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Back in Time, Part 1: How the 5.9L Cummins Revolutionized the Diesel Pickup Segment

Ford may have always had the best selling trucks in the diesel segment, but that hasn’t kept the competition from tinkling in its Cheerios from time to time. One repeat offender has been Dodge/Ram—and the Auburn Hills manufacturer has been doing it for 30 years. Ever since Dodge debuted the 5.9L Cummins in its ’89 ¾-ton and 1-ton D-series trucks, Ford has continually had to answer the bell. The result has been a perpetual game of one-upmanship between the two—and we’ve enjoyed every minute of it.

Setting the Stage

While the horsepower and torque war is a close race in modern times, three decades ago things were quite different. For example, the 5.9L Cummins debuted with 400 lb-ft of torque, significantly more twist than Ford’s Navistar International 7.3L IDI V8 could produce (338 lb-ft). By comparison, GM’s fuel-sipping, Detroit Diesel-built 6.2L IDI V8 could only muster 240 lb-ft. The huge gap in torque between brands definitely played into Chrysler’s favor and was a major talking point for selling what was, for all intents and purposes, a prehistoric truck platform (Chrysler’s AD chassis was used from ’72-‘93). But boy was the Cummins enticing…

Dodge Actually Changed the Rules in ‘89 (Not ‘94)

Years before the “rules have changed” slogan became inseparable from the newly-redesigned Ram line, Dodge revolutionized the truck segment with the commercial duty 6BT 12-valve Cummins. After the arrival of the turbocharged, direct-injected inline-six mill led to exponential sales growth in what was once a truck line in its death throes, Ford took note. Ford and Navistar would add a turbo to the 7.3L IDI by ’93 and follow it the next year by debuting the direct-injected 7.3L Power Stroke. GM would up its game in 1992 with the release of the turbocharged 6.5L IDI V8.

So how do each of the Big Three’s engines from the late '80s stack up against each other? Better yet, why was the 5.9L Cummins so much better than the V8s offered by Ford and GM? Keep reading, we’re about to cover all of that and more, and stay tuned for Part 2 of our series, where we move into 1994: the 7.3L Power Stroke vs. the P-pumped Cummins.

Smallest Displacement, Most Torque

1989 Cummins 6BT Diesel

From a work truck standpoint, the Cummins was miles ahead of the IDI V8 diesels that Ford and GM offered at the time. While the 5.9L’s I6 design featured less moving parts and a 4.72-inch stroke that resulted in big torque production, direct injection and turbocharging also helped make the powerful inline extremely efficient. From just 359 cubic inches, the Cummins produced 400 lb-ft of torque, and many believe it was underrated. By comparison, Ford’s Navistar-built 7.3L displaced 444 ci yet only cranked out 338 lb-ft, and GM’s 6.2L gleaned just 240 lb-ft from 379 cubes. Aside from the fuel they burned, the Cummins was a different animal in nearly every way.

Anvil-Like Construction

Cummins 6BT Forged Steel Connecting Rod

A forged-steel crankshaft with induction-hardened fillets, 14mm main cap bolts, forged-steel I-beam connecting rods, tapered piston pin bushings with massive 1.57-inch diameter floating piston pins and six head bolts per cylinder make the original 5.9L 6BT one tough customer. Throw in the fact that the camshaft, injection pump, oil pump and everything else on the front of the engine were gear driven via heat-treated, ductile iron helical gears and you begin to see why most Cummins mills went well beyond the 350,000-mile B50 life. Confident in its industrial oil-burner, Dodge offered a 7-year/100,000-mile warranty standard from the get-go.

The Only Turbodiesel in the Segment

Cummins 6BT Turbo Diesel

Given its robust design and burly hard-part makeup, the 5.9L was more than ready to benefit from the use of forced induction. The Holset H1C turbocharger that came bolted to the engine’s exhaust manifold produced 18 psi of boost off the showroom floor. Thanks to an 18cm2 exhaust housing, the H1C provided quick response at low rpm. Unlike the naturally aspirated V8s in the Ford and GM trucks, performance of the Cummins-powered Dodge’s was virtually unaffected by elevation changes. Originally void of an intercooler, the 5.9L would gain an air-to-air unit midway through the ’91 model year (along with a looser, 21cm2 exhaust housing).

Direct Injection (DI)

1989 Cummins Direct Injection Piston

Beyond the fact that being turbocharged set the 5.9L apart from the competition, the engine itself was direct-injected. This means that, rather than fuel and air mixing in a swirl chamber prior to entering the cylinder (indirect injection, or IDI), fuel from the injector is sprayed directly on top of the piston. Each Mexican Hat style piston incorporates its own combustion chamber and the injection event itself is much more efficient in terms of atomization and complete burn than the indirect injection method.

Mechanical Bosch Injection System

Cummins Bosch VE Injection Pump

Partnering with Bosch (the other two went with Stanadyne), the Cummins’ injection system was highlighted by the VE rotary, distributor style injection pump and mechanical pop-off style injectors. The mechanical pump is what governs the 5.9L’s engine speed and from the factory it was fitted with a 2,500-rpm governor spring—right where it achieves its 160 hp peak power rating. In contrast, Ford’s 7.3L IDI made peak horsepower at 3,300 rpm (185 hp) and GM’s 6.2L made its 130 hp at 3,600 rpm.

Before Cummins, Ford’s 7.3L IDI V8 Was King

Navistar International IDI Ford Diesel V8

If you take yourself back to the time of brick nose Fords, this would’ve been a fuel efficient, workhorse type alternative to the 460 ci big block gas V8. Prior to the 5.9L Cummins’ arrival in ’89, it ruled the roost in the two-horse diesel truck market. Based on the 6.9L that preceded it (’82-‘87) but with a few improvements, the 7.3L IDI V8 featured a stroke of 4.11 inches, a bore of 4.18 inches, forged-steel connecting rods and five ½-inch diameter head bolts per cylinder. Its 21.5:1 compression ratio was helpful in low-end torque production (its 338 lb-ft peak occurred at just 1,400 rpm), but its indirect injection system was, at best, adequate for the times.

Turbo IDI

Ford Navistar International Turbo IDI Diesel

After observing the success Chrysler had with its Cummins/Dodge truck program (inadequate supply with high demand actually forced Chrysler dealerships to stop accepting orders for a period), Ford and Navistar went back to the drawing board for both a short-term and a long-term answer to Chrysler’s newfound diesel dominance. Midway through the ’92 model year, the 7.3L IDI received a bump in torque (358 lb-ft), followed by a turbocharged version in ’93-’94. The turbo IDI, as it’s often referred, brought a wastegated, fixed geometry Garrett turbocharger into the mix, along with a 190 hp and 388 lb-ft rating. Select hard-part changes were also made, internally, but the turbo IDI still fell short of the kind of performance Cummins owners were seeing. At this time, Ford was more or less biding its time until the 7.3L Power Stroke program was completed.

GM’s 6.2L IDI V8

GM 6.2L diesel print ad

First available for ’82 model GMs, the 6.2L Detroit Diesel was the original “reliable” diesel engine offered in the American pickup segment (we’ll save the 5.7L Olds diesel story for another time). But while reliable, you weren’t going anywhere quickly with the 130 hp, 240 lb-ft V8 under the hood. To be fair, GM and Detroit Diesel designed the 6.2L IDI more around fuel efficiency than mountain moving—and it is here that this engine shines most. Highway fuel economy can easily top 20 mpg in an empty, 1-ton. However, as you can imagine, with even Ford’s slouchy 6.9L IDI handily outperforming the 6.2L upon its release in 1983, the 5.9L Cummins rendered it virtually non-existent in 1989. GM’s answer to the Cummins and Ford diesels was the more powerful 6.5L IDI, but it too was incapable of closing the gap between it and the Cummins.

Attempts to Improve the IDIs

GM's Detroit Turbo Diesel IDI

Both the 6.2L IDI and Ford’s 7.3L IDI engines could be livened up considerably with the addition of a turbocharger. Aftermarket companies such as Banks Power (pictured above) and Hypermax Engineering offered complete, bolt-in turbo systems that effectively doubled horsepower output. In 1989, Banks even landed a deal to supply GM with the factory turbo option it wanted to offer its customer base. But while the addition of a turbocharger did help the aging IDI mills along, they were still a long way from directly competing with the 5.9L Cummins.

*This series is meant to highlight the pivotal moments in the diesel truck wars. They were sparked by the release of the 5.9L Cummins in the summer of 1988, which prompted Ford (and eventually GM) to answer the bell. In each manufacturers’ quest to be the best, diesel engine technology, fuel systems and even the trucks themselves have only gotten better throughout the years. In the next installment, we’ll examine how Ford got back on top of the game with the 7.3L Power Stroke.

If you want to know how to get the most our of your 5.9L Cummins, look through our budget diesel mods.

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