BREAK­ING POINT

HOW MUCH IS TOO MUCH FOR THE ALMIGHTY CUM­MINS?

Diesel World - - Contents - BY MIKE MCGLOTHLIN PHOTOS BY MIKE MCGLOTHLIN & COURTESY OF JIM RENDANT

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.

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