The Cummins. It’s the engine that brought inline-six architecture, direct injection, the P-pump, and true, industrial-level strength to the ¾-ton truck market. In either 5.9L or 6.7L form, its stroke (4.72-inch, and then 4.88-inch) dwarfs the competition and produces the most usable off-idle torque of any engine in the segment. Higher in the rev range, it’s always been able to hold its own in the horsepower department as well. Not surprisingly, the majority of all aftermarket endeavors are geared toward the ’89-present 5.9L and 6.7L Cummins mills.

But why was the Cummins so overbuilt? For starters, it was originally intended for off-highway, round-the-clock use. At the outset, it was designed to power tractors, wheel loaders, mobile cranes, and gen-sets—with production of the 6BT starting as early as 1983. With Cummins looking to score an even larger contract than the one it landed with Case, Chrysler—which happened to be shopping for a diesel engine to power its 250 and 350 series Dodge trucks—came along at the perfect time. Equivalent to hitting the jackpot, the Cummins option revived the automaker’s dying truck line while also redefining everything a heavy-duty pickup could be.

Starting with its iron foundation and then moving on to the burly parts that reside within it, in the following pages we’ll explore why the Cummins is so robust and how you can make yours even better.

The 6BT 5.9L Cummins unquestionably revolutionized the diesel option in the pickup truck market when it debuted in ’89 model year Rams. However, plans for the Cummins/Case joint venture engine were on the drawing board long before that, with engines destined for Case off-highway equipment in production by 1983. Believe it or not, the 4BT (its four-cylinder cousin) went into production prior to the six cylinder.
Simple, robust, and proven, the original 6BT Cummins was designed around the use of a cast-iron, deepskirt, sleeveless block, and this remained the case until the present 6.7L, which utilizes a compacted graphite iron crankcase. The block’s casting entails oil pump and oil cooler cavities, the oil pump, crankshaft, camshaft, and accessory drive system are all gear-driven via heat-treated, ductile iron helical gears.
The Cummins’ immense low-end twist isn’t as much about its
inline-six design as it is the length of the stroke of its crankshaft. A 4.72-
inch stroke allowed Cummins to extract as much as 610 lb-ft of torque out
of 359 cubic inches at the OEM level, and the 4.88-inch stroke of the 6.7L
provides for 1,000 lb-ft right off the showroom floor at the present time.
Made from forged-steel, the crankshaft’s fillets and journals are treated to induction hardening for optimum wear behavior and durability. The crankshaft is secured in place via seven main bearing caps (14mm diameter main bolts on ’89-’97 engines, 12mm on ’97.5-later), which all but rules out crankshaft flex. A single connecting rod journal is located between each main bearing journal.
In street-driven applications, the connecting rods inside the Cummins (even the fractured-cap versions that made the cut beginning in ‘03) have long proven capable of handling 800 to 900-rwhp (and double that in torque). The I-beam forged-steel rods found in ’89-’02 5.9L’s are said to be good for as much as 1,500 hp in competition applications, after being treated to shot-peening, micro-polishing, and stronger 7/16-inch rod bolts. The small end of each rod accommodates a sizeable, 1.57-inch diameter floating wrist pin.
One of the Cummins’ biggest strengths has always been its use of six head bolts per cylinder. On 5.9L mills especially, where a fixed geometry turbo is employed and less torque is typically produced than what the 6.7L does, a blown head gasket is an extremely rare occurrence at mild to moderate power levels. From ’89 to ’18, all 5.9L and 6.7L Cummins engines were fitted with 12mm diameter head fasteners, but the debut of the 1,000 lb-ft 6.7L in ’19 brought with it larger, 14mm diameter head bolts.
Along with bringing turbocharging and direct injection to the pickup segment, Cummins was also the first to offer an intercooled diesel. As part of meeting the lower NOx standards that took affect on January 1, 1991, an air-to-air charge air cooler debuted aboard ’91.5 model engines, along with a larger 21cm2 exhaust housing on the H1C turbocharger (vs. 18cm2 on ’89-’91 engines).
Unlike the competition, the Cummins has employed Bosch injection systems exclusively throughout its production run. Things started with the Bosch rotary VE pump and mechanical pop-off style injectors from ’89-’93 (top left). From there, the Bosch P7100 (P-pump) was combined with higher pressure pop-off style mechanical injectors from ’94-’98 (top right). Then the Bosch VP44 was paired with mechanical injectors embodying the highest pop-off pressure yet from ’98.5-’02 (middle). Beginning in ’03, the switch to Bosch’s high-pressure common-rail system was implemented, and a CP3 (bottom right) was used in conjunction with solenoid valve style injectors. While minor improvements and overall pressure was increased with the 6.7L Cummins, the CP3 remained in the mix until ’19. Going forward, the 6.7L Cummins will make use of a Bosch CP4.2.
In the turbocharger department, Cummins owns Holset (presently known as Cummins Turbo Technologies) and has for more than 30 years. So it only makes sense that Holsets have been used exclusively on 5.9L and 6.7L Cummins power plants. This kind of in-house control is a big reason why the 6.7L Cummins was able to debut exhaust brake functionality, thanks to the well-matched and thoroughly tested HE351VE VGT, way back in mid-2007. Thanks to its head start, by the time GM and Ford introduced turbo braking, Ram had all but perfected it.
In January of 2019, Ram and Cummins dropped a major bombshell: ’19 model Ram 3500’s would be offered with a new high output 6.7L option that turned out 1,000 lb-ft of torque—the first manufacturer to hit the four-digit torque mark. But this was no revamping of the same-old 6.7L that had existed since ’07.5. Instead, it was an all-new power plant, complete with a compacted graphite iron (CGI) block, hydraulic lifters, larger diameter head bolts, a 29,000-psi common-rail system, and a state-of-the-art Holset VGT that produces more than 30 psi of boost in stock form.
Even the toughest engine in the world won’t tolerate a steel dowel pin-turned wrecking ball wondering around inside of it. This is why all ’89-’02 Cummins owners should address what’s known as the “killer dowel pin” before it ever has a chance to strike. From the factory, all 12-valve 6BT’s and ISB engines were fitted with a dowel pin in the block to correctly align the timing gear housing. Over time, the pin can work itself loose, drop down into the cam, pump, or even crank gears and cause all kinds of mayhem. Several aftermarket companies sell comprehensive killer dowel pin solution kits, Diesel Power Products, Industrial Injection, Haisley Machine, and Pure Diesel Power being among them.
For both VE and P-pumped 12-valves, engine speed is drastically limited in stock form. This is precisely why governor spring kits are so popular for these engines in the aftermarket. While both mechanical pumps can provide a tremendous increase in fueling with a few simple tweaks, you can’t fully capitalize on the added horsepower without bringing more usable rpm into the equation. Check out XDP or Industrial Injection for the best governor spring kit for your needs.
Power Driven Diesel’s AFC Live brought on-the-fly tunability to the P-pumped Cummins and in doing so revolutionized the street-driven ’94-’98 Dodge Ram aftermarket. The product allows you to fine-tune your max fueling, your fuel rate, and even provides a full-fuel override switch for when it’s time to race. AFC Live is also available with a custom AFC foot to ease the installation process, along with a max travel kit that’s matched to the amount of boost your engine sees.
EFI Live proved to be a game-changer when it was introduced for the ’06-’07 5.9L back in 2011. Almost overnight, the access and insight tuners gained into the Bosch ECM allowed them to make big horsepower reliable, along with bringing tremendous drivability back into the equation. EFI Live support has since spread to most late model 6.7L power plants. Across both platforms (the late 5.9L and most 6.7L’s), you can squeeze 525-rwhp out of virtually any ’06-newer truck with stock injectors, the factory CP3, the OEM turbo, and a good lift pump.
For ’98.5-’02 24-valve owners, adding an electric aftermarket lift pump isn’t simply a recommendation, it’s necessary if you want to get any kind of longevity out of the VP44. Be it a frame-mounted, engine-mounted, or an in-tank pump, consistent supply pressure and steady volume can keep a VP44 alive for 200,000 miles or more. Speaking of in-tank lift pumps, Fleece Performance Engineering recently extended its PowerFlo line to the 24-valve Cummins. Its comprehensive system provides a dual gerotor lift pump with integral fuel sender that supports 800 hp, push lock hose and the necessary AN and hose barb fittings, a filter base with a 3-micron Donaldson filter, and a distribution filter block.
Most Cummins mills can live their entire lives seeing 40 to 50 psi of boost on stock head bolts, and we’ve even seen the factory bolts handle 80 psi without stretching in race applications. However, when fuel and air mods take a turn toward serious, the possibility of blowing a head gasket can always become a reality. For holding down the fort, ARP head studs are the fasteners of choice. The company’s standard head studs (ARP2000 material) are a sound upgrade, but for big boost builds, its Custom Age 625+ head studs are recommended (and feature 260,000- psi tensile strength). For all-out competition, 14mm studs are also available, with drilling and tapping of the block being required in order to run them.
Not only can a Cummins handle a lot of abuse in stock form, but the factory turbochargers that came on them can as well. Each Holset can handle in excess of 30 psi of boost long-term, and the HX35 is known to live for years while dealing with 40 psi or more on a regular basis. As far as stock turbo mods go, compressor upgrades such as the 60mm and 62mm wheels from Gillett Diesel Service make a significant improvement on the H1C (’89- ‘93). For the HX35, various exhaust housing and compressor wheel mods can yield solid gains, and complete performance replacements such as the Fleece Performance Engineering 63mm Cheetah (shown) transform it into a 650hp-capable, stock-appearing charger.
For newer Holset models such as the HE351CW, Fleece and Tater Built offer impressive factory-based, direct replacement performance turbo options. And if you own an ’07.5-‘18 6.7L Cummins, Fleece has you covered there as well. Its Cheetah version of the HE351VE VGT utilizes a 63mm forged-milled compressor, is machined and accepts a high-flow turbine wheel, offers stock-like spool up, and supports 700 hp at the wheels.
In recent years, T4 flange exhaust manifolds have become all the rage on high-powered 5.9L’s and especially 6.7L mills. With the great strides made in turbo technology, torque converters, tuning, and even manifold design, an S400 series BorgWarner turbo can be run as a single with minimal drivability drawbacks. This is especially true for the 6.7L Cummins, where its bigger displacement coupled with a second-gen style exhaust manifold allows an S400 to be spooled almost as quickly as the VGT it replaced (also known as second-gen swap kits).
While every automatic placed behind the Cummins is arguably on the edge of its torque-holding capabilities at stock power levels, several highly capable aftermarket companies do a superb job of making them nearly indestructible. For the 46/47RH, 47RE, and 48RE fourspeeds, Sun Coast, Firepunk Diesel, and Goerend offer some of the best parts or complete transmissions you can buy. From high-powered street trucks to 2,000+ hp drag racers, each of them has a ton of R&D in perfecting Chrysler’s four-speeds.
Whether you’re looking to make 3,000 hp or just want plenty of insurance in your engine build, the Cummins aftermarket produces some of the beefiest connecting rods you’ll find anywhere. CP Carrillo’s H-beam rod is immensely popular, precision machined from a 4330M alloy forging, and proven to hold up in competition engines. Another extreme rod option is manufactured by Wagler Competition Products. Wagler’s heavy-duty units are made from forged-steel and come standard with ½-inch ARP rod bolts (vs. the factory size of 7/16-inch).
One of the most affordable ways to stop main cap walk is by tying all of them together with a girdle. Industrial Injection’s commercially available girdle kits have been used in countless high-horsepower builds, cost less than $1,000, and come with your choice of 12mm or 14mm ARP XL main studs. The Gorilla girdle (as it’s known) also adds rigidity to the block by tying into the the oil pan rails.
As camshaft technology has progressed for the Cummins, the one-size-fits-all approach from the old days has long disappeared. Whether you’re looking for just a little more exhaust flow, lower EGT, a bump in fuel economy, less smoke, or you’re looking to maximize your power beyond 4,000 rpm, there is a grind for everyone. For example: Colt Cams offers six different cam options for 5.9L and 6.7L owners. Its cams range from mild, 3,000-rpm intended applications all the way up to race engines with deep valve pockets designed to see 6,500 rpm.
The aftermarket has also made significant improves in the cylinder head arena. For durability purposes, thread-in style internal freeze plugs, heavy-duty valve guide liners, fire-rings, and heat-treated oversize valve seats are normal upgrades. For optimum horsepower, performance valve jobs, larger valves, heavy-duty valve springs, and 5-axis CNC porting is par for the course. Believe it or not, Fleece Performance Engineering sells an off-the-shelf performance series 24-valve cylinder head that supports 1,100-rwhp and can be shipped directly to your door. D&J Precision Machine is another major player in the cylinder head game, and offers a Stage 4 common-rail head that flows a whopping 320 cfm on the intake side (vs. 165 cfm stock).

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