How much power will the Duramax en­dure?

Ultimate Diesel Builder's Guide - - Contents -

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. While it's true that you'll kill a stock 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. 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.


The weak­est link in the LB7 Duramax’s ro­tat­ing assem­bly is its con­nect­ing rods. Like all Duramax en­gines prior to the new L5P, 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).

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 550-600 hp (1,100-1,200 lb-ft) range. 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% over noz­zles, a stro­ker CP3, and a 66mm turbo live sev­eral years at 700 rwhp. The key? EFILIVE tun­ing that was cus­tom-tai­lored to limit lowrpm 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.


With the LLY 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.

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.

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 550-600 rwhp.


Start­ing with the ’06 model year GMS, the weak­est link in the bot­tom end 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.

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.

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.

In stock form, the Duramax is a heck of an en­gine. Ben­e­fi­cial fea­tures like six 14mm head bolts per cylin­der, cylin­der in­duc­tion hard­en­ing, high-flow­ing, heat-dis­si­pat­ing cast-alu­minum cylin­der heads, and com­mon-rail in­jec­tion make it not only ideal for mak­ing horse­power—but last­ing while do­ing so. How­ever, like any en­gine, the stock in­ter­nals can only han­dle so much. We’ll spell out where you should draw the line with a stock bot­tom end Duramax.

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.

Be­cause we’re not into blan­ket state­ments that as­sume all LB7 and LLY en­gines will die at the 550-600rwhp mark, this truck rep­re­sents the ex­cep­tion to the rule. Laced with 60% 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 Du­ra­max­ 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.

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.

While con­nect­ing rod fail­ure is al­ways a pos­si­bil­ity when you push an LB7 or LLY truck far enough, af­ter­mar­ket tun­ing (namely EFILIVE) has come a very long way over the years. Premier cal­i­bra­tors such as Adrenaline Truck Per­for­mance, Danville Per­for­mance, Du­ra­max­, Fleece Per­for­mance En­gi­neer­ing and PPEI have suc­cess­fully mas­tered this soft­ware and made stock rod ’01-05 en­gines live at high horse­power, thanks to be­ing con­ser­va­tive with torque pro­duc­tion at lower en­gine speeds.

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.

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 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 650rwhp setup.

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.

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.

Af­ter this ’07 GMC be­gan to smoke, idle rough and pro­duce ex­ces­sive crankcase pres­sure, the owner knew 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.

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