3 where it is pressurized and sent into the fuel rail. Once in the rail, fuel is distributed into each of the six fuel injectors. The revision allowed the engine to stay EGR-free for at least a few years.

The new fueling strategy included solenoid-activated injectors, an updated cylinder head, and a stronger 5.9-liter engine block. The injectors were 8-hole (2003-2004) and 5-hole (2004.5-2007) designs which, once again, helped with emissions. The new cylinder head featured a rocker box that accommodated the injectors and crossover tubes as well as heavier-duty intake and exhaust valves.

From 2003 to 2007, Cummins offered two different turbochargers for its 5.9-liter engine. A Holset HE341CW was used in 2003. It features a mechanically controlled internal wastegate that holds higher-than-stock boost pressures. However, to better comply with emissions regulations, the 2004.5 to 2007 engines came with Holset's HE351CW. This 'charger is similar to the HE341CW but the wastegate is electronically controlled. This new turbocharger allows for a noticeable boost in performance and lowered emissions output.

Finally, CP3-equipped 5.9-liters are the easiest Cummins engines to modify. Their ECMs are so easily manipulated that making over 500 horsepower and 1,000 lb-ft of torque is a breeze.

When it comes to the engine, the revisions over the years have made it tough as nails. When making power, the problem lies within the drivetrain for trucks of this vintage.

The 48RE four-speed automatic transmission that backs common-rail 5.9-liter engines is a common fail point when you start adding power. It's even problematic on unmolested, stock powerlants.

Emission concerns have played a huge part in the evolution of the 5.9-liter Cummins. Because of that, we've seen Cummins adapt and overcome, which resulted in better versions of the engine. The 5.9-liter is and always will be one of the most iconic diesel engines in the automotive industry.

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