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, and the oil pump, crankshaft, camshaft, and accessory drive system are all gear-driven via heat-treated, ductile iron helical gears.
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.
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 as such 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 op- tion revived the automaker’s dying truck line while also redefining everything a heavy-duty pickup could be.
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.
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.
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.