How Factory Bottom Ends Are Kept Alive At Huge Torque Levels

Whether the platform is Power Stroke, Duramax or Cummins, experienced tuners, knowledgeable mechanics, and well-informed enthusiasts know exactly how much power they can squeeze out of their preferred power plant. Thanks to being overbuilt from the factory, the engines produced for use in the truck segment—namely those manufactured by Ford, Navistar, GM, and Cummins—all offer exceptional breathing room for growing horsepower. This means an exceptional amount of power can be stacked on top of a stock bottom end without negative repercussions—or even a need to make many (if any) reinforcements under the valve cover(s). It’s the automotive equivalent of getting away with murder.

 So how do we get away with doubling and, in some cases, even tripling the factory horsepower and torque on a stock rotating assembly? Tuning. Tuning. Tuning. With a proficient calibrator on your side, along with careful parts selection, you can live at the unofficial limit of what your particular engine can handle almost indefinitely. The biggest key to making it possible is the ability to effectively limit what diesel engines are notorious for: torque. While torque is one of diesel’s biggest advantages, it’s also what leads to the kind of cylinder pressures that bend rods, crack pistons, and even split blocks. This time, we’re exploring how and why tuners do what they do—and what your specific engine’s factory short block is capable of enduring.

As we already alluded to, excessive cylinder pressure (i.e. big torque) at low rpm is the biggest killer of diesel engines. Avoiding rod-bending torque is best executed through conservative injection timing. If properly done, the torque curve is moved further out in the rpm range. Then at higher engine speeds, fuel can be poured on, achieving horsepower goals up top without completely sacrificing torque down low. Granted, you’re still playing with fire anytime you double or triple the stresses imposed on a stock bottom end, but this fine-balance of limiting torque without killing high-rpm performance has been proven to work for years now.
Luckily, since word began to spread about how the act of limiting torque through tuning could save engines, tens of thousands of engines have likely been kept alive. At this point, with tuning just about down to a science, we know where to draw the line on horsepower and especially torque for each engine platform. The days of over-torquing an engine, sending a rod through a block, destroying pistons, and lifting heads at moderate power levels can all be avoided.
It’s been fairly well documented that the 6.0L Power Stroke and the 6.4L Power Stroke engine’s factory bottom ends are beyond stout. Thanks to their use of a factory bed plate, the 6.0L’s tendency to be more of a high-rpm performer, and the 6.4L’s connecting rods being the biggest and strongest ever offered in the Power Stroke lineage, it’s the 7.3L and 6.7L power plants that can encounter problems in the sub-700-rwhp range. In particular, the factory forged-steel rods that came in ’94.5-’00 and select ’01 and ’02 7.3L’s are good for 600 to 650-rwhp and roughly 1,200 lb-ft. The weaker, powdered metal rods found in ’01, ’02, and ’03 engines tend to be on borrowed time anywhere beyond 500-rwhp and 1,000 lb-ft.
The 570-rwhp 7.3L Power Stroke that belongs to yours truly sports a 224,000-mile, forged-rod stock bottom end that has been kept alive at this power level for nearly a decade. For more than 100,000 miles the engine has been turning out at least double its factory horsepower rating, and over the last 60,000 miles the 7.3L has been producing more than 500 hp. The key to making it live? Highly-refined PCM tuning that calls for limited torque at low rpm, the right supporting modifications, and driving sensibly have all contributed to its longevity. For towing and cruising, a 400-rwhp file is selected on the Hydra chip. At the track, things get cranked up to the aforementioned 570-rwhp.
7.3L Ford Power Stroke Diesel
A prime example of what a forged-rod bottom end 7.3L is capable of handling with the right tuning and parts combination can be found in Matt Maier’s OBS Ford. Dual high-pressure oil pumps, 350cc, 200-percent nozzle hybrid injectors, a competition electric fuel system with regulated return, an S467.7 turbo, and tuning to keep timing in check until higher engine speeds contributed to a setup that cleared 640 hp on the chassis dyno. This power level also made it possible for the truck to run 7.70-second eighth-miles on fuel, which it did for years before finally bending a factory connecting rod.
Speaking of instances of getting away with murder…how about making 1,226 hp at the rear wheels with a factory forged-rod 7.3L? The monstrous horsepower number came by way of a ghetto fogging on the dyno, one of the hardest, instant-power hits an engine can be forced to cope with. The 1,226hp hit was accompanied by 2,107 lb-ft of torque. For some perspective, the average 7.3L-powered ’94.5-’97 Ford typically makes 170 to 180 hp at the wheels, along with roughly 350 lb-ft of torque.
This is what happens if you fail to limit low-end torque for a highly-modified, forged-rod 7.3L Power Stroke. Despite what some tend to believe, keeping a stock bottom end alive has less to do with luck than you might think. Over the past two decades, ECM calibrators have figured out the limits of each engine platform and then perfected their crafts. That’s not to say engines never fail, but it’s much less the tuner’s fault than the operator when outright engine failure occurs.

6.7L Power Stroke

6.7L Ford Power Stroke Diesel Engine
Despite scares of having weak connecting rods at first, the 6.7L Power Stroke’s rotating assembly has proven itself more than capable of handling 650-rwhp—not a hard number to achieve with a high-pressure fuel pump upgrade and turbo swap on the first-generation engines (’11-‘14). While the ’11-’16 engines’ powdered metal rods are weaker than the ’17 model year versions, they can still tolerate 650-rwhp or more provided that torque is kept in check.
Where ’11-’14 6.7L Power Stroke owners got into trouble was when the add-a-turbo kits hit the market. Combined with the added fueling from twin high-pressure fuel pumps (often a belt-driven CP3 added to the factory CP4.2 back in those days), the dual compressor and quick-reacting GT32 SST Garrett VGT in the valley not only became a huge restriction, but it produced gobs of low-end torque with aggressive tuning in the mix.
Beginning with ’17 model 6.7L Power Strokes assembled within a specific VIN range, a revised connecting rod brought increased strength into the fold—or as we like to call it in the aftermarket: more breathing room. Most notably, the updated rods featured a larger wrist pin and a stronger, visibly beefier beam. They were soon put to the test by horsepower chasers where they proved themselves more robust, and are now offered as a budget-friendly, heavier duty-style rod for mild power level engine builds.
With peak torque occurring at 2,800 rpm and a solid horsepower number being run out until 3,400 rpm, some good tuning is on display in this dyno graph. To keep torque at bay on a late-model 6.7L Power Stroke engine, the calibrator doesn’t get serious with fueling until the engine is out of the big torque danger zone.
Powerstroke Diesel Engine
A’17 F-350 Super Duty produced the previous, 713hp and 1,281 lb-ft dyno graph, and it did it with fairly minimal mods. Compound turbos consisting of a Precision atmosphere charger and a 63mm VGT in the valley, a 10mm stroker CP4.2 in place of the factory high-pressure fuel pump, and custom-tailored tuning was all it took to get the truck past the 700-rwhp mark.
If measures aren’t taken in a 6.7L Power Stroke’s tuning to limit torque, this can be the result. The OEM rod that there is apparently nothing left of was asked to handle too much inside the ’14 model year engine. Rolling the dice with compound turbos, a stroker CP4.2, and bigger nozzles on top of a stock bottom end didn’t pan out for the owner. However, according to the truck’s dyno sheet, changes could’ve been made to at least give the rods a better chance of surviving the ride.
6.6L Duramax Diesel Engine
It’s obvious GM was looking for a lightweight rotating assembly in its first generation run of the Duramax, RPO code LB7, but that may have come at the expense of the connecting rods’ strength. Said to be the weakest rods employed in a Duramax, the units found in the LB7 (as well as the LLY that followed) can start to bend at or before the 600-rwhp mark. However, aggressively tuned (and operated) engines can bend rods around 550-rwhp or when 1,100 lb-ft of torque or more is regularly seen.
With less material in the lower section of the beam than what you’ll find in an LBZ and LMM rod, the forged-steel LB7 and LLY rods aren’t as structurally sound as the rods that would be used later. However, later versions of the Duramax didn’t feature the higher compression ratio the LB7 and LLY engines did (17.5:1). This means higher peak cylinder pressure (and higher torque loads) were at work in the early years of the Duramax lineage, which could further help explain why their rods fail sooner than in any other engine.
Not to disprove everything we’ve just laid out for you, but some engines are true freaks of nature and simply cannot be killed. Case in point, Buddy Callaway’s GMC Sierra is powered by a 300,000-mile, stock bottom end LB7 that runs in the 5.90 Index class of ODSS drag racing. For those that don’t know, even an ultra-light vehicle competing in this category needs at least 1,200-rwhp to be competitive. Somehow, someway, Callaway’s LB7 has survived having a 12mm CP3, 200-percent over injectors, and nitrous thrown at it.
It’s a well-known fact that 650-rwhp puts factory LBZ and LMM Duramax pistons on the hot seat. However, with tuning that creates dyno graphs like this, an ’06-’10 engine can be made to live while turning out very respectable numbers. With peak torque intentionally being produced at 3,000 rpm rather than 2,000 rpm or even 2,500 rpm, the pistons see considerably less torque than they would be exposed to at lower rpm. The feat was accomplished by holding off on timing advance until later in the power band, where it’s ramped up progressively yet quickly enough to produce 1,157 lb-ft and a solid 700 hp from 3,200 to 3,600 rpm.
We can’t count how many times this one has played out for LBZ and LMM owners. In most of those cases, two turbos were parked above a bone-stock bottom end and more than 650-rwhp was being made. This textbook style crack occurred along the center line of the wrist pin beneath it, as they always seem to do. If you decide to throw caution to the wind or just simply choose to chance it, at least do yourself a favor and be on the lookout for a usable core Duramax that would make a good rebuild candidate.
Compound 6.6L Duramax Turbo
The aforementioned 701hp dyno graph would not have been possible without the Duramax mill’s billet S475 over 63mm VGT compound arrangement, its 10mm stroker CP3 and 45-percent over injectors, or its built transmission. However, the biggest piece of the puzzle was the spot-on ECM calibration that’d been written using EFI Live software. With it, the stock bottom end, LMM-powered ’09 GMC Sierra 2500 HD lived on for several years serving as a daily driver and occasional tow mule.
When it comes to stock LB7/LLY rods or factory LBZ/LMM pistons, good tuning can make or break a setup living on the edge, but one other exception to keeping an engine alive lies in the purpose-built niche. In applications where the engine only sees immense strain intermittently, such as in drag racing or truck pulling, factory ’01-’10 Duramax short blocks seem to hold up better than those that are driven or towed with on a daily basis. With a 12mm CP3, 150-percent over injectors and a 67.7mm turbo aboard his stock bottom end LB7, Austin Aschemann’s ’04 Sierra has the potential to make 800 hp, yet the engine has lived just fine with only being pulled a few times a week.
Thanks to its long stroke allowing the piston plenty of room to get away from the cylinder pressure bearing down on it, a Cummins—of any variety—can handle more torque than the Power Stroke or Duramax V-8’s we’ve discussed thus far. But even though their threshold for pain is higher, they too have their breaking points. In the case of the old 5.9L’s, the factory forged-steel, I-beam rods can handle well north of 1,000rwhp as long as the engine primarily lives upstairs. However, the factory rod bolts can become a major problem at roughly 800-rwhp. As for an untouched short-block, somewhere between 1,500 to 1,800 lb-ft of torque seems to be the limit.
The compound turbo’d, highly-fueled 5.9L Common-rail that produced these numbers was purposely kept just out of the low-torque danger zone—and torque peaked at a very safe, 3,300 rpm on top of that. However, it’s unknown how long the Cummins lasted at this power level. As they do with any engine, multiple turbos bring elevated boost and drive pressures into the fold, which takes its toll on factory hard-parts. If drivability isn’t top priority, big single turbos—which naturally expose an engine to lower boost, drive, and torque levels—get the nod for allowing a stock bottom end Cummins to live at high horsepower.
In truck pulling, especially aboard purpose-only trucks, a Cummins with a factory rotating assembly tends to hold up to big power much longer than an engine that lives on the street and gets subjected to a full range of rpm and varying loads over the course of thousands of miles. Eric Loy has been campaigning his ’05 Dodge dually in the dirt for more than a decade. Although his 5.9L sports a fire-ringed head and ARP head studs, the factory crank, rods, and pistons are still employed in the crankcase. Try squeezing 800 hp and more than 1,500 lb-ft out of a common-rail 5.9L in a daily driver and you likely won’t get a decade of use out of the engine.
Power Driven Diesel
By intentionally not avoiding big torque at U.C.C., teams are definitely playing with fire. As Power Driven Diesel found out in 2021, a well-built rotating assembly can hold its own all day long, but when the block splits at the mains there will be fireworks. When the block gave way mid-way through the violent dyno run, PDD’s infamous P-pumped monstrosity of a second-gen had already sent 2,369 hp and 3,039 lb-ft of torque to the wheels. (Photo Courtesy of S&S Diesel Motorsport’s Christian Mahoney).
Industrial Injection - Shredder Engine
Due to how the scoring is calculated for the dyno portion of U.C.C. with torque being factored in with horsepower for an overall total, competitors don’t have the luxury of avoiding making big torque if they want to win. Here, even engines built with the best components in the aftermarket are placed on the ragged of what they can handle. As you can imagine or as you might’ve seen for yourself in person, nitrous oxide plays a big role on the rollers at U.C.C.—and a hard hit of N2O is extremely stressful on an engine. Rather than introduce nitrous progressively, in order to score a big torque number it’s required at a lower rpm. The biggest key is to bring nitrous into the equation only after the engine is sufficiently loaded and the proper amount of boost is on tap. If you think strategy plays a huge role in the dyno competition at U.C.C. you’re right!
12v Cummins Engine
Pro Tip: If you choose to roll the dice on a stock bottom end, at least do yourself the favor of procuring a spare engine or block in case catastrophe strikes. After all, when you’re pushing the limits you’ll likely know exactly what you’re doing. This makes for the perfect time to begin rounding up parts for an eventual engine build—which brings peace of mind to the horsepower game.
Haisley Machine - Rock Hard-Ram
Torque limitations are a common theme throughout the diesel performance world—even in engines built to handle 3,500 hp or more. Despite being assembled with the strongest, most cutting-edge parts on the planet, even a Sigma-pumped, triple-turbo’d Super Stock Cummins is intentionally kept above 4,000 rpm during the course of a pull. The reason? There isn’t as much torque up there. Granted, there are other reasons these engines are spun 5,000 and even 6,000 rpm (big turbos requiring high engine rpm and vehicle ground speed, to name a few), but the fact that they don’t see 6,000 lb-ft (or even 5,000 lb-ft) of torque is part of what keeps them not only alive but reliable.
Cummins 5.9L Diesel Injector
it’s a wise way to protect your investment. On a smaller scale you can do the same thing with a street-driven, milder horsepower setup. For example, instead of going with 30-percent over injectors to get to 625-rwhp with your LBZ Duramax, go with 60 or 100-percent overs and de-tune them. The larger nozzles’ ability to inject quicker means less duration is required to achieve your horsepower goals, and the engine will see less heat and stress in the process.

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