Connecting Rods: How Much Can Your Duramax Handle?

Killing a Duramax

Since we gave the Power Stroke guys a little insight, as to what their powerplants could handle, it’s time to take a look at what pushes the stock hardware of the Duramax mills to the brink. If you’re wondering how much horsepower can a stock Allison 1000 handle? While it’s true that you’ll kill the Allison automatic long before you hurt the Duramax in front of it, we’re assuming all supporting mods have been performed (most notably, a built transmission and lift pump) for the purposes of illustrating what the 6.6L V8’s stock rotating assembly can handle.. So how much boos can a stock Duramax handle? In our experience, you can get away with doubling the power output of each generation of the Duramax, but venturing beyond that has a tendency to bring out the breakage gremlins at an accelerated rate.

Because torque (i.e. cylinder pressure) is the primary killer of rods and pistons, it’s difficult to directly link a horsepower number to each engine’s weak link. However, as a general rule of thumb, a common consensus exists for the horsepower thresholds listed here (and the torque that goes along with those numbers). To be sure, aggressive tuning, poor parts selection, and a careless driver can spell disaster at any power level. And while plenty of Duramax owners have successfully run bigger turbos, upgraded CP3s, and large injectors on top of a stock bottom end, by and large, most enthusiasts don’t get away with doing it for very long. Read on to find out where the danger zone is for your ‘01-present GM’s connecting rods.

LB7 Rod

The weakest link in the LB7 Duramax’s rotating assembly is its connecting rods. Like all Duramax engines, the rods are made from forged steel and feature a cracked cap design. However, less material exists in the bottom of the beam area when compared to ‘06-‘10 engines. The lack of meat in this critical area helps explain why the rods are prone to bending at lower horsepower and torque levels, as well as why LB7 rods weigh less than the units you’ll find in LBZ and LMM models (1,155 grams vs. 1,173 grams).

We’d say compression was definitely down in the LB7 still sporting this rod. No matter the setup, limiting the amount of torque the rods see via tuning is the only real way to keep them straight (whether it’s an LB7, LLY, LBZ, LMM or LML). And even then, the outcome isn’t certain.

Now, in knowing that the LB7 rods are lighter and feature less structure in arguably the most critical area of the beam, add in the fact that this engine featured the highest compression ratio (along with the LLY) ever offered. With a 17.5:1 compression ratio (vs. 16.8:1 on LBZ, LMM, and LML mills), the LB7 and LLY engines naturally see higher peak cylinder pressures (elevated torque levels). Couple that with a weaker connecting rod and you can draw a pretty strong conclusion as to why the LB7 rods fail in the 550 to 600 hp (1,100 to 1,200 lb-ft.) range.

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While we tend to talk in terms of horsepower, it’s the torque of these engines that does all the damage. Whether it’s created by fuel volume, aggressive timing, a blast of nitrous, or a quick spooling turbo, extreme torque (of any kind) at low rpm can destroy the factory connecting rods and pistons. After this stock bottom-end LB7 had large injectors, a big single, and a hard-hitting nitrous system thrown at it, it didn’t take long for the rods to exit the block.

On the contrary, there are always die-hard Duramax owners willing to push the limits and prove everyone wrong. We’ve seen a 150,000-mile, stock bottom-end LB7 equipped with 60-percent over nozzles, stroker CP3, and a 66mm turbo live several years at 700-rwhp. The key? EFI Live tuning that was custom tailored to limit low rpm torque, yet pour on the timing and pulse width up top. It’s important to note here that the truck’s owner knew exactly what he was doing—and that the fun could come to an abrupt halt at any time.

DANGER ZONE: 550 – 600-rwhp

Because we’re not into blanket statements that assume all LB7 and LLY engines will die at the 550-to-600-rwhp mark, this truck represents the exception to the rule. Laced with 60-percent over injectors, a stroker CP3, and a Work Stock 66mm turbo, it withstood more than 700 hp for quite some time. Thanks to the strong grasp the folks at DuramaxTuner.com have on EFI Live software (and maybe a little luck), the stock short-block LB7 allowed the truck to run 11.7s in the quarter-mile, dominate the local Work Stock pulling class, and still drive around—problem-free—on the street.

Using state-of-the-art cylinder pressure test equipment, Mark Broviak of Danville Performance knows exactly where he needs to be when tuning a stock bottom end Duramax for both maximum power and utmost longevity. Based on all of his tests and findings over the years, he tells us the bottom line is that cylinder pressure higher than 3,000 psi will kill any stock bottom end, be it an LB7, LLY, LBZ, LMM, or LML.

LLY Rod

With the LLY Duramax sharing the same connecting rod as the LB7, it’s no surprise that this component is also the weakest link in the second generation of the Duramax. And, like the LB7, compression once again checks in at 17.5:1, meaning higher peak cylinder pressure (i.e. torque) beating down on it. But the LLY introduces one more ingredient to the weak connecting rod recipe.

Without a doubt, the most popular aftermarket connecting rod for the Duramax is the forged-steel H-beam from Carrillo. At 1,090 grams apiece, they’re lighter than any factory Duramax rod, and are said to be the strongest, hence the frequency with which they’re used in engine builds and their high availability.

The LLY is particularly at risk of rod failure because of its use of a variable geometry turbocharger (the LB7 came with a fixed geometry IHI unit). Variable geometry turbos offer quick spool up (response), which equals instant torque. So, with the combination of quick spool up and higher compression already inherently producing elevated cylinder pressures in the LLY, you can imagine how close to the brink the rods are once enthusiasts add fuel and upgrade to an aftermarket VVT charger.

While the LLY came equipped with the largest compressor wheel diameter of any Duramax built to date, its variable geometry design (VVT) coupled with the engine’s 17.5:1 compression ratio lends itself to naturally having higher peak cylinder pressure. And with already having weaker connecting rods than what you’ll find in the LBZ and later mills, rod failure can come sooner rather than later in this generation of the 6.6L.

In the case of both the LB7 and LLY engines, rod failure usually isn’t noticed immediately. In fact, it’s not uncommon for someone to drive for hundreds (if not thousands) of miles on a bent rod or two. Depending on how much the rod(s) has bent, a slight drop in compression in that cylinder(s) will be the only way to tell. Many times, a minimal bend won’t even lower compression enough to be noticeable to the average truck owner. Only when the truck begins to hiccup or smoke, or worse, does the owner know a major problem exists. It’s not uncommon to find a slight bend in most (if not all) connecting rods upon teardown of an LLY powered truck churning out 550 to 600-rwhp.

DANGER ZONE: 550 to 600-rwhp

Good tuning and a large single turbo setup can be your best friend if you’re trying to make big power on a stock bottom-end Duramax. The lag associated with larger, fixed geometry turbos lends itself to producing less torque down low (keeping the rods and pistons from experiencing excessive cylinder pressure), yet yields great horsepower in the mid-range and top-end.

The I-beam style connecting rod found in the LBZ and LMM engines is said to be capable of handling 50 to 100 more hp than the stock rods found in the LB7 or LLY. For this reason, they make their way into a lot of budget builds where LB7 or LLY owners who’ve bent the factory rods are looking for a reliable 550 to 650-rwhp setup.

LBZ Rod

Starting with the ’06 model year GM’s, the weakest link in the bottom end of the LBZ Duramax shifted to the cast-aluminum pistons. With a lower compression ratio than the LB7 and LLY mills (16.8:1 vs. 17.5:1), the LBZ and LMM have lower peak cylinder pressure on their side. In addition, the connecting rods entail roughly 10 percent more cross sectional area than LB7 and LLY rods. The added mass in the lower portion of the beam (where rods tend to bend) makes them more ideal for handling torque. Thanks to these OEM changes, the aftermarket was able to get away with pushing the LBZ and LMM powered trucks to the 650 hp (1,200 to 1,300 lb-ft.) realm before encountering piston failures.

After this ’07 GMC began to smoke, idle rough (like it was missing), and produce excessive crankcase pressure, the owner new he’d smoked a piston. He would eventually opt for Carrillo forged rods, Mahle Motorsport cast-aluminum 16.5:1 thermal coated pistons, and moved forward with a much more reliable 700-plus hp setup.

Several theories help explain why the piston is the weak link in LBZ engines. First, poor quality control of the casting process is said to have existed when GM switched to a Korean piston foundry prior to the LBZ engine’s production. It’s been said that inconsistencies were present in the metallurgy of the piston casting, making them prone to cracking. A second theory is the LBZ/LMM piston’s utilization of wrist pin bushings compromises the integrity of the cross sectional strength over the wrist pin area. Essentially, there is not enough meat in the wrist pin area, and 99 percent of all cracks occur along the centerline of the wrist pin. The final theory suggests that inadequate cooling leads to the piston’s demise.

Once a crack spreads across the length of the top of the piston, this is often the result. Cracked piston scenarios can do considerable damage to cylinder walls. An overbore was definitely needed to clean up this LBZ’s number 5 cylinder.

Cracked piston scenarios lead to excessive crankcase pressure (blowby), a miss in the engine, and inordinate amounts of smoke exiting the tailpipe. While the danger zone for an LBZ engine begins in the low 600s to 650 hp, we’ve seen our fair share of stock bottom end trucks making 700, and sometimes a little more. Sled pullers that are only ran hard a couple nights a week will typically last longer than a daily driven street truck, one that hits the dragstrip often, and certainly a truck that’s used to tow.

DANGER ZONE: 650 rwhp

The 16.5:1 Mahle Motorsport cast-aluminum replacement pistons require an LB7 style wrist pin. This is ideal, being that one of the major weak points in the factory LBZ/LMM piston is thought to be its lack of material in the wrist pin area due to the use of wrist pin bushings.

LMM Rod

With the same rods and pistons as what you’ll find in the LBZ, the LMM Duramax possesses the same weak link as the ’06-’07 trucks do: the cast-aluminum pistons. The theories as to why these pistons fail remains the same as well, but it doesn’t hurt to add excess stress from dealing with extreme EGT and high mileage fatigue to the picture—all of which can combine to cause a cracked piston (even at stock horsepower and torque levels).

Known for their ability to handle more stress than LBZ/LMM pistons, many enthusiasts opt for a cut and coated version of the LB7/LLY piston in budget builds. The pistons shown here came from SoCal Diesel, feature de-lipped fuel bowls, valve reliefs, and drop compression down to 16.1:1.

A lot of cracked piston scenarios we come across in LBZ and LMM trucks stem from the owner knowing the risks, yet trying to get the most out of his parts combination without crossing that red line. For instance, knowing that the danger zone was in the neighborhood of 650-700 rwhp and piecing together a setup good for 625 rwhp. The only problem is that that 650-hp number isn’t a line in the sand. As we’ve mentioned, some LBZ and LMM pistons survive 700 or more horsepower while others bite the dust at 600. There is simply no way to know when it will happen.

Quick-lighting compound turbo arrangements such as this one, where a stock Garrett VVT and BorgWarner S475 are employed, can kill stock bottom ends in short order. The faster spool up leads to vastly more cylinder pressure (torque) at low rpm, which is the primary culprit in connecting rod and piston failures. We’ve also seen a handful of LBZ and LMM owners switch from a single, stock-based VVT turbo to a set of compounds, only to waste a piston not long afterward.

Ironically enough, a lot of budget Duramax builds (LB7-LMM engines) entail the use of LBZ/LMM rods, but employ LB7/LLY-based pistons. Cut and coated (de-lipped) LB7 pistons were a hot item in the days before Mahle Motorsports and other piston manufacturers offered an aftermarket piston option—and for affordability reasons, they’re still a relatively popular choice. This type of build is typically safe for the 650 to 700-rwhp range, which is as far as a lot of enthusiasts want to go anyway (financially speaking).

DANGER ZONE: 650 rwhp

LML Rod

Now several years after its production run, the verdict is still out to what exactly the LML Duramax HPlimit is. While we haven’t heard or seen a lot of rod or piston failures on stock bottom-end engines yet, it doesn’t mean they’ll handle more power than what the LBZ or LMM’s will. In fact, the pistons are very similar to the LBZ and LMM engines, which means the 650hp range may just put you on the carnage radar.

Although the LML Duramax shares a similar piston to what you’d find in the LBZ and LMM engines, GM did away with the wrist pin bushings that were thought to be the key weak point (according to those in the aftermarket). This might explain why more LML powered trucks are surviving the 650 to 700-rwhp range on stock internals.

One edge the LML piston might have over the LBZ and LMM versions is that its design eliminates wrist pin bushings. While GM claims it did this to achieve a lighter weight rotating assembly (along with a lighter crank and lighter connecting rods), it added strength back into this area of the piston. We’ve seen several 700-rwhp LML’s sporting the stock bottom end, a few of which were running compound turbo arrangements. The key to these trucks living at this point has been torque limitation at low rpm via spot-on EFI Live tuning. DW

DANGER ZONE: 650 to 700-rwhp

In case you’re wondering, the verdict is still out on how much horsepower and torque the stock bottom ends will handle on the ’11-newer LML mills. With rods similar to what you’ll find in the LBZ and LMM’s and slightly stronger pistons, it looks promising, we caution anyone building an engine past the 700-rwhp mark.

Sources:
Danville Performance
317.745.8587
DanvillePerformance.com

EFI Live
EFILive.com

General Motors
GM.com

SoCal Diesel
661.775.5620
SoCalDiesel.com

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Pinaki Bhattacharya

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