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Why the 8.3L Cummins Is Tougher Than The 5.9L

Written By, Mike McGlothlin


Over the years, much debate has stemmed from comparisons between Cummins’ 5.9L B-series (6BT) engine and its 6CT power plant (the 8.3L). While the 5.9 Liter Cummins served time in light-duty applications, it was also included as a base engine option in many medium-duty truck, off-highway, and marine applications—segments where the bigger, premium 6CT was often offered as well. Deciding which of these two workhorses to run could be especially difficult in the RV segment. There, the 5.9L was underpowered but often achieved better fuel economy, while the 8.3 Liter Cummins could power over virtually any hill without losing speed but did so at the expense of consuming more diesel.

Despite many drivers, fleet owners and operators swearing by the smaller 6BT, the 8.3L is the true medium-duty Cummins engine part of the two. In addition to its higher torque producing capabilities, which results in superior performance under heavy workloads, the 6CT’s overall design better lends itself to a million-mile, 20,000-hour life cycle. The bigger I-6 also has fewer weak links than its little brother, most notably the fact that it’s free of the potentially catastrophic killer dowel pin. We’ll cover the former, and more of the parts and pieces that make it more desirable (and durable) than the 5.9 Liter Cummins, below.

Free Of The KDP

Unlike the 5.9L Cummins, the 8.3L didn’t call for a locating dowel to be used during assembly at the factory. On the 5.9L assembly line (6BT and ISB), a steel dowel pin measuring just 5/16-inches in diameter was pressed into place in order to more accurately install the front gear housing. Over time, the dowel pin can work itself free from the block, fall into the gear train, and cause all sorts of mechanical mayhem. No such issue is a worry with the 8.3L. On the factory 6CT production line, a different method was employed for mating the front gear housing to the block.

Wet Sleeves: What Separates Light-Duty From Medium-Duty

The 8.3 Liter Cummins is a wet sleeve engine, making it similar to another popular and venerable power plant from the same era, the unkillable International DT466. Its mid-stop wet liners enable coolant to circulate around each cylinder, which controls heat and ultimately helps keep the bores perfectly round. But not only does wet sleeve technology virtually guarantee a longer-lasting engine, each cylinder can also be repaired individually and without removing the engine. In contrast, the 5.9 Liter Cummins is of a parent bore design, requiring complete Cummins engine part extraction and overbore work in the event of an overhaul.

No Strings Attached To The P-Pump

Both the mechanical 6BT and 6CT were offered with the Bosch P7100 series injection pump (i.e. P-pump). However, while these engine’s P-pumps might look similar at first glance, they aren’t interchangeable. On the 5.9L, the P7100 receives its oil supply through a fitting on the inboard side of the pump body. On the 8.3L, oil feeds into the front of the P7100 housing through the engine’s front gear housing. With no external oil feed line, which can pose an extra leak point, this again speaks to the heavier duty nature of the 8.3L’s design.

P-Pump Similarities

Where the 8.3L and 5.9L do share common ground is in the P7100’s overall size and function. Either version’s fueling and tunability can be altered in a myriad of ways—all of which are the same. The air fuel control (AFC) handles the pump’s rate of fueling at low turbo boost pressure, the rack plug limits the fuel volume that’s allowed to feed into the plungers and barrels, the fuel plate is in charge of the P7100’s maximum fueling output, and the delivery valves control the amount of fuel that travels to the injectors. Last but not least, the governor springs effectively control the engine rpm the pump will fuel to.

One Less Seal To Worry About

Pesky oil leaks are par for the course on virtually any high mileage diesel engine, so eliminating any potential leak point is always welcome. In the 5.9 Liter Cummins world, oil leaks at the tappet cover are a highly common sight. With age, vibration, and the expansion and contraction that comes with heating and cooling cycles, the tappet cover gasket corrodes and cracks with time, causing a mess down the side of the block. With no tappet cover to speak of on the 8.3 Liter Cummins, oil leaks are never a concern—at least at this location.

One Piece Valve Cover

Another area where oil leaks are minimized on the 8.3L is through its use of a one-piece valve cover. On the 5.9L Cummins engine part, each individual cylinder is capped off with its own valve cover. This means six valve covers, six gaskets, and six potential leak points. With age on the gaskets (and/or a clogged crankcase breather), a considerable oil leak can develop on top of a 6BT cylinder head. From a serviceability standpoint, the 8.3L’s single valve cover makes for quicker access to the valvetrain. With either version of the 12-valve 8.3L or 5.9 Liter Cummins, where valve adjustments must be performed more regularly than on later, 24-valve engines, this is yet another win for the medium-duty 8.3L.

A Torque Monster

Naturally, thanks to its larger displacement (and primarily its longer, 5.31-inch stroke) the 8.3L Cummins is significantly torquier than the 5.9L. In fact, it boasts what is roughly a 3:1 torque-to-horsepower ratio, whereas the 5.9L’s ratio checks in much closer to 2:1. This is typical of a medium-duty intended engine vs. a light-duty one. And it’s also why the 8.3L prefers lower rpm operation, peaking between 2,100 and 2,500 rpm in most applications. But the 6CT’s dominance over the 6BT doesn’t end there. Its internal hard parts are bigger, the block casting is thicker, and its overall heft is a direct reflection of it, tipping the scales at more than 1,600 pounds (which is Class 8 engine territory).

A Go-To Engine For Farm Equipment

With big low-rpm torque comes great luggability, and the 8.3 Liter Cummins shines in farm tractor applications. Back in its heyday, more than half a dozen manufacturers offered the 6CT in their tractors, among them were Case IH, AGCO, New Holland, McCormick, and even Massey Ferguson. In these agricultural applications, where the engine was often governed no higher than 2,200 rpm, the 8.3L proved itself as a reliable, day-in, day-out workhorse with very few problems, and its ROI was among the best in the business. So long as fuel tinkering was left at a minimum (limiting the engine’s exposure to excessive EGT), the 8.3L could even be turned up to tackle more acreage in a quicker window of time.

Where The 8.3L Isn’t Tougher (High RPM)

While it’s great when being lugged at low speed, the 6CT’s longevity is known to nose-dive when it’s operated at high rpm, especially for prolonged periods of time. The best example of this can be found in old Case IH combines, where the 8.3L often saw a sustained engine speed of 2,600 rpm. Connecting rod bolt failure, which can result in a rod leaving the block, is always a worry in combine applications with more than 7,000 hours on the meter. As further proof that engine speed can make or break the 8.3L Cummins, when these engines were backed off to 2,200 rpm the engine’s life expectancy doubled.

More On The 6CT’s Performance In Combine Applications

Horror stories abound when it comes to the 6CT’s propensity for failure in combine applications, but some issues—such as blown head gaskets—can be traced back to factors other than higher engine speed. Tighter overall packaging, poor coolant flow (primarily due to the 90-degree radiator hose leaving the water pump), and a filthy operating environment don’t do the 8.3 Liter Cummins any favors (or any engine for that matter). In summation, the life of a 6CT in a combine harvester was one that often consisted of lack of airflow across the radiator (or a radiator chock full of debris), poor coolant flow, higher coolant temps, and high rpm, which is rarely a favorable recipe for a medium-duty engine.

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