When it comes to comparing indirect injection to direct injection, IDI possesses very few advantages. Aside from being more cost-effective than direct injection (at the OEM production level), a direct injection diesel is superior in virtually every other way. The biggest advantage the 5.9L Cummins had over its IDI counterparts was the fact that, thanks to being void of pre-chambers, it enjoyed far greater thermal efficiency. Combine that with the significantly higher injection pressures that are required in a direct injection system and you get better fuel economy, cleaner emissions, and even quicker starting performance

How The 5.9L Cummins Changed The Diesel Pickup Segment Forever

It’s the mid 1980s and you’re on the hunt for a new -ton or larger truck… The only manufacturers offering diesels are Ford and GM—and while they’re torquey, the indirect injection, non-turbo power plants don’t quite keep pace with the larger gas engine offerings of the day. Although the idea of getting double the mileage of a big-block appeals to you, the thought of being passed by anything and everything with a trailer behind you is a deal-breaker. The proven, big-cube gasoline V-8 wins out in the end. Just a decade later, justifying the purchase of a gas-powered -ton pickup is significantly harder to do. This is a direct result of Dodge getting into the diesel game back in 1989, which revolutionized the truck segment.

Up until Dodge entered the fray, Ford and GM were practically on cruise control, enjoying the added sales that came from those interested in the compression ignition side of life. When the Cummins option graced Dodge trucks, it was more than a contender. It dominated the class and made diesel a much more attractive choice for consumers. In a series of firsts, the Cummins brought direct injection, turbocharging, and 400 lb-ft of torque to the diesel pickup segment. This time, we’re cracking open the history book for a look back at the engine that ignited the torque wars between OEM’s, initiated the never-ending battles for class-leading payload and towing capacity, and even introduced us to diesel performance as we know it today: the 5.9L 6BT Cummins.

 

Although General Motors’ Detroit Diesel-produced 6.2L V-8 was the fuel economy king in the late 80s, Ford’s Navistar-built 7.3L engine shown here was the more powerful of the two, and also the better seller. The latter produced 185 hp at 3,000 rpm and 338 lb-ft at 1,400 rpm to the 6.2L’s ratings of 130 hp at 3,600 rpm and 240 lb-ft at 2,000 rpm. Both V-8 diesels were naturally aspirated and made use of indirect injection (IDI), which means that—thanks to featuring pre-chambers (or swirl chambers) in the cylinder heads—combustion actually began in the swirl chamber before spreading into the cylinder beneath it.
When it comes to comparing indirect injection to direct injection, IDI possesses very few advantages. Aside from being more cost-effective than direct injection (at the OEM production level), a direct injection diesel is superior in virtually every other way. The biggest advantage the 5.9L Cummins had over its IDI counterparts was the fact that, thanks to being void of pre-chambers, it enjoyed far greater thermal efficiency. Combine that with the significantly higher injection pressures that are required in a direct injection system and you get better fuel economy, cleaner emissions, and even quicker starting performance.
Not only was turbocharging a first for the diesel pickup segment, but it’s one of the reasons the smaller displacement 5.9L (359 ci) was able to blow its rivals out of the water. The 6BT produced more torque than either IDI V-8 by far, and it also enjoyed a better horsepower per cubic inch ratio than Ford’s 7.3L Navistar. On top of that, the turbo was the difference maker at altitude, where horsepower loss was minimal despite the lower air density that comes with elevation.
But how did the first-gen Cummins sell? To the surprise of Chrysler’s marketing department, which didn’t exactly expect the diesel to be a hit, a supply-limited 16,000-plus Cummins-powered Dodge trucks were sold that first year. In only its second year of production, Chrysler had to limit Cummins orders to 30,000 units. The demand for the vehicle played a large role in Cummins establishing an engine production plant specifically for the automaker. That facility is known as Cummins Midrange Engine Plant, or CMEP, located in Walesboro, Indiana. Cummins engines produced for Dodge trucks have been produced here since 1992. Prior to that, the 6BT was assembled at Cummins’ Rocky Mount, North Carolina plant.
With its 4.72-inch stroke, the most in the segment, the 5.9L 6BT produced gobs of low-end grunt, and the most in its class by a significant margin. On this engine dyno graph, notice that 350 lb-ft of torque or more is being produced from 1,300 rpm through 2,500 rpm, where peak horsepower is made. Although the original 5.9L Cummins was no high-rpm performer out of the box, its torque curve made it extremely drivable and more than capable of out-towing the competition.
The 5.9L Cummins’ peak torque of 400 lb-ft checked in at 1,700 rpm. Horsepower topped out at 160 hp, achieved at 2,500 rpm. At that engine speed, you were up against the Bosch VE injection pump’s governor, which overall made the Cummins a lower rpm operator than its rivals. Then again, by the time it was time to shift, the Dodge had already made much more headway than a Ford or GM.
While making an inline-six diesel fit in a gas truck’s engine bay might seem like an engineering marvel, Cummins’ 6BT was intentionally made the shortest I-6 oil-burner in existence in the 1980s. Its ability to be packaged in tighter engine bays and compartments was a way of attracting various truck and equipment makers to the 6BT platform—and it ended up convincing Chrysler to use it. However, with manpower severely lacking within the ranks of Chrysler, Cummins stepped up as the automaker’s engineering outside contractor on the first-gen trucks. That is to say the engine manufacturer took on nearly all the tasks of engineering, designing, and testing its engine within Dodge ¾-ton trucks.
It all starts with a solid foundation, and the 6BT Cummins was designed around the use of a deep-skirt, cast-iron block for utmost rigidity. Its parent bore design (i.e. dry-sleeve, liner-less cylinders) is shared with the 4BT, and not with the bigger 8.3L C series engine (6CT) which utilized wet sleeves. A conventional cam-in-block crankcase accommodated the camshaft with no press-in bearings being required, and also boasted seven main bearings.
Made of forged-steel and weighing in at approximately 125 pounds, the 5.9L Cummins’ crankshaft was pretty serious. It featured induction-hardened fillets and journals to promote optimum wear resistance and the aforementioned seven main bearings ensured that bend stress and deflection was never an issue. The 6BT’s main bearing journal size was determined in order to guarantee fillet stress was kept below 35,000 psi, and that oil film thickness could be optimized at maximum engine load.
You may have heard or seen the practice of a competition Cummins block visiting a shop to be machined to accept 14mm main cap fasteners (usually studs). On ’89-’97 5.9L engines, 14mm main bolts were factory hardware. Between the 14mm diameter main bolts, induction hardened forged-steel crankshaft, and seven main bearings, Cummins made darn sure the most important piece of the 5.9L engine (the crank) was never going to be a weak link.
Another bit of engineered overkill exists in the 6BT’s connecting rods. Of an I-beam design and made from forged-steel, the oversized con-rods were specifically designed to handle the 5.9L’s abundance of low-rpm torque. In the aftermarket, these factory rods have been used in engines making almost 10 times the horsepower rating the original 6BT had. They featured an angle-split rod cap design and used 7/16-inch rod bolts. The small end accommodated a large, 1.57-inch diameter floating wrist pin to reduce bearing loads, and at 0.407-inches thick (10.34mm), the wrist pin was extremely robust.
With direct injection comes the combustion chamber integrated directly in the piston. Because the original pistons used in the 5.9L Cummins were intended for a non-intercooled engine, the fuel bowl geometry is different from later versions (’91.5-‘93). Due to their need to stand up to higher in-cylinder temperatures than what later pistons would see, early pistons were marine-derived pistons designed to work with a 145-degree injector spray angle. The pistons were made from cast-aluminum and featured a three ring design, with a keystone top compression ring for optimal sealing and reduced risk of sticking.
A solid tappet camshaft was chosen to actuate the valvetrain on the 5.9L Cummins. The cam was made from chilled cast-iron, which brings with it high strength, hardness, toughness, and wear resistance qualities. To reiterate, the camshaft was also void of press-in bearings. The tappets too were chilled cast-iron pieces.
The tappets actuated the valves by way of push tubes (shown), and both the intake and exhaust valve rocker arms were made of a type of nodular iron with a highly advantageous strength-to-weight ratio. Individual, die-cast aluminum rocker covers were said to have been chosen with both cost and structural integrity in mind. The 6BT Cummins’ tappet cover was made of stamped steel. The cover’s primary purpose was to seal the opening in the block, although it also incorporated a crankcase breather.
Ultimate durability was also part of the cylinder head’s design. The crossflow style, cast-iron piece featured two valves per cylinder, but helical intake ports were employed to achieve the high-swirl goals for meeting both horsepower goals and emissions requirements. On the exhaust side, the valves were positioned as far away from water jacket areas as possible in an effort to better drive the turbocharger.
Also lending the cylinder head to longevity were induction-hardened valve seats for optimum wear resistance and six head bolts per cylinder, which was a practical guarantee that customers would never experience head gasket failure. By comparison, the 7.3L Navistar IDI powering Ford’s trucks made use of five head bolts per cylinder. The 5.9L Cummins’ fasteners measured 12mm in diameter, with two bolts being shared with the adjacent cylinder. In addition, one head bolt per cylinder was used to clamp the rocker pedestal assembly (shown) to the cylinder head itself.
In true, industrial form, all accessories were gear-driven off the front of the 5.9L Cummins, including the power steering/vacuum pump and oil pump. The crank gear drives the cam gear, which in turn drives the injection pump gear. The gear train was made up of six austempered ductile-iron helical gears, which were heat-treated for superior strength properties.
The 6BT utilized a serpentine belt system rather than a V-belt arrangement, which was still being employed on the rival 7.3L IDI at the time. The serpentine system drove the fan clutch, alternator, water pump, power steering pump, A/C compressor (if equipped), and made use of an automatic tensioner. A high-strength, 8-rib, poly-V serpentine belt made it all possible.
Surrounding the gear train was a molded aluminum timing gear housing. The timing gear housing was located using the infamous “killer dowel pin” and supported the injection pump and accessory drive components. Sealing everything off was a stamped steel front cover that incorporated the crank seal. Fasteners of varying lengths secured the front cover to the timing gear housing.
Unlike the Navistar/Ford 6.9L and 7.3L IDI’s and the Detroit/GM 6.2L, Cummins didn’t require glow plugs to accomplish timely cold-weather startups with the 6BT. Instead, a 12-volt intake air grid heater was employed, which pre-heated the incoming air and arguably worked much better than glow plugs in competing engines. As for starting, a single battery with 1,025 cold cranking amps—in conjunction with a 4hp, gear-reduction starter—was originally employed to turn the 17.0:1 compression engine over. After production began, dual 750 CCA units would replace the 1,025 CCA single battery.
Despite the compact fit of the 6BT within Dodge D/W engine bays, a 1.5-inch thick, downflow style radiator core could still be fitted, which proved highly efficient in rejecting heat. A 7-blade, 20-inch diameter fan (with each blade measuring roughly 2.5-inches in width), operated via a belt-driven viscous clutch (which turned 1.35 times that of engine speed), and a box-type fan shroud completed a sound cooling system.
After pressurizing fuel, the VE pump distributed fuel to six mechanical injectors, also supplied by Bosch. The pop-off style injectors used in the ’89-’93 6BT Cummins were set to open at 245 bar (or 3,553 psi) from the factory. Non-intercooled engines in ’89-’91 Dodge trucks were equipped with 4-hole nozzle injectors with a 145-degree spray angle, while intercooled engines (’91.5-‘93) would feature 6-hole units with a 155-degree spray angle.
High-pressure, direct injection combustion called for a more robust fuel system than what was being used in the IDI competition. It was 100-percent mechanical, but the Bosch VE, a rotary (or distributor style) injection pump was employed due to the higher pressures it could achieve. The axial piston VE pump used a vane-style supply pump to internally pressurize fuel as high as 17,400 psi. By comparison, the Stanadyne DB2 mechanical distributor style injection pump on Ford’s Navistar 7.3L IDI was rated at a peak of 6,700 psi—but in reality operated at much lower pressure than that.
As many already know, Holset turbochargers have graced every version of the Cummins, be it a 6BT, ISB, or common-rail 5.9L, or the VGT’s on the 6.7L. The O.G. version found on the 6BT—a fixed geometry, journal bearing charger called the H1C—is shown here. On non-intercooled engines (’89-‘91) the H1C featured a 7-blade, compressor wheel with a 50mm inducer diameter, along with an 18 cm2 turbine housing. In factory form, the 5.9L Cummins could force the H1C to produce between 18 to 20 psi of boost.
Beginning with ’91.5 engines, which again were intercooled, a version of the Holset H1C turbo with an 8-blade, 54mm compressor wheel inducer was in play. This H1C also used a looser, 21 cm2 turbine housing. However, due to a number of customers complaining of turbo lag and lack of power, the turbocharger’s 18 cm2 turbine housing was reintroduced midway through the ’92 model year production run.
The biggest historical change in the 5.9L Cummins’ lineage was the addition of the Bosch P7100 for the ’94 model year. In the aftermarket, huge performance potential could be unlocked from the mechanical P-pump. In OEM form, the injection pump change brought with it a bump in horsepower (175 hp) and torque (420 lb-ft), so long as the engine was paired with the NV4500 five-speed manual transmission option. Later on, those figures would swell to 215 hp and 440 lb-ft of torque.
Without question, Chrysler and Cummins’ move in 1989 forced the hand of the other truck manufacturers to get serious about their diesel options. With direct injection in the mix, turbocharging on the table, and 400 lb-ft of torque being made with fewer cubic inches, the 5.9L Cummins was in a category all its own from ’89-’93. In ’93, Ford introduced a turbocharged version of the Navistar 7.3L IDI which produced 190 hp (pictured), but still fell short of the Cummins in the torque department with 388 lb-ft. It wasn’t until the Navistar-built 7.3L Power Stroke—a direct injection, turbocharged, and electronically controlled V-8—came into the picture in Ford trucks in ’94.5 that a playing field more closely resembling “level” was reached.
Emissions regulations coupled with the impreciseness of mechanical injection would eventually spell the end for the 12-valve platform, at least for Dodge trucks. By mid-year ‘98 the ISB 24-valve took over. However, the 6BT provided a launching pad for what we have today: immensely powerful diesel engines that can move mountains (literally) right off the showroom floor. It’s not out of line to believe that the ’89-’98 5.9L Cummins was singlehandedly responsible for driving the rise in popularity of light-duty diesels in America. In the world of diesel pickups, very few consumers are unfamiliar with the Cummins name.
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