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Cummins History, Lesson 1: ’89-’93 5.9L

For 30 years, Cummins and Chrysler have been engaged in one of the most successful mutual business ventures in automotive history. Over that time, Cummins has supplied its legendary diesel engines to Chrysler for use in ¾-ton and larger trucks. The Cummins name is credited with having saved Dodge trucks from extinction back in the late ‘80s and more than an 80-percent take-rate applies to the 2500 models Ram sells today. That percentage only goes up with 3500 models. So what was the first Cummins-powered Dodge like? By today’s standards, primitive—but in ’89 the 5.9L I6 was light-years ahead of the competition.

The first-generation 5.9L inline-six Cummins engine came stock with 160hp and 400 lb-ft, courtesy of a Holset H1C fixed geometry turbo. The cast-iron block and forged steel rods were well known for their durability, on top of producing big power from its 359 ci of displacement.

Even though the 6BT Cummins debuted in the late ‘80s aboard an obviously-dated AD Dodge chassis, it was nothing short of revolutionary. The industrial-intended inline-six packed a turbocharger, direct injection and 400 lb-ft of torque—something neither Ford nor GM offered on their V8 diesels. Join us for a quick walkthrough of this iconic, cast-iron mill’s robust construction, simple design and its one pitfall. The original 12-valve Cummins predates the P-pumped version, but it definitely set the tone in the diesel truck segment for years to come in terms of power, fuel efficiency and long-term durability.

Speaking of P-pump, stay tuned for Lesson 2, where we delve into the superlative ’94-’98 5.9L. It’s up next!

’89-’93 5.9L Hard Facts

Engine: 6BT Cummins Valvetrain: OHV, two-valves per cylinder, single cam
Configuration: I6 Injection System: Bosch mechanical, direct injection
Bore: 4.02 inches Injectors: Bosch
Stroke: 4.72 inches Injection Pump: Bosch VE
Displacement: 359 ci Turbocharger: Holset H1C fixed geometry
Compression Ratio: 17.0:1 Intercooler: Air-to-air (’91.5-‘93)
Block: Cast-iron with forged-steel crankshaft Emissions Equipment: None
Rods: Forged-steel, I-beam Horsepower: 160hp at 2,500 rpm
Pistons: Cast-aluminum Torque: 400 lb-ft at 1,700 rpm
Head: Cast-iron with six head bolts per cylinder (with sharing), cast-aluminum intake manifold    

Prototype 1 of 5

1989 Dodge D250 Cummins

You’re looking at one of the first Cummins-powered Dodge D250s ever built. Though its trucks were a bit dated by ’89, the Cummins option piqued the interest of thousands of consumers, with some 18,000 first-year models being sold (vs. the 10,000 trucks Chrysler’s marketing executives had predicted). While it’s not abnormal for a new engine program to take a few years to get up and running, this one was in the works for more than seven. Between finding a suitable engine supplier, choosing an engine that would actually fit in its AD platform trucks, building non-running prototypes, re-engineering key components to integrate the engine and then testing the snot out of the road-ready version(s), Chrysler’s first Cummins-powered Dodge, known by many as the “first-gen Cummins,” was launched in the summer of 1988.

Simple Design, Burly Parts

1989 Cummins 6BT Diesel Engine

Designed with both rigidity and simplicity in mind, the 5.9L Cummins 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. Originally, the 6BT was produced as a joint-venture between Cummins and Case in Rocky Mount, North Carolina. However, beginning in 1992 all engines would be built at Cummins Midrange Engine Plant (CMEP) in Walesboro, Indiana, a facility that’s dedicated to building engines specifically for Ram trucks. Initial production of the 6BT for Dodge is believed to have started in late October, 1987. To turn the 17.0:1 compression beast over, a 1,025 CCA single battery accompanied the engine. Later in the 6BT’s production run, dual 750 CCA batteries replaced the single unit.

Big-Rig Torque Curve

1989 Cummins 6BT Dyno Graph

While the 5.9L Cummins’ 160hp may not seem impressive by today’s standards, at the time of its release it was right in the thick of things, with GM’s 6.2L diesel producing just 130hp and Ford’s Navistar 7.3L turning out 185hp. However, the 6BT would handily beat out GM and Ford in torque. Thanks to its 4.72-inch stroke, the 5.9L could come up with 400 lb-ft at just 1,700 rpm (vs. GM’s 240 lb-ft and Ford’s 338 lb-ft). As you can see from the engine dyno graph above, most of the 5.9L’s power band consists of making 360 lb-ft of torque or more from 1,300 rpm all the way to 2,400 rpm.

Forged-Steel Rods

Cummins Forged Steel Connecting Rod

To cope with the kind of cylinder pressure the rotating assembly would be exposed to in this low-rpm, big-torque application, forged-steel connecting rods were selected. Of I-beam design, they utilize 7/16-inch rod cap bolts. To say there was a certain level of insurance built into the Cummins’ rods would be an understatement. After shot-peening, micro-polishing and equipping them with stronger rod cap bolts, they’re capable of withstanding 2,000 lb-ft of torque or more in competition applications.

Durable Pistons

1989 Cummins Piston

Cast-aluminum, direct-injection (DI) pistons provide for fuel to be injected directly into the piston bowl (no pre-chamber combustion event, as was the case on the IDI Ford and GM engines). The 17.0:1 compression pistons make use of a three-ring design that incorporates a keystone top ring and a sizable, 1.57-inch diameter floating wrist pin. Early, non-intercooled 5.9L pistons (’89-‘91) are said to be stronger than later versions due to the need to stand up to higher in-cylinder heat. The piston bowl’s geometry was altered to enhance intake swirl beginning with intercooled, ’91.5 engines.

12-Valve Cylinder Head

1989 Cummins 12 Valve Cylinder Head

The 5.9L’s cast-iron, cross-flow cylinder head entailed two valves per cylinder and anchored to the block via six bolts per cylinder, with one head bolt per cylinder used to fasten the rocker-lever assembly in place. The intake and exhaust valve rockers were made of ductile iron and all valve seats were induction-hardened. A cast-aluminum intake manifold routes air into the intake ports. Beginning on ’91.5 6BTs, revised head bolts with additional thread length for more block engagement were used, as well as a head gasket with a thicker crush ring around the cylinder bore.

Bosch VE

Bosch VE Cummins Injection Pump

At the heart of the 5.9L sat a Bosch VE rotary injection pump driven off the camshaft gear. A mechanically controlled, axial-piston pump, the VE utilizes a vane-style supply pump to pressurize fuel internally, with supply pressure increasing along with engine speed. A single distributing plunger is tasked with creating injection pressure as high as 17,400 psi in stock form, with the distributing plunger also controlling timing. Fuel makes its way to the injectors via six separate delivery valves located in the pump’s housing.

VE Tweaks

Cummins Diesel VE Fuel Pin

As a completely mechanical injection pump, the VE can be tweaked with simple hand tools by way of adjusting the maximum fuel screw on the backside of the pump. Other common free or cheap mods include grinding the pump’s fuel pin or buying an aftermarket one for harder fueling as boost rises and a 3,200 rpm governor spring, which allows the engine to keep building power beyond the factory-governed 2,500 rpm.

Mechanical Injectors

Dodge Cummins Bosch Fuel Injectors

Mechanical fuel injectors (also from Bosch) keep the rest of the 6BT’s fuel system simplistic. Once its pop-off pressure is achieved thanks to the fuel pressure being sent its way via the VE pump, its few moving internal parts force fuel out the nozzle and into the cylinder. These injectors are known to last at least 200,000 miles, but with proper maintenance can last twice that long before an overhaul is necessary. From ’89 to ’91, the injector bore within the cylinder head measured 9 mm, but starting in ’91.5 (through ‘93) this diameter was reduced to 7 mm to accommodate slightly different injectors.

The Holset H1C

Cummins Holset H1C Turbocharger

A fixed geometry, journal bearing Holset H1C model turbocharger graced all ’89-’93 5.9Ls, but there were a few variances throughout the engine’s production run. Most notably, there were turbine (exhaust) housing and compressor wheel differences. On early engines an 18 cm2 turbine housing was employed, then a 21 cm2 housing was used beginning on ’91.5 mills. However, due to customer complaints of low power and increased lag, the tighter, 18 cm2 turbine housing was reinstated from ’92.5-’93. From the factory, the H1C produced 18 psi of boost.

Beware the “Killer” Dowel Pin (KDP)

Cummins Diesel Killer Dowel Pin Damage

The only major drawback on the first 5.9L Cummins (and all ’89-’02 engines for that matter) is something that often occurs with age. To locate the timing gear housing during assembly, a small dowel pin was pressed into the block. After years of vibration and heat cycles, the dowel pin (which is made of steel) can work itself out of the block. Once free, the dowel pin can make contact with the cam gear, as well as the injection pump and crank gears, which can result in catastrophic engine damage. While it can be laborious to gain access to the dowel pin, it’s worth venturing inside the timing gear housing to physically lock the KDP permanently in place.

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