Back in 1981, Dodge unveiled the first-generation Ram trucks and vans. Everything was fine and dandy until Chrysler realized the need for something to increase sales in Dodge Ram lineup. Competition in the truck market was very stiff between Ford and General Motors, and it was hard for Dodge to carve out a name for themselves in this market.
So, in 1989, they teamed up with Cummins to offer a diesel engine in their pick-up trucks. The Cummins name is credited with having saved Dodge trucks from extinction back in the late ‘80s and more than 80% of RAM 2500 trucks sold today use a Cummins power plant. That number is even higher for 3500 models.
The 6BT Cummins
When the 6BT Cummins was debuted in 1989, it was a game-changer. Even though it was stuffed into the outdated AD Dodge chassis, it gave the Ram trucks a new foothold in the pick-up truck market. The 6BT packed a turbo, direct injection, and a whopping 400 lb-ft of torque, which exceeded anything Ford and GM were offering at the time with their V8 diesel engines.
With the new Cummins option for Ram trucks, Dodge began to see an increase in sales for the first time since the Ram’s launch in 1981. It should be noted that the Cummins equipped Ram was not the first Dodge pickup truck to have a Diesel engine option. Back in 1978 and 1979, the D-Series trucks were available with a naturally-aspirated Mitsubishi engine, however, those trucks didn’t sell very well.
The original 12-valve Cummins predates the P-pumped version, but it set the tone in the diesel truck segment for years to come in terms of power, fuel efficiency, and long-term durability. In the first year, the Cummins engine option was equipped on over 18,000 trucks, which was nearly double what Chrysler initially expected.
Offering 160hp, the 5.9L Cummins is pretty weak by today’s standards, however, at the time of its release, GM’s 6.2L diesel produced a mere 130hp and Ford’s Navistar 7.3L output 185hp. This put the Cummins right in the middle of its two competitors as far as horsepower, however, the Cummins output 400 lb-ft of torque, while GM’s diesel output 240lb-ft and Ford’s output 338lb-ft.
The Cummins higher torque output is part of what made it incredibly attractive in the late 80s and early 90s. Aside from the awesome power output, the first-gen 5.9L Cummins featured a super simple and super reliable design.
The 6BT’s foundation begins with a sleeveless cast-iron block with an integrated oil cooler and oil pump cavity, as well as a camshaft bore that doesn’t call for pressed-in bearings. For increased wear resistance, the forged-steel crankshaft’s fillets and journals were treated to induction hardening and the crank was anchored in place via 14mm main cap bolts.
At the heart of the 5.9L Cummins is a Bosche VE rotary injection mechanical fuel pump which is driven by the camshaft gear. A mechanical fuel system is very outdated by today’s standards where everything uses an electronic fuel system, but for the 5.9L Cummins, this was an awesome feature.
The pump created injection pressure up to 17,400 psi in stock form. One of the cool parts of the mechanical fuel injector system is the mechanical part of it. Unlike modern diesel engines which control fueling through the ECU, requiring an ECU flash to increase. The Bosche VE mechanical fuel pump can be tweaked with some basic hand tools and completely change how the 5.9L Cummins performs.
On all first-gen 5.9L Cummins, you’ll find a fixed geometry, journal bearing, Holset H1C turbocharger. There were some variances of this turbo from 1989 to 1993, however, they’re all pretty similar for the most part.
Underneath the cast-iron head, you’ll find cast-aluminum, direction-injection pistons that feature a large piston bowl for fuel to be injected directly into. Underneath the beefy pistons, you’ll find forged-steel I-beam connecting rods which are necessary for coping with the massive torque at low RPMs. These stock connecting rods are capable of supporting up to 2,000lb-ft of torque.
Updated 6BT in 1994
With stricter emissions standards set to take place in 1994 and the need for an updated Ram truck, the 5.9L was modified with some updated equipment for the launch of the second-generation Ram trucks. For the most part, the updated 5.9L Cummins was similar to the ones used from 1989 to 1993, however, the fueling system got a significant upgrade.
The addition of the Bosche 7100 pump, also known as the P-Pump, allowed the fuel system to have higher injection pressure which cut down particulate matter produced in-cylinder (and exiting out the tailpipe). Aside from the P-Pump, different injectors, revised pistons, a wastegated turbocharger, and a large intercooler were all additions to the updated 5.9L Cummins.
At this point, with the launch of the second-gen Ram trucks, the 5.9L Cummins was revolutionizing the 3/4-ton and larger truck market. Up until Ford released the 7.3 Powerstroke in 1994, the Cummins was the leader in torque output for five years straight!
Although the P-pump was originally added for emissions purposes, the massive increase in flow and pressure also means the P-pump can provide massive performance boosts when modified. While the VE pump on the early Cummins only had one plunger, the P-pump has six plungers, one for each cylinder. Nearly every internal component on the P-pump can be modified for increased power.
The 1994 Cummins powered Ram Trucks used a modified version of the Holset H1C turbocharger found on the earlier Cummins. In 1995, that turbo was scrapped entirely in favor of the HX35W, better known as the HX35. This turbo is known for being super reliable, even when pushing twice as much boost as it was originally intended for.
Aside from the upgraded turbocharger and P-pump fuel system, the updated Cummins also received more emissions friendly pistons, which featured a revised piston bowl design for more complete combustion. The camshaft was also updated to improve durability.
By the time the late 90s rolled around, the 5.9L Cummins was once again struggling to meet emissions standards, which forced another update of hardware to keep up with stricter emissions standards. Midway through the 1998 model year, the 5.9L ISB, better known as the 24-valve, was released.
The updated 24-valve engine featured electronically controlled fuel injection, compared to the mechanical fuel injection the 5.9L Cummins had been using up until that point. It also featured a 24-valve head, an updated turbocharger, new fueling components, and some other hardware updates, all as an effort to improve emissions and performance.
The result of the four-valve head and VP44 electronic injection pump combination was substantially improved airflow. This helped improve emissions, but more importantly, it meant more power. More specifically, up to 245hp and 505lb-ft depending on the year.
Inside the 24-valve, you’ll find the same I-beam connecting rods used in previous 5.9L Cummins engines, with further improved pistons. The four-valve head massively improved airflow and coolant flow. The 24-valve head featured 60 PPI valve springs, reshaped exhaust ports for improved exhaust flow, and six 12 mm head bolts per cylinder. A one-piece valve cover with a reusable gasket also replaced the 12-valve’s six individual units, which simplified servicing and accessing the valvetrain.
The Holset HX35W was carried over from the ’94-’98 12-valve engine, and it was used on all 5.9L ISBs with a manual transmission and ’98.5-’00 engines with the 47RE automatic. For emissions purposes, the automatic-equipped engines were fitted with the HY35W in ’01 and ’02, which was a little more restrictive on the exhaust side, which led to quicker spool up but less flow at higher rpm. Both turbos use a T3 turbine inlet flange, but the HX35W’s is divided while the HY35W’s flange is non-divided.
Unfortunately, the updated 24-valve engine wasn’t without its issues. Every version of the 5.9L we’ve talked about so far suffers from the killer dowel-pin problem, however, the 24-valve also suffers from the number 53 block problem. Certain ’99-’01 model year blocks were prone to cracking. Approximately 100,000 crankcases, produced by TUPY, were cast with thinner water jacket walls than what was found on earlier and later model engines.
The problematic blocks are identifiable via a “53” casting number visible on the driver side front of the block. Cracks typically begin beneath the freeze plugs, which can lead to coolant leakage when the engine is worked hard or exposed to increased cylinder pressure. These blocks are not guaranteed to crack, but it was a big enough problem that Dodge ended up replacing quite a few cracked blocks under warranty.
Yet again, in 2002, the 5.9L was getting close to failing to meet emissions standards and Dodge needed an updated version to meet the stricter standards. This is where 2003 to 2007 common-rail version of the 5.9L comes into play. Where the 5.9L was once a fully mechanic engine, the common-rail version was entirely dependent on electronics.
The new, electronically-controlled common-rail fuel system from Bosch featured a CP3 high-pressure fuel pump and solenoid-actuated, multi-event injectors, which made the Cummins quieter, cleaner and more powerful than ever before.
Even though there was a massive update that added tons of electronics, at the end of the day, it was still a 5.9L 24-valve Cummins. Although it was born from the need for better emissions, the common-rail version of the 5.9L also offered a massive increase in performance, bumping power to 305hp, and 555 lb-ft.
The new Bosch CP3 featured on the updated 5.9L was a radial-piston pump comprised of three cam-driven plungers, a high-pressure and low-pressure circuit, a hardened-steel housing and a fuel pressure regulator. Unlike the VP44 pump on earlier engines, the CP3 pump’s primary responsibility is to produce and regulate high-pressure fuel for the injectors to use. It isn’t directly involved with controlling injection timing or fuel volume. While it’s still driven by the front gear train, the CP3 isn’t timed to either the crankshaft or camshaft.
The common-rail 5.9L Cummins also featured new Bosch, solenoid-activated injectors, which were fired electronically, rather than mechanically like the earlier 5.9L engines. On ’03 and ’04 engines produced before January 1, 2004, each injector carried out two injection events per combustion cycle, however, later 2004 engines used new injectors featuring a third injection event that was employed to meet the 2004 EPA-mandated emission regulations.
Like previous 5.9L Cummins updated, the common-rail engines featured newly designed pistons that were designed to work well with the new injection system. This was accomplished by revising the fuel bowl area for more efficient combustion. Anticipating significantly more in-cylinder heat, Cummins added an oil cooling gallery on 2004.5 and later pistons.
In 2003, Cummins replaced the Holset HX35W in favor of the HE341CW. The new fixed geometry turbocharger utilized journal bearings, a seven-blade compressor wheel with a 56mm inducer, and a 58mm exducer. Like previous 5.9L turbos, the new turbo was wastegated to control boost pressure. This turbo was later replaced by the HE351CW, which featured minor changes including an electronically controlled wastegate.
Other changes in the 2003 to 2007 5.9L engine include a revised 24-valve head, as well as lots of other minor upgrades. Ultimately, the common rail 5.9L Cummins was no longer capable of meeting emissions standards and was replaced by the 6.7L Cummins in 2007.
The 6.7L Cummins
Similar to the LMM Duramax and 6.4L Powerstroke, the “clean diesel” era brought new emissions components to the 6.7L which hadn’t been seen in previous Cummins engines. The two major additions include an exhaust gas recirculation system and a diesel particulate filter. Unfortunately, the new emissions components made the 6.7L arguably less reliable than the 5.9L Cummins.
Of course, the increase brought with it an entirely new block, internals, and architect. The deep-skirt block features a rigid, Siamese bore design that the aftermarket would come to love thanks to its superb strength. In case you didn’t already know a Siamese bore design, just entails a lack of channels for coolant to circulate directly between cylinders, which improve cylinder stability and its an overall stronger design.
With the increased displacement and new design, the 6.7L Cummins was far more powerful than the 5.9L Cummins it replaced, outputting 350hp and 650lb-ft. Interestingly enough, the 6.7L Cummins was the first Cummins found in Ram trucks where the automatic transmissions had a higher output over the manual transmission models.
A feature that could be found on the 6.0L Powerstroke and the LLY Duramax was a variable geometry turbocharger, and the 6.7L was the first Cummins to feature this kind of turbo. The Holset HE351VE and it’s variable geometry design helped give the 6.7L Cummins lots of low-end and top-end power, while also doubling as an exhaust brake to help bring the truck to stop.
Unfortunately the increased stroke and use a variable turbocharger pushing lots of boost lead to significantly more head gasket failures on the 6.7 compared to previous Cummins engines. It’s not to say the 6.7 blows head gaskets like crazy, but it’s more often than the 5.9 Cummins.
A similar version of the Bosch common-rail fuel system found on the 5.9L was also on the 6.7L, but produces more peak pressure and makes use of slightly redesigned injectors to handle the added pressure. The revised injectors are said to be more reliable than the versions used on the 5.9L common-rail. When compared to 5.9’s fuel system, the 6.7L’s fuel rail, rail feed lines, and injector lines also measure 50-percent larger.
As I said before, the 6.7 was the first Cummins to feature an EGR and diesel particulate filter. Unfortunately, the EGR system was known for sticking and suffering from clogged EGR coolers when the 6.7 was initially released. The diesel particulate filter, better known as DPF, worked by trapping particulate matter in a chamber downstream in the exhaust. The accumulated soot is burned off through a process called regeneration, which uses diesel fuel to raise the temperature to more than 1,000 degrees F to incinerate the contents of the DPF.
Today, the 6.7L Cummins has overcome most of its early reliability issues, however, it features even more emissions controls now than it did before. Even with stricter emissions, the modern 6.7L Cummins outputs an insane 1,000 lb-ft of torque thanks to various upgrades.