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Performance Roadblocks of the 6.7L Cummins

As a direct result of tightening emission standards and consumer demand for more powerful engines, the 6.7L Cummins replaced the common-rail 5.9L midway through the 2007 model year. The 408 ci inline-six featured a bigger bore and longer stroke than its 5.9L predecessor had, and it packed 350 hp and 650 lb-ft of torque right off the assembly line. The 6.7L Cummins still made use of a Bosch common-rail injection system, but peak pressure increased from 23,200 psi to 26,000 psi and a variable geometry turbocharger (VGT) was employed for the first time. On top of that, the 68RFE automatic transmission debuted in 2007.5 Dodge Rams, the first time a six-speed auto was offered in its ¾-ton and larger trucks.

Unfortunately, the 68RFE was hardly an improvement over the 48RE it replaced in terms of torque-holding capacity—and not even fortifying one means it will last (more on that below). But if you can manage to get the 68RFE to live while harnessing additional power, sooner or later the longer stroke of the 6.7L Cummins (and the added cylinder pressure that comes along with it) will bring you closer to your next performance roadblock: a blown head gasket. Contributing to the 6.7L’s propensity to blow a head gasket, the factory VGT also becomes a point of restriction before you can even realize the full horsepower potential of the higher flowing fuel system.

For a closer look how each of the problems mentioned above, along with how to move beyond them, keep reading.

Roadblock #1: Automatic Transmission

Chrysler 68RFE Transmission

It’s the same old, same old with the ’07.5-present Cummins-powered Rams: before you can even get your plan for more horsepower off the ground you’ll find yourself having to address the factory shortcomings of the automatic transmission. To be sure, the 68RFE can handle more horsepower and torque than the 48RE could in OEM form, but if you abuse it it can fail just as fast. Low line pressure, slipped overdrive clutches, excessive heat and valve body cross leakage are just a few of this transmission’s weak links. With quality transmission control module (TCM) tuning, the stock 68RFE can support 500rwhp and 1,000 lb-ft or more, but it won’t survive boosted, four-wheel drive launches for very long—and maybe not even once if the Low/Reverse sprag explodes (which is common).

TCM Tuning Helps

Cummins Transmission Control Module

To reiterate, with sound TCM calibrating accompanying an ECM file that nearly doubles the 6.7L Cummins’ factory horsepower, the stock 68RFE can live under the right foot of a sensible driver. We’ve even seen one hold up in a truck making 570rwhp and more than 1,100 lb-ft. In time however—and even with fine-tuned shift points, higher fluid pressure and improved converter lockup events in the mix—the 68RFE will still be your 6.7L Cummins’ biggest performance roadblock.

The 68RFE Conundrum

68RFE Six Speed Automatic Transmission

Tearing a transmission down and stuffing it full of better parts usually yields great results if the work is performed by a reputable builder. But here’s the kicker for 6.7L Cummins owners: even after being built using the best aftermarket parts available (which isn’t exactly cheap), the 68RFE is still notorious for not holding up, long-term, in trucks making 700-rwhp or more. Some live while others don’t, which likely means that things boil down to who’s in control of the accelerator. Even so, when your mods progress beyond a turbo upgrade on stock fuel, you’d better make plans to build up the 68RFE and take your chances if you want to enjoy the truck’s power reliably.

Roadblock #2: Head Gasket Issues

6.7L Cummins Blown Head Gasket

While the 6.7L’s longer stroke and bigger torque figures are major perks, the added cylinder pressure that comes along with it leads to a problem rarely encountered by 5.9L owners: a blown head gasket. Granted, 6.7L’s don’t pop head gaskets left and right (many engines go 200,000 miles), but when you combine the added cylinder pressure with the high drive pressure produced by the restrictive variable geometry Holset turbocharger and stretch that abuse out over tens of thousands of miles, eventually the head is going to lift.

Head Studs & Beyond

ARP Head Studs Cummins Cylinder Head

Faced with a blown head gasket, and after resurfacing the head and inspecting the trueness of the block, most 6.7L Cummins owners opt to install ARP head studs when things go back together. At this point, the average tune-only (500 to 530rwhp) ’07.5-newer Ram is good to go for the long-haul. For those with more ambitious power goals, the head can be fitted with stiffer valve springs, thread-in style freeze plugs and even machined to accept fire-rings.

Roadblock #3: Factory Turbo

Holset HE351VE Variable Geometry Turbocharger Cummins

Though the Holset HE351VE turbo found on the 6.7L Cummins is larger than the HE351CW employed on the 5.9L (60mm vs. 58mm compressor), as a variable geometry charger the HE351VE is arguably more restrictive. In some aggressively tuned applications, this turbo’s drive pressure can be double what boost pressure is, and this kind of 2:1 ratio is a big no-no in the world of turbocharging. Of course, at this point not only is the turbo near its breaking point, but it’s well out of its efficiency range.

Fixed Geometry Turbo Upgrade

Second Gen Turbo Swap 6.7L Cummins

Replacing the factory VGT with a T4 flange, a fixed geometry turbocharger is one of the best ways to wring every ounce of power out of the 6.7L Cummins’ stock fuel system. In addition to the extra airflow providing a 30 to 50hp gain, exhaust gas temperature (EGT) drops and reliability is improved thanks to the fixed geometry unit’s better flow and inherent simplicity. As a bonus, the 6.7L’s added displacement over the 5.9L means it’s capable of driving a large frame turbo such as a BorgWarner S400 very well. You don’t sacrifice much in the way of spool up in order to see significant power gains at higher rpm.

Roadblock #4: Stock Fuel System

2012 Ram 2500 Cummins Lift Pump Sending Unit

When pushed to its absolute limit, the OEM fuel system on the 6.7L Cummins (lift pump, CP3, injectors) can support as much as 530rwhp and roughly 550rwhp with a turbo upgrade. To go beyond that point, bigger injectors are required, which in turn will call for a larger displacement CP3, which will have to be supplied fuel from a higher flowing lift pump. Each upgrade supports the other. It is here that things get expensive—and it’s why most ’07.5-present Cummins owners call it a day and remain in this power range.

Upsizing the Fuel System (From Tank to Engine)

Bosch CP3 High Pressure Fuel Pump Cummins

For those that simply can’t resist pursuing more power, your upgraded lift pump, injectors and CP3 should be purchased according to your horsepower goal. For instance, a truck making more than 600rwhp needs a lift pump that’s capable of flowing 150-gph. As for CP3’s, a 10mm stroker pump can support up to 800rwhp and will fit most needs, but 12mm pumps and 14mm units are also available for hotter setups. Injector options run the gamut from 50hp to 200-percent over (and even larger than that) but may require a properly-sized CP3 in order to support them, as well as precise ECM tuning to dial everything in. For sound lift pump options, check out the 170-gph PowerFlo in-tank unit from Fleece Performance Engineering or a complete low-pressure system from FASS. For high quality injectors and CP3’s, give Exergy Performance and S&S Diesel Motorsport a look.

Looking to take your 6.7L Cummins to the next level but need to do it on a budget? Check out our Budget Mods recipes for ’07.5-newer Rams here.

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