The heavy-duty truck market is one of the most competitive segments in the automotive industry. With Ford and Ram delivering strong offerings in their own right, GMC is looking to level the playing field with the 2017 Sierra HD. The truck’s redesigned Duramax 6.6L V-8 turbo-diesel is something loyal GMC buyers will appreciate.
“With nearly 2 million sold over the past 15 years, customers have forged a bond with the Duramax diesel based on trust and capability,” said Dan Nicholson, Vice President, GM Global Propulsion Systems.
Typical with HD trucks, the new GMC Sierra will see a multitude of difficult jobs. Robust payloads like bricks, seed corn bags, lumber, and stacked pallets are common in the bed of any given Sierra. It’s not unusual to see them towing heavy horse trailers with enclosed living quarters or a 5th wheel camper.
“The new Duramax takes those traits to higher levels,” Nicholson said.
The 2017 GMC Sierra HD is SAE-certified at 445 horsepower (332 kW) and net 910 lb-ft. of torque (1,234 Nm), marking a 19 percent increase in max torque over the current Duramax. Innovative designs contributed to the increase in performance.
“Nearly everything about the Duramax is new, designed to produce more torque at lower rpm, and more confidence when trailering or hauling,” said Gary Arvan, Chief Engineer.
An enhanced, higher flow oiling circuit provides a dedicated feed for the turbocharger, meaning increased turbo pressure and faster oil delivery. Larger piston-cooling oil jets, at the bottom of the cylinder bores, now spray up to twice the amount of engine oil into oil galleries under the piston crowns. As a result, operating temperatures are lower for vehicle efficiency. The integrated oil cooler for 2017 has 50 percent greater capacity.
The new Duramax features a sealed Venturi Jet Drain Oil Separator, a segment first. It gathers the fine mists of oil entrained (to draw in and transport by the flow of a fluid) in the blow-by gas. It also uses a small portion of the boosted air generated by the turbo, pumping the collected oil back to the engine oil sump for re-use. The Venturi Jet Drain Oil Separator returns oil during full-load operation, without the risk of carryover into the cylinders during combustion.
Most people today are amazed by how quiet a modern diesel truck is. The new Duramax has a unique two-piece oil pan, with a combination of aluminum and steel. The upper aluminum area gives the pan a level of rigidity but GMC engineers found an aluminum only pan actually conducts noise. Thus, a lower steel section was incorporated to cut noise and vibration. Further modifications reduce engine idle nose by 38 percent.
“You’ll notice the refinement improvements the moment you start the engine, and appreciate them as you cruise quietly down the highway – with or without a trailer,” Arvan said.
The new Duramax benefits from a stronger rotating and reciprocating assembly. A forged micro-alloy steel crankshaft anchors the assembly for maximum durability and usable life. Cut-then-rolled journal fillets strengthen the junction where the journals (the round sections on which the bearings slide) meet the webs that separate the main and rod journals. Upgraded connecting rods reside on a new, 45-degree split-angle design. This allows the larger-diameter rod bearings to pass through the cylinder bores during engine assembly. They’re forged and sintered with a powdered metal alloy and a fractured-cap design for more precise cap-to-rod fitment.
Stronger cast aluminum pistons finish the rotating assembly, with a taller crown area and remelted combustion bowl rim. During the manufacturing process, remelting is performed on the aluminum pistons. The bowl rim area is reheated after casting and pre-machining, creating a much finer and more consistent metal grain structure, reducing thermal fatigue and increasing overall strength.
Finally, the Duramax’s pistons are absent of pin bushings to cut weight and increase engine response.
The new Duramax maintains its first-in-class aluminum cylinder head design, with six head bolts and four valves per cylinder. Aluminum is used for weight reduction, while the six-bolt design inherently handles the turbo’s high compression. A stiffer head structure with more direct coolant flow is achieved by a new aluminum head casting. The double-layer water core design separates and arranges water cores in layers accordingly.
The 2017 GMC Sierra HD Duramax utilizes a common-rail direct injection fuel system with new solenoid-type injectors. High fuel pressure of 29,000 psi (2,000 bar) provides ideal fuel atomization for a cleaner burn and reduced particulate emissions. The new injectors support up to seven fuel delivery events per combustion cycle, contributing to better overall vehicle efficiency.
The 2017 GMC Duramax benefits from a new electronically controlled, variable-vane turbocharger. Boost pressure jumps to 28 psi for more raw power on demand while increasing the effectiveness of the diesel exhaust brake. The new turbo uses a more sophisticated variable vane mechanism, allowing a 104-degree F (40 C) increase in exhaust temperatures. As a self-contained unit, it decouples movement from the turbine housing for operation at higher temperatures, giving the new Duramax better power numbers with lower cylinder pressures.
Additionally, with less internal leakage, more exhaust energy can be captured during exhaust braking, especially handy if you’re towing a heavy trailer down a steep grade.
Although not everything about the 2017 GMC Sierra HD is entirely new – and that’s a good thing. The Duramax holds true to its cast-iron foundation with induction-hardened cylinder walls and five nodular iron main bearings. It’s the same 4.05-inch (103mm) and 3.89-inch (99mm) bore and stroke dimensions as the current Duramax, meaning when you pop the hood, the familiar 6.6L (403 cu.-in./6,599 cc) displacement will greet you.
A deep-skirt design with four-bolt, cross-bolted main caps uphold the block’s strength and enable a more accurate location of the aforementioned rotating assembly. Another constant is the availability of power, with 90 percent of peak torque hitting at 1,550 rpm, sustained through 2,850 rpm.
Naturally, the 2017 GMC Sierra HD is paired to the Allison 1000 six-speed automatic transmission. Although a new torque converter was added, the patented elevated idle cab warm-up has not changed.
Manufacturing & Awards
The new Duramax 6.6L engine is produced with locally and globally sourced parts in Moraine, Ohio, as part of GM’s joint venture with Isuzu. The 2017 GMC Sierra HD is manufactured in Flint, Michigan by the UAW Local 598. In 2014, the GMC Sierra HD received honors from J.D. Power in their Vehicle Dependability Study.
Carl Anthony is Managing Editor of Automoblog and resides in Detroit, Michigan