When it comes to gas-guzzling engines, the American automotive industry is known for building some of the best V8s of all time. However, US manufacturers have also developed some legendary diesels, and the best of them all remains the 5.9-liter Cummins.

Since then, Cummins has been responsible for the best-performing and most reliable diesel engines built in the United States, which a wide range of industries have been relying on.

Unquestionably, its most famous products are the powerplants designed for America's modern-day workhorse: the

The game-changer

Photo: Cummins

With a beefy

The first-gen 5.9-liter Cummins (6BT) wasn't the only diesel engine available for light-duty pickup trucks, but it was a game changer that worked wonders for boosting sales of Ram trucks. Compared to the V8 diesels from

Available with either a five-speed manual or a heavy-duty version of the A727 three-speed automatic, it made 160 hp and a whopping 400 lb-ft (542 Nm) of torque.

Built between 1989 and 1993, the 5.9-liter quickly became the go-to choice for all Dodge Ram customers, proving it was the most reliable and

Making a great engine even better

Photo: Dodge

However, the most famous upgrade of them all was the replacement of the old camshaft-driven, rotary-style VE injection pump with a new Bosch inline unit codenamed P7100 - better known as the P-pump.

With all the new hardware, the 1994-1998 second-gen became more powerful, reliable, fuel-efficient, and emission-friendly. Depending on the model year and the transmission it was mated to, the engine could crank out up to 215 hp and 440 lb-ft (597 Nm) of torque.

The move to 24 valves

Photo: Mecum

Its cylinder head was reworked and featured four valves per cylinder, while the fuel-injection system was now electronically controlled. The system also gained a new high-pressure electronic rotary pump called Bosch VP44, which became the engine's Achilles heel.

Though the 5.9 generally remained a staple of reliability, and its output increased to 235 hp and 460 lb-ft (624 Nm) of torque (when linked to a

The VP-pump ISB was discontinued after the 2002 model year. However, a year before it rode into the sunset, Dodge offered a high-output version equipped with several improvements, including a bigger Holset HY35W turbocharger. Mated to a

The Common Rail version

Photo: Mecum

This time, the formidable powerplant received a revised block, cylinder head, valve train, turbocharger (since 2003), and injectors. Still, the most significant upgrade was Bosch's innovative high-pressure Common Rail fuel injection system, which also meant a new injection pump codenamed CP3 that proved much more reliable than the VP.

In addition to reliability, fuel efficiency, and output figures increased again. In 2003, the engine's maximum ratings stood at 305 hp and 555 lb-ft (752 Nm) of torque. Then, midway through the 2004 model year, the high-output 5.9 called Cummins 600 made 325 hp and 600 lb-ft (813 Nm) of torque.

The Common Rail ISB 5.9-liter remained available until the end of the 2007 model year. Then it was

Legacy and tunability

Photo: Dodge

It's proven more than capable of outliving the trucks it was equipped on, and it's highly sought-after by both Ram fans and diesel enthusiasts who recognize its greatness but argue that it was available on mediocre trucks.

Although it was a workhorse engine, throughout the years, both categories of enthusiasts have realized that the 5.9 had immense tuning potential thanks to its ultra-solid architecture. Thus, they began tuning and swapping it into various trucks (and even cars) for

Thus, 5.9s with significant horsepower and torque gains have become fairly common. For example, extracting around 600 horses and 1,000 lb-ft (1,355 Nm) of torque from a Common Rail ISB is easy, requiring just a remap, a

In conclusion, as electrification is taking over, the 5.9-liter Cummins will probably go down as one of the best diesel engines ever built.

Since I mentioned 5.9 swaps into cars, I'll leave you with an awesome YouTube video by Westen Champlin, who installed a heavily-tuned, 1,500-hp 6BT into his Mustang.

Founded 104 years ago (1909) in Columbus, Indiana, by a bright, young mechanic named Clessie Cummins and banker William Glanton Irwin, the Cummins Engine Company (now Cummins Inc.) started by developing diesel engines - which was a relatively new concept back then -, but it became a successful industry leader several decades later, during the road building boom of the post-WWII era.Since then, Cummins has been responsible for the best-performing and most reliable diesel engines built in the United States, which a wide range of industries have been relying on.Unquestionably, its most famous products are the powerplants designed for America's modern-day workhorse: the pickup truck . If we have to single out the best one, it's, without a shred of doubt, the venerable 5.9-liter turbocharged inline-six that powered Ram Ram trucks for nearly two decades.Part of the B-Series engine family designed for multiple applications, the 5.9-liter made its automotive industry debut in the first-generation Dodge Ram trucks back in 1989 as a fuel-efficient alternative for gas-powered V8s.With a beefy cast-iron block featuring a forged crank with seven mains, a 12-valve cast-iron cylinder head, and a gear-driven cam drive, it was built to withstand years of hardcore punishment.The first-gen 5.9-liter Cummins (6BT) wasn't the only diesel engine available for light-duty pickup trucks, but it was a game changer that worked wonders for boosting sales of Ram trucks. Compared to the V8 diesels from Ford and GM, the inline-six was the only unit that featured direct fuel injection courtesy of Bosch. Furthermore, it was sensibly more fuel-efficient and delivered more torque since it ditched natural aspiration for a Holset H1C turbocharger.Available with either a five-speed manual or a heavy-duty version of the A727 three-speed automatic, it made 160 hp and a whopping 400 lb-ft (542 Nm) of torque.Built between 1989 and 1993, the 5.9-liter quickly became the go-to choice for all Dodge Ram customers, proving it was the most reliable and fuel-efficient diesel engine available in the light-duty pickup truck segment.The mighty 5.9-liter got its first significant update in 1994 when the second generation of Ram trucks was introduced. Though it featured the same block, conrods, and 12-valve cylinder head as the initial version, the engine received many upgrades, including redesigned pistons, higher-flow fuel injectors, or an enlarged intercooler that made it better across the board.However, the most famous upgrade of them all was the replacement of the old camshaft-driven, rotary-style VE injection pump with a new Bosch inline unit codenamed P7100 - better known as the P-pump.With all the new hardware, the 1994-1998 second-gen became more powerful, reliable, fuel-efficient, and emission-friendly. Depending on the model year and the transmission it was mated to, the engine could crank out up to 215 hp and 440 lb-ft (597 Nm) of torque.With emission restrictions tightening, at the halfway point of 1998, the 5.9 got its first significant redesign and received a new designation: ISB.Its cylinder head was reworked and featured four valves per cylinder, while the fuel-injection system was now electronically controlled. The system also gained a new high-pressure electronic rotary pump called Bosch VP44, which became the engine's Achilles heel.Though the 5.9 generally remained a staple of reliability, and its output increased to 235 hp and 460 lb-ft (624 Nm) of torque (when linked to a manual gearbox ), the VP pump had the habit of failing, which was a nagging issue for the legendary engine.The VP-pump ISB was discontinued after the 2002 model year. However, a year before it rode into the sunset, Dodge offered a high-output version equipped with several improvements, including a bigger Holset HY35W turbocharger. Mated to a bombproof NV5600 six-speed manual, this 5.9 made 245 hp and a massive 505 lb-ft (685 Nm) of torque.For the second part of the 2002 model year, Dodge introduced the new, third-generation Ram trucks, and several months later, another revised version of the 5.7-liter ISB followed.This time, the formidable powerplant received a revised block, cylinder head, valve train, turbocharger (since 2003), and injectors. Still, the most significant upgrade was Bosch's innovative high-pressure Common Rail fuel injection system, which also meant a new injection pump codenamed CP3 that proved much more reliable than the VP.In addition to reliability, fuel efficiency, and output figures increased again. In 2003, the engine's maximum ratings stood at 305 hp and 555 lb-ft (752 Nm) of torque. Then, midway through the 2004 model year, the high-output 5.9 called Cummins 600 made 325 hp and 600 lb-ft (813 Nm) of torque.The Common Rail ISB 5.9-liter remained available until the end of the 2007 model year. Then it was discontinued entirely, passing the torch to its bigger, more advanced B-Series sibling, the ISB 6.7-liter.Today, sixteen years after it was discontinued, the 5.9-liter Cummins is arguably the most legendary diesel engine ever produced on American soil.It's proven more than capable of outliving the trucks it was equipped on, and it's highly sought-after by both Ram fans and diesel enthusiasts who recognize its greatness but argue that it was available on mediocre trucks.Although it was a workhorse engine, throughout the years, both categories of enthusiasts have realized that the 5.9 had immense tuning potential thanks to its ultra-solid architecture. Thus, they began tuning and swapping it into various trucks (and even cars) for off-road , on-road, and race applications, which led the industry to develop a wide range of aftermarket upgrades.Thus, 5.9s with significant horsepower and torque gains have become fairly common. For example, extracting around 600 horses and 1,000 lb-ft (1,355 Nm) of torque from a Common Rail ISB is easy, requiring just a remap, a bigger turbo , a higher-pressure lift pump, and higher-flow injectors. But that's not to say that the older 6BT versions are inferior when it comes to power potential. With more comprehensive tuning, the block can handle up to 1,500 hp and around 2,000 lb-ft (2,711 Nm) of torque.In conclusion, as electrification is taking over, the 5.9-liter Cummins will probably go down as one of the best diesel engines ever built.Since I mentioned 5.9 swaps into cars, I'll leave you with an awesome YouTube video by Westen Champlin, who installed a heavily-tuned, 1,500-hp 6BT into his Mustang.

5.9L vs. 6.7L: Which Cummins Is Really Better?

• December 9, 2016
• Story By Mike McGlothlin

When it first arrived on the scene for ’07.5 model year Dodge Rams, the 6.7L Cummins had a lot to live up to. After all, the 5.9L it replaced was highly revered for its million-mile durability, vast performance potential, and an endless supply of aftermarket parts and support. Luckily, more than 40 percent of the 6.7L’s internals were carry-over parts from the 5.9L, and the injection system remained virtually the same.

The common-rail 5.9L Cummins was produced from 2003-2007 and featured a fixed-geometry turbo. It put out 325hp and 610 lb-ft of torque by the end of its run, along with 359 ci of displacement coming from a bore of 4.02 in and stroke of 4.72 in. The 6.7L Cummins started its run for 2007.5 model year Ram trucks with 350hp and 650 lb-ft of torque. It used a variable geometry turbo and had a bore of 4.21 in and stroke of 4.88 in, with 408.2 ci of displacement.

However, as far as emissions control devices are concerned, the 6.7L would be saddled with the full gamut — something 5.9L owners knew absolutely nothing about. The brand new, 6.7L Cummins came with exhaust gas recirculation (EGR) and a diesel particulate filter (DPF). On 2013 model year Rams, selective catalytic reduction (SCR) would be added to the mix, which as part of the exhaust after treatment system would require the use of diesel exhaust fluid (DEF) to reduce NOx emissions.

And now the trade-offs begin! Due to the aforementioned emissions control components, fuel economy is noticeably lower on 6.7L-equipped trucks as well as its overall reliability. However, the 6.7L’s use of a variable geometry turbocharger makes it much more responsive than the 5.9L at low rpm, not to mention that it incorporates a very effective exhaust brake function. The 6.7L’s larger displacement also helps produce considerably more torque, which is ideal for truck owners that spend a lot of their time hooked to a trailer.

While plenty of folks still believe the venerable 5.9L is the end-all, be-all Cummins, many have embraced the fact that the 6.7L is basically a bigger, better version of the 5.9L. But, is the 6.7L actually the better engine? To try to settle this 50/50 split, we’ve spelled out each engine’s strong suits and key weaknesses below.

*FYI: With so many versions of the 5.9L being produced (12-valve with mechanical injection, 24-valve with electric-over-mechanical injection, 24-valve with common-rail injection), we’re concentrating on the last 5.9L engine produced for the purposes of this article: the 24-valve, common-rail version produced from 2003-2007.

HARD FACTS

  5.9L Cummins Specs 6.7L Cummins Specs DISPLACEMENT 359ci 408.2ci BORE 4.02 inches 4.21 inches STROKE 4.72 inches 4.88 inches COMPRESSION RATIO 17.2:1 17.3:1 INJECTION SYSTEM Bosch high-pressure common-rail Bosch high-pressure common-rail INDUCTION

Holset HE341 (’03-‘04) or HE351CW (’04.5-‘07) fixed geometry turbocharger

Holset HE351VE variable geometry turbocharger

FACTORY HORSEPOWER

325 hp (2005-2007)

350 hp (2007.5-2012), 385 hp (2013-2016)

FACTORY TORQUE

610 lb-ft (2005-2007)

650 lb-ft (2007.5-2012), 850 lb-ft (2013-2014), 865 lb-ft (2015), 900 lb-ft (2016)

 

High-Pressure Common-Rail

5.9L Cummins engines built between 2003-2007 and all 6.7L mills utilize a high pressure common-rail fuel system. At the heart of the system sits a Bosch CP3 injection pump, which is responsible for creating in excess of 23,000 psi of fuel pressure and then maintaining that pressure. It’s rumored that the version of the CP3 that graces the 6.7L flows more, but we’ve never been able to confirm this (even Bosch agrees it’s just hearsay).

Per Bosch’s data on the 5.9L and 6.7L CP3, the pump flows 180 to 185 lph at 3,500 rpm. However, the common-rail system aboard the 6.7L Cummins does have a fuel rail, fuel rail feed lines, injector lines and crossover tubes that measure 50 percent larger than what you’ll find on the 5.9L. Because the 6.7L’s injection system operates at slightly higher pressure than the 5.9L’s, a pressure relief valve with an additional 3,000 psi rating is used.

Similar Injectors

While the injectors used in both the 5.9L and 6.7L Cummins appear similar in dimension, the version used in the 6.7L is designed and rated for higher pressure (and also requires lower voltage to operate them).

Unlike the 5.9L injectors, the units used in the 6.7L are programmed to each specific truck’s ECM, and injectors can’t be swapped into different cylinders (which was possible on the 5.9L).

Fixed Geometry Turbo (5.9L)

For the sake of a stock vs. stock turbo comparison, the Holset HE351CW found on ’04.5-’07 5.9L Cummins mills is by far simpler and less technologically advanced than what graces the 6.7L.

The HE351CW features a 58mm compressor wheel (inducer), a 58mm turbine wheel (exducer), employs an internal wastegate and is extremely durable, as long as it’s not subjected to 45 psi of boost or more on a regular basis. However, its fixed geometry design means there is no getting around turbo lag at low rpm.

Variable Geometry Turbo (6.7L)

Good responsiveness at any engine speed is a good way to describe what variable geometry turbochargers provide, and the Holset HE351VE on the 6.7L Cummins is no exception. This turbo features a 60mm compressor wheel (inducer), is void of a wastegate and features Holset’s variable nozzle technology. By varying the exhaust flow across the turbine wheel (via the nozzles), the turbo can perform like a smaller unit at lower engine speed (to aid responsiveness) and then act like a much bigger turbo at higher rpm. This effectively offers optimum performance at any point in the engine’s power band, from idle to redline.

While the HE351VE sounds like the best of both worlds, it does have some reliability issues. And, the problems the HE351VE has aren’t lost on any variable geometry style turbo design: carbon, soot and coking in the turbine side hampering the operation of the nozzles (i.e. causing them to stick). If the nozzles remain stuck in the open position, you’ve got a laggy truck until the tach shows a good amount of rpm. If they stick closed, you’re driving a truck that is really only usable at lower rpm. It’s widely known among 6.7L owners that the best way to avoid ending up with a sticking turbo is to leave the exhaust brake on at all times (which exercises the nozzles).

Exhaust Brake Function (6.7L)

That’s right, we said exhaust brake. Technically a turbo brake, the HE351VE’s variable geometry design can be used to create considerable braking power, thereby saving considerable wear on the truck’s service brakes. By changing the nozzles’ positioning to yield more restriction, the 8,000-pound heft of an ’07.5-present Ram can be slowed surprisingly quickly.

As far as the exhaust brake department goes among all newer trucks (including ’11-present Fords and ’11-present GM’s), the Holset HE351VE is second to none in its ability to decelerate a vehicle. To cut to the chase, the HE351VE’s aggressive and effective exhaust brake function makes 6.7L powered Rams towing machines.

Emissions System Failures (5.9L Exempt)

While acronyms like “EGR” and “DPF” were a foreign language to 5.9L owners (unless you count a catalytic converter), 6.7L owners are well aware of how they function, and many have experienced a sticking EGR valve (due to carbon and soot buildup) or a plugged DPF.

A dirty, sticking EGR valve is shown here, and we’ll note that Cummins specifies it’s necessary to clean the EGR valve (and EGR cooler) every 67,500 miles in order to maintain trouble-free functionality. The 5.9L definitely has the advantage in this category due to its lack of emissions devices throughout its production run.

Blown Head Gasket (6.7L)

Thanks to its increased stroke over the 5.9L (4.88 inches vs. 4.72 inches), the 6.7L makes considerably more torque at low rpm, especially when combined with the aforementioned quick-lighting variable geometry turbocharger. Unfortunately, this means more cylinder pressure is present in the cylinders, which is the primary culprit for a surprisingly common problem on the 6.7L: head gasket failure. A lack of sealing area between the cylinders and water jackets (due to the 6.7L’s larger bore) is also to blame for its frequent, blown head gasket scenarios.

Engines that tow regularly are known to lift the head anywhere near the 200,000-mile mark, while modified engines typically one much sooner. By comparison, head gasket failures were few and far between on the 5.9L Cummins, and we’ve even seen owners of 12-valve 5.9L’s retorque the factory head bolts, add compound turbocharger systems and make 100 psi of boost without experiencing a head gasket issue.

Solid Foundation

Nearly a decade into its production run, the 6.7L block has become the preferred foundation in diesel motorsports — mainly due to its Siamese cylinder bore design and the displacement advantage it provides.

In the upper echelon of truck pulling and drag racing, several companies add sleeves to the cylinders, a deck plate up top (to keep the tops of the cylinder bores from distorting) and increase the stroke of the crank. 6.7L blocks treated to this type of machining typically wind up with 390 cubic inches of total displacement, produce anywhere between 1,800 and 3,000 hp and do so in ultra-reliable fashion.

5.9L Pros & Cons

PROS:

• 325hp/610 lb-ft stock (2005-2007 model years)
• Fixed geometry turbo is more reliable than the 6.7L’s variable geometry unit
• No emissions equipment other than catalytic converter
• More sealing area + less cylinder pressure than the 6.7L (torque) = fewer head gasket failures

CONS:

• Smaller displacement (359ci) resulting in less low-end torque
• Less cubes means it’s harder to light a bigger turbo
• Fixed geometry turbo is laggier than the 6.7L’s variable geometry unit
• Lower pressure, smaller fuel rail and injection lines on common-rail system

6.7L Pros & Cons

PROS:

• Larger displacement and longer stroke than the 5.9L means more low-end torque and better ability to spool a larger turbo
• 350hp to 385hp stock (depending on model year and transmission)
• 650 lb-ft to 900 lb-ft of torque stock (depending on model year and transmission)
• Variable geometry turbo is very responsive (no lag) and incorporates a turbo brake
• Higher pressure, larger fuel rail and injection lines on common-rail system

CONS:

• Much more prone to head gasket failure
• Variable geometry turbo is less reliable than the 5.9L’s fixed geometry unit
• EGR system is prone to failure
• DPF is prone to failure
• SCR system adds a failure point (‘13+ trucks)

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• Read this diesel engine comparison article to see if the 7.3L or 6.0L Power Stroke is better.