Back in Time, Part 8: 6.7L Power Stroke Vs. LML Duramax

• August 18, 2019
• Story By
  Mike McGlothlin

Despite the bailouts of 2009 and the shaky economy that followed, the summer of 2010 proved an exciting time for truck lovers. Not only was GM set to debut the all-new LML Duramax, but Ford was prepared to release its long-awaited 6.7L Power Stroke—the V8 it had designed, developed and produced 100-percent in-house (and nicknamed Scorpion). Though each company’s new power plant was going to be capable of meeting the 2010 0.20g/bhp-hr NOx emissions standard, the big rumor was that significantly higher power ratings were also expected. Would this be the year manufacturers breached the 700 lb-ft mark? And who would be the first manufacturer to offer 400 hp?

Perhaps not wanting to show its full hand upon the initial launch of its new engine, Ford announced its 6.7L would produce 390 hp and 735 lb-ft of torque in February of 2010. GM quickly responded by releasing the numbers for its LML Duramax: 397 hp and 765 lb-ft. Ford, having not been a front-runner in the horsepower or torque game since 2003 and not wanting its brand-new, state-of-the-art Power Stroke to be overshadowed, waited until mid-summer to up its power ratings to 400 hp and 800 lb-ft of torque. When this happened, all early 6.7L customers could bring their Super Duty’s to the dealership of their choice for a free PCM reflash, which gave them the additional 10 hp and 65 lb-ft of grunt.

To find out how Ford got back in the game, how GM made the LML Duramax better than the LMM that preceded it and when Ram fired shots of their own, keep reading.

Ford’s 6.7L: The Power Stroke That Couldn’t Fail

To say there was a lot of hype surrounding the release of the 6.7L Power Stroke would be an understatement. With all the problems with the 6.0L, no shortage of emissions system quirks on the 6.4L and even a couple ongoing lawsuits between Ford and Navistar, a lot was riding on FoMoCo’s in-house diesel engine endeavor (that’s right, no Navistar involvement whatsoever). Looking to get it right the first time, Ford brought several firsts to the segment, including a strong yet light(er) compacted graphite iron block, reverse-flow cylinder heads, a valvetrain with four valves per cylinder, four rockers and four pushrods per cylinder, a single sequential turbocharger and a water-to-air intercooler.

All the Right Hard Parts

With durability a very high priority, Ford secured the forged-steel crankshaft in place with 6-bolt nodular iron main caps, reduced rocker arm pivot wear with the four-pushrod/four-rocker per cylinder arrangement and brought back a six head bolt per cylinder pattern—something the Power Stroke name hadn’t seen since the 7.3L. The heads themselves were cast from aluminum (a first for the Power Stroke brand) but with a reverse-flow design. This means the exhaust manifolds are situated in the engine’s lifter valley. The heads receive fresh air via the valve cover inlets.

Dual Compressor, Single Turbo

The 6.7L Power Stroke’s induction system was unique as well, bringing water-to-air intercooling and a dual compressor wheel turbocharger to the diesel pickup segment. Called a single sequential turbo (SST), the Garrett GT32 SST was chock full of technology. The intake side entailed two 46mm inducer compressor wheels, positioned back-to-back, and two inlets on the compressor housing. On the exhaust side, variable geometry and a wastegate were employed. For utmost turbine shaft support, a ceramic ball bearing center section was used.

Hot-Side EGR Valve & SCR System

While the 6.7L had a diesel particulate filter, active regeneration functionality and a diesel oxidation catalyst, the big news was the addition of selective catalytic reduction (SCR) and the new EGR system it was using to fight NOx emissions. Instead of sending exhaust gases through the EGR cooler and then allowing the EGR valve to direct them into the intake stream, a hot-side EGR valve was utilized. By managing hot exhaust gases prior to sending them through the EGR coolers (yes, the 6.7L Power Stroke used two) the amount of soot, carbon and grime buildup on the EGR valve was significantly reduced. The SCR system, or urea injection, used a dosing valve positioned upstream of the SCR catalyst to introduce diesel exhaust fluid (a mixture of 32.5-percent urea, 67.5-percent water) into the exhaust system. When exhaust gases react with DEF, ammonia and carbon dioxide is formed.

No More Second Fiddle—Best-in-Class HP and Torque

Dead set on being the front-runner again, Ford publicly divulged the 6.7L Power Stroke’s introductory power rating of 390 hp and 735 lb-ft of torque knowing full well that it might have a battle on its hands if General Motors bested those figures. And, following GM’s release of the LML Duramax’s 397 hp and 765 lb-ft numbers, Ford put together a new PCM calibration and subsequently announced a new power rating for the 6.7L Power Stroke. Beginning August 3, 2010, all diesel-powered ’11 Super Duty pickups left the assembly line packing 400hp and 800 lb-ft. Ford even offered a free reflash to all of its early 6.7L Power Stroke customers who’d purchased 390 hp, 735 lb-ft versions.

LML Duramax: A 6.6L Prepped for More Twist

Any time a diesel engine manufacturer casts a stronger block it’s a pretty good indication that increased torque is on the way. With plans to add 105 lb-ft more to its Duramax platform, GM made several improvements on the LML. First, the main bearing profiles were altered to promote added oil film thickness. Second, the wrist pin bushings used in the LBZ and LMM engines were done away with and revised wrist pin ends were developed for improved piston support. Thirdly, an oil pump capable of moving 11-percent more oil volume than what was found on the LMM was added to the mix.

CP4.2 & Piezo Injectors

Both the LML Duramax and the 6.7L Power Stroke featured common-rail injection systems with Bosch piezo injectors and the new, 30,000-psi CP4.2 high pressure fuel pump (Ford’s system is pictured above). The new injection system helped make the LML the quietest Duramax to date, along with the most fuel efficient given the circumstances. Despite being saddled with an exhaust after treatment system that called for fuel-using regeneration cycles, LML-powered GM HD’s saw an 11-percent increase in fuel economy over outgoing ’10 LML models.

Emissions System Changes

Like Ford, GM had to add selective catalytic reduction to its new engine in order to meet federal NOx standards. In adding the SCR system to the LML Duramax, GM was able to lower the 6.6L’s NOx output by more than 60-percent. Unlike Ford, which had mounted its DEF tank behind the fuel tank and the filler neck next to the diesel filler neck behind the fuel door, GM located its low-slung, 5.3-gallon DEF tank along the passenger side of the frame and the corresponding DEF filler neck under the hood. To get away from post injection fueling in order to pull off regeneration events in the exhaust after treatment system, GM switched to downstream hydrocarbon injection (or HCI). With HCI, a ninth fuel injector was used to spray fuel directly into the exhaust system (pre-DPF), which still used excess fuel to pull off a DPF cleaning, but washed cylinder walls and diluted engine oil were no longer a side-effect of the regen process.

Exhaust Brake Debut

While this series hasn’t spent a lot of time dealing with maximum towing and payload capacities, throughout the horsepower and torque wars these figures increased substantially. When GM introduced the LML in the summer of 2010, it did so in conjunction with a much beefier chassis under its 2500 and 3500 series trucks—the latter being capable of towing as much as 21,700 pounds when properly equipped. To help bring all that mass to a halt more effectively, GM added exhaust brake functionality to its Garrett GT3788VA VVT turbocharger. Designed to be a more restrictive turbocharger on purpose, the LML-specific GT3788VA incorporates the shortest variable vanes ever offered on a Duramax (11.24mm vs. 15mm on the LLY, for example). Though not as powerful as the exhaust brake offered on the 6.7L Cummins, GM’s first crack at this technology was still very effective and much more refined in feel and operation than Rams.

800 LB-FT Cummins

While the news for ’11 model trucks was relatively quiet from the Ram camp, in the middle of winter the team at FCA dropped a bombshell. When ordered in conjunction with the 68RFE six-speed automatic, beginning in February of 2011 the 6.7L Cummins would come with 800 lb-ft. To handle the added stress, the 68RFE was treated to a beefier torque converter and an improved shift strategy. The Cummins’ 350 hp rating remained.

Curious where the torque war sits as of 2019? Ram’s 1,000 lb-ft 6.7L Cummins is king at the moment, but we have a sneaking suspicion Ford’s 2020 Power Stroke will debut with more twist than that. Find out why right here!