HOW MUCH IS TOO MUCH FOR THE ALMIGHTY CUMMINS?
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
AT 800 HP WE LIKE TO AT LEAST STICK A GOOD ROD BOLT IN THE 12-VALVE RODS. —JERRY FREY, SCHEID DIESEL
WE’VE FOUND THAT THE COMMON-RAIL ROD’S LIMIT IS AROUND 1,800 LB-FT. —CHASE FLEECE, FLEECE PERFORMANCE ENGINEERING
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.
THE RED LINE
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 fire 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.
THE TRUCK RAN AT 900+HP FOR FOUR YEARS AND 40,000 MILES, BUT I KNEW I WAS LIVING ON THE EDGE THE WHOLE TIME.”
—JIM RENDANT, CALIBRATED POWER SOLUTIONS
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.
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 find 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 fire-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.
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 figure, 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-confirmed 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.
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 sacrificing a little low-end torque, the life of the factory connecting rods is preserved and no sacrifice is made in peak horsepower production.
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.