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 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 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