Here at Diesel World, we spend a lot of time pointing out the weak points and pitfalls associated with Power Stroke and Duramax ownership. We even brought you exclusives on each of those V8’s breaking points last year. But now it’s time to revisit the breaking point theme and shed some light on what exactly sends a Cummins over the edge. That’s right, even the venerable inline six—the Chevy small block of the diesel performance world—has its limits, and with the factory rotating assembly in place it’s easier to destroy one than you might think. So what exactly is the Cummins’ threshold for pain? As far as stock connecting rods are concerned, it’s just like any other diesel engine where torque (i.e., cylinder pressure) is the greatest threat to its survival. When you’re dealing with a highly modified 5.9L or 6.7L, it becomes a perpetual dance of avoiding peak cylinder pressure while maximizing horsepower in order to keep them alive. We’ll discuss the ways in which shops and enthusiasts do this, be it through low compression, custom tuning that limits low-rpm timing advance, or high rpm being the sole method of operation. BIG TORQUE, BIG PROBLEMS As mentioned (and as most of you know), the big torque that makes diesel so appealing is also what wreaks the most havoc on its internals. And since extreme cylinder pressure (i.e., torque) is so easy to come by with an inline six, these mills are constantly bombarded with stress. Were it not for the long stroke of the Cummins (where the piston and rod can escape some of the cylinder pressure by traveling downward), we’re sure there would be a lot more catastrophic engine failures in the sub-800hp range.

While the forged-steel (nonfractured- split design) connecting rod found in the ’89-02 5.9L Cummins will bend long before it breaks, the fracturedsplit rods found in ’03-present 5.9L and 6.7L mills (shown) are known to be a bit more brittle and break clean off. What’s the breaking point for this rod? According to the common-rail gurus at Fleece Performance Engineering, 1,800 lb-ft seems to be the line in the sand, although poor parts selection, bad tuning, and an aggressive driver can kill an engine at virtually any power level.
After speaking with Fleece, we learned that you can get away with playing with fi re for a little while, but not forever—and that a built engine is the only way to make big power reliably. For the common-rail block shown, a parts combination that yielded 900 hp and (more importantly) 1,800 lb-ft of torque lasted just a few short months before all hell broke loose.
One advantage a P-pumped 12-valve engine has over a common-rail (in terms of surviving cylinder pressure) is the fact that timing is fi xed. With infi nitely variable timing advance capability built into the electronic side of the commonrail engine, torque can be brought in much sooner via timing advance and boost whereas most competition-ready P-pumped applications are set up to run at high rpm (more than 4,000 rpm, well out of the torque danger zone).
The forged steel connecting rod found in ’89-02 5.9L Cummins mills is stout to say the least. They’re known to be capable of handling in excess of 1,000 hp at near-stock compression ratios before bending, and we’ve even seen the block split before the rods failed in several engines. According to Jerry Frey of Scheid Diesel Service, somewhere around the 1,000hp mark they start to lean toward getting a good aftermarket rod in a mechanical engine.
While the factory 12-valve rods can handle 1,000 hp, Scheid Diesel does recommend upgraded rod bolts anytime you’re in the neighborhood of 800 hp. Scheid offers these heavy-duty 12-point rod bolts, made by ARP, which are 23% stronger than the stock fasteners.
As a testament to how much abuse the factory 12-valve rods can handle, look no further than this 5.9L. Built to compete in the owner’s local 3.0 pulling class, a set of polished, shot-peened, and balanced stock rods lived through nearly two full seasons (2011-2012) and 56 hooks at the 1,200 to 1,300hp level before the number 3 rod said goodnight. The key to making the engine survive this long was the driver always making sure it spun 4,500 rpm or more while going down the track, as the higher engine speed kept it from seeing dangerous levels of cylinder pressure. Without a doubt, living at high rpm and having ultra-low compression (12:1) helped this engine avoid seeing the kind of torque that would’ve ripped it apart much sooner.
One surefire way to add stress to a Cummins’ bottom end is to add a compound turbo confi guration to the mix, especially one that employs a small high-pressure charger such as the factory turbo or a 56-to-62mm S300. The 6.7L shown was fi tted with 100% over injectors, a 12mm CP3, and a compound arrangement that made use of an S480 but retained the factory variable geometry turbo. While the truck was responsive and a blast to drive on the street, going from zero to 100 psi of boost almost instantly proved to be too much for the factory rods. This engine met its fate inside of 10,000 miles.
When an engine is lugged into lower rpm, cylinder pressure (i.e., torque production) increases. So when you’re depending on your engine to stay in the upper range of its power band—such as this P-pumped 6.7L making use of stock 12-valve rods in a sled pulling application—bad things can happen if the engine gets dragged down below 4,000 rpm. Under extreme load, the connecting rod broke near the small end and ejected the wrist pin. On its catastrophic journey out of the block, it would also take out the camshaft.
Thanks to much-improved tuning software now on the market (namely EFI Live), melted piston scenarios are seldom seen on common-rail engines these days. However, there was a time where you were more likely to melt a piston than break or bend a rod in a common-rail 5.9L (especially the ’04.5-07 engines). Most of the piston melting was due to engines being subjected to a combination of devices that both ramped up injection timing. The latter scenario coupled with an aggressive driver’s right foot could melt a piston in no time.
Even a well-built, well-tuned engine can self-destruct if exposed to prolonged cylinder pressure and extreme heat. After a year’s worth of street miles, a dozen sled pulls, several trips to the dragstrip, and countless dyno runs, the QSB marine pistons in this ’06 5.9L began to melt. At roughly 1,300 rwhp, this is proof that nothing (not even a built engine) will last forever if it’s regularly exposed to 100 psi of boost, 1,600+ degree EGT, and north of 2,000 lb-ft of torque.
Long before a 6.7L Cummins needs a stronger connecting rod, the head gasket will likely give you trouble. Thanks to extra cubic inches creating more cylinder pressure than you’ll fi nd in a 5.9L, this is the first weak link most 6.7L owners encounter. Some opt to simply resurface the head and then secure it with head studs (shown), while others fi re-ring it, upgrade the valve springs, install larger valve seats, opt for thread-in style freeze plugs, and throw in chromoly pushrods while they’re at it.
Once EFI Live tuning became available for the ’06-07 common-rail Cummins and aftermarket calibrators began to hone their skills, enthusiasts were better able to tiptoe around the danger zone on high-torque, stock bottom end 5.9Ls (and sooner after that, 6.7L engines). With this better tuning software available, melted piston scenarios were drastically reduced. However, not even EFI Live could stop the engines that were living on the ragged edge from eventually succumbing to connecting rod failure. Good tuning or not, if you’re pushing the envelope (approximately 1,800 lb-ft) with a common-rail engine there is no telling when rod failure will strike.
The key to successfully campaigning a stock rod 12-valve Cummins in sled pulling is to make sure the engine spends all its non-idle time at high rpm, which is where most P-pump owners like to keep their engines anyway. However, if the truck doesn’t carry its target rpm down the track and gets pulled under the 4,000rpm range, the bottom end will see more load than it’s used to (and in some cases, more than it can handle).
Believe it or not, a larger nozzle injector can actually help a common-rail engine live longer, provided competent ECU tuning is brought into the equation. With larger nozzles, shorter duration is required to make power, which means less timing advance is needed to get fuel in-cylinder at the perfect time. According to Fleece Performance Engineering, 300% over injectors are great for stock rod common-rail owners looking to push the limit, as only 1,000 to 1,300 microseconds of duration is required to make respectable power and the minimal injector on-time that’s required is much easier on the engine.
Anytime you stack compounds, dual CP3s, and 100% injectors on top of a 200,000-mile common-rail 5.9L, it behooves you to invest in the safest tuning possible. For the owner of this ’06 Dodge 2500, EFI Live was the software of choice to keep the bottom end from scattering. While a tow-friendly S362 over S475 compound turbo arrangement won’t make huge horsepower, its relatively small sizing can produce big torque down low—which had to be tamed if the factory rods were going to survive.
This dyno graph, obtained from the aforementioned ’06 Dodge, illustrates the practice of safe tuning to a tee. Notice that peak torque isn’t being made until 3,100 rpm. This is because astute ECU calibrating, courtesy of Calibrated Power Solutions, doesn’t ramp up timing until higher engine speeds (vs. pouring on the fuel at low rpm and putting the factory rods under tremendous stress). By sacrifi cing a little low-end torque, the life of the factory connecting rods is preserved and no sacrifi ce is made in peak horsepower production.
Without a doubt, campaigning a large single turbo (along with good tuning) makes life easier for stock rods. A charger that spools later in the rpm band isn’t conducive to producing big torque numbers but will allow for an impressive horsepower fi gure, hence the reason Jim Rendant was able to get away with making more than 900 rwhp for more than 40,000 miles.
Of course, there are always those willing to push the limits. Jim Rendant was one such enthusiast who put his stock bottom end ’06 Dodge to the ultimate test. After sporting 150% over injectors from Exergy Performance, dual CP3s, a single S475, and making a track-confi rmed 990 rwhp for more than four years, Rendant decided to push the envelope even further by adding more fuel and compound turbos to his 5.9L.
Eventually, 110 psi of boost, 1,200 rwhp, and more than 2,000 lb-ft of torque (courtesy of 250% over Exergy injectors and an S464/S588 compound setup) would lead to the end of Rendant’s 195,000-mile stock bottom end. In the midst of a low-10-second pass, the number 5 rod left the block in catastrophic fashion. Despite blowing up at the eighth-mile mark (in which the truck nabbed a 6.73 at 107 mph), Rendant’s Dodge would go through the traps on the brakes yet still collect a timeslip that showed 10.91 at 100 mph.
Carrillo is a household name in the diesel aftermarket, and it’s a widely popular (and highly available) choice for street-driven and competition Cummins builds. The company’s forged-steel H-beam rods are capable of withstanding well north of 2,000 hp, the torque that goes along with it, and with upgraded MP35N rod bolts they can handle more than 6,000 rpm without issue.
Low compression, big single turbos, and engines that don’t see a lot of timing advance or aren’t driven at low rpm are all ways enthusiasts attempt to avoid the kind of torque that kills connecting rods. At least two of the latter options are always at work in sled pulling—even though most entry-level pullers are now sporting built engines and don’t have as much concern about rod failure.


The general industry consensus is that once you breach the 800hp mark, you need to be thinking about aftermarket connecting rods. And at the very least, you need to know you’re playing with  re at this point. On these pages, we have provided examples from street-driven to competition-only, and P-pump to common-rail engines that met their fate at the hands of excessive torque. Some were ahead of their time, while some lasted way longer than they should’ve, but all of them let go in the 1,800 to 2,100 lb-ft range. DW


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