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A Tale Of Two Legends: How The Cummins 855 Big Cam Evolved Into The N14

Written By, Mike McGlothlin


When looking back at heavy-duty, on-highway applications from the 1970s and 1980s, it’s easy to see that Cummins owned the road. By 1983, Cummins’ share of the big-rig market had grown to an incredible 63-percent, up from 29-percent in 1975. Much of this success is due to the engine manufacturer’s 855 Big Cam, an inline-six diesel power plant named after its cubic inch displacement and its “larger than average” camshaft diameter. These 14.0L cast-iron Cummins mills operated at very low rpm, made tremendous low-end torque, and returned best-in-class fuel economy. Soon, Cummins 855 Big Cam owners would also discover their engines were rock-solid-reliable, million-mile contenders.

As the years wore on, the sixth and final version of the 855 Big Cam (the 88 Big Cam IV, to be exact) gave way to the N14—an engine similar in overall design but that was designed with future diesel emission requirements in mind. Cummins’ use of electronic controls made this possible, with the N14 even meeting the approaching federal emission standards of 1991 ahead of schedule. And despite being up against the likes of the Detroit Series 60 and Caterpillar 3406E in the turn-of-the-century era, the N14 held its own in power, fuel economy, and reliability. It was formally retired from on-highway use in 2002.

The following covers the evolution of the 855 Big Cam platform, when it morphed into the N14, and how the N14 evolved over the course of its 15-year production run.


Why Is It Called A Big Cam?

In case you were wondering how the Big Cam got its name, it’s because its camshaft diameter is 2.5 inches (vs. the common 2-inch diameter). The key benefit of the larger cam meant that it permitted a shorter injection period—the payoff being more efficient combustion and increased fuel economy. It’s also important to note that this design came to light following the 1973 oil crisis when, essentially overnight, horsepower became much less important than fuel efficiency. The other piece of the 855 Big Cam’s fuel-sipping puzzle was big low-rpm torque production, low overall engine speed, and the use of low-rpm drivetrains. As for the fuel system, the 855 Big Cam would introduce top-stop unit injectors.


Big Cam II: “King Of The Road”

The original Big Cam was a hit, but the Big Cam II arguably made the 855 ci Cummins legendary. Introduced in 1979, the revised Big Cam engine featured Cummins’ signature pulse-tuned exhaust manifold and also debuted the demand flow cooling lube system. In 1980, mechanical variable timing (MVT) was added in order to meet California-only emissions requirements. That same year, a twin-turbo version (which was technically a two-stage compound arrangement) was released and produced 475 hp—the highest horsepower 855 Big Cam to date. On the opposite side of the spectrum, the “Fleet 270” version of the 855 was introduced in 1981, an engine which was intended to return optimum fuel economy and that was governed to 1,600 rpm.


Big Cam III

Cummins Big Cam III On Highway DieselSignificant changes took place between 1981 and 1982 model 855’s, and the Big Cam III featured direct fuel feed (DFF) injectors, revised pistons, and a three-pass aftercooler (vs. a single-pass unit, previously). The engine’s oil and lubrication system was improved with the addition of a two-pass oil cooler (vs. the previous, single-pass configuration), integrated, full-flow and bypass spin-on lube filters (vs. single elements on the Big Cam I and II), and a stamped steel oil pan replaced the cast-aluminum version used in prior years. The Big Cam III would also be the first Cummins engine to begin using Holset turbochargers exclusively.


Big Cam IV

855 Cummins Big Cam IV Heavy Duty DieselA new camshaft design came about in 1984 with the introduction of the Big Cam IV (up to that point, the Big Cam I, II, and III had all used the same basic cam). Specifically, the Big Cam IV 400 camshaft added more lift to the valves. Other changes that surfaced during the Big Cam IV’s production run included low flow cooling (with a two-pass core), a unitized aftercooler, and Compu-Check quick-connect fittings. It was during this timeframe—and thanks in large part to the Big Cam’s stellar fuel economy, reliability, and power—that Cummins enjoyed 63-percent of the North American heavy-duty truck market.


New Big Cam IV

Class 8 Semitruck Cummins 855 Big CamThe fifth rendition of the 855 Big Cam came with indications that electronics were on the horizon. Block castings began to offer a provision to mount an ECM, and despite none of this being available at the time it was clear that wiring harnesses and electronic control modules would be in Cummins’ future. The New Big Cam IV also boasted the company’s LF3000 combo spin-on lube filter arrangement. On the fuel side of the equation, and because electronics weren’t yet available to curb exhaust emissions, Cummins added Step Timing Control (STC). The STC system, which consisted of the fuel injectors, oil control valve, and a plumbing and check valve, was geared toward fine-tuning the Big Cam’s injection timing. Specifically, reducing white smoke at cold-start was a top-priority.


88 Big Cam IV

88 Big Cam IV Cummins Diesel EngineIn an effort to work toward meeting the more stringent federal particulate matter (PM) regulations set to take effect beginning in 1991, Cummins treated the Big Cam to considerable fuel system changes. The 88 Big Cam IV debuted the company’s reduced drain flow (RDF) injectors, fuel injectors which provided a substantial increase in injection pressure over previous model engines. On paper, the higher injection pressures solved the in-cylinder PM problem, but out in the real world it was quickly apparent that Cummins hadn’t perfected the new fuel system. It would be the first time the Big Cam lineage suffered a sizable blow to its reputation for reliability.



Cummins N14 Celect Plus 525 HP DieselAlthough production commenced for the N14 Cummins in 1987, it was formally launched in 1990. It’s been said that the N14 was so similar to the 855 Big Cam that it could’ve simply been called the Big Cam VII. It shared the same bore and stroke (5.50-inch x 6.00-inch), incorporated wet sleeves, used modular 4-valve cylinder heads, also employed unit injection, and even shared the same oil pan. Throughout its production run, the N14 was subjected to various incremental changes to meet ever-tightening diesel emission standards. After meeting the 0.25 g/bhp-hr PM requirement for 1991 and the more stringent 0.10 g/bhp-hr PM standard for 1994, Cummins began working toward the gradual NOx limit goal of 4 g/bhp-hr for 1998.


Improving On The 855’s Design

359 Peterbilt Cummins N14Despite its similarities with its predecessor, the N14 was treated to a host of changes, improvements, and upgrades, the most noticeable of which was the engine’s use of an air-to-air intercooler (shown here). Inside, the crankshaft rod journals were upsized from 3-1/8-inch diameter to 3-1/2-inch diameter and treated to larger bearings. Within the valvetrain, the rocker shafts became separate pieces (there was no longer any sharing between cylinders), the cam follower shafts were enlarged, the rollers were bigger, and the injector lobes on the camshaft were made wider. One other notable change surfaced in the water manifold being integrated with the rocker boxes.


Evolving Hard Parts & Electronics

Electronically Controlled Cummins N14 Celect PlusEarly versions of the N14 were produced with the mechanical pressure time (PT) fuel system onboard. However, by the early 1990s full-on, electronic engine control was the order of the day (primarily to meet the coming emission standards)—and that electronic engine control system is known as Cummins CELECT. By 1994, and with the aforementioned 0.10 g/bhp-hr PM standard taking effect on January 1, Cummins switched to the articulated steel piston design which, in addition to improved durability, provided more complete in-cylinder combustion. In 1996, Cummins released the N14 Celect Plus, the “Plus” referencing the engine’s enhanced electronic controls. The major change here was that the ECM could now broadcast on the popular J1939 CAN bus system.


N14 Celect Plus

Red Top Cummins N14 Diesel EnginePerhaps the most revered of the N14 engines, Celect Plus models could be had in 525 hp form. These beasts were also capable of turning out 1,850 lb-ft of torque right off idle (1,200 rpm) and to this day enjoy a reputation for million-mile durability. Many Cummins engineers and countless truck owners consider the N14 Celect Plus the best all-around truck engine ever produced. Other than their red top valve cover, they are distinguishable due to their diagonal exhaust manifold bolt layout (vs. the previous, horizontal layout). The N14’s final year of production for on-highway use in the United States was 2002. It was ultimately retired in favor of the ISX.

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