By Bruce W. Smith
Towing a 26-foot, 9,600-pound Jayco Eagle HT fifth-wheel trailer up the twisting 8 percent grade between Paradox and Telluride, Colo., can put a strain on a pickup truck as the elevation closes in on 10,000 feet. But behind the wheel of the 2017 GMC Sierra 3500 Denali dually I recently drove, the travel trailer barely affected driving performance, requiring little more than a light touch of throttle to maintain speed.
The all-new 6.6-liter LP5 Duramax turbo-diesel under the Sierra Denali 3500 hood is one of the strongest engines on the road in a heavy-duty pickup, with a factory rating of 445 horsepower (tops in its class) and 910 pounds-feet of torque (second place in its class). Combined with a well-appointed interior and sound levels rivaling a library, GM's marquee full-size heavy-duty pickup should be on the radar of anyone who tows big trailers long distances.
"The typical Denali buyer is a self-made individual that is a disciplined achiever, highly organized, with high standards," said GMC Marketing Manager Stu Pierce. "They typically tow big trailers in both their business profession and as part of their personal lifestyle, all of which is what attracts them to GMC Sierra Denali HDs."
The Denali trim level is undeniably the flagship of the GM heavy-duty pickup lineup when it comes to style and luxury, but it's the new 6.6-liter V-8 Duramax LP5 diesel engine that will be the attention grabber for anyone thinking of stepping into a one of GM's 2017 HDs, be it a GMC or a Chevrolet at any trim level.
Inside the Duramax LP5
While little has changed structurally with the 2017 HD pickups, including their appearance, the changes to the hood and what lies beneath the diesel are significant. The new hood sports a multistage air-induction hood scoop that now provides more than 50 percent of the air to the LP5's intake, supplementing the previous generation's intake located in the fender well (as before). The induction system provides the coolest air source into the charge air cooler, which is especially helpful during the summer.
That supply of fresh, cool air dumps into a new BorgWarner turbo that flows more air than the outgoing, less powerful LML Duramax. In fact, in addition to the new turbo, 90 percent of the parts on the LP5 are also new. The only common part shared between the two turbo-diesel engines is the 90-degree V-8 block.
The LP5 turbo-diesel has new heads, valve-train parts, rods, pistons, camshafts, crank, turbo and injectors. They combine to give this latest-generation Duramax 19 percent more torque (910 pounds-feet compared with 765) and 12 percent more hp (445 versus 397) than the previous engine. The new engine delivers 90 percent of its peak torque by 1,500 rpm.
According to Eric Stanczak, vehicle chief engineer of GM full-size trucks and SUVs, who accompanied me on that Colorado drive, the LP5's heads flow 20 percent more air, the internal sealing is tighter, the rods are stronger and the forged crankshaft has larger journals. This combination of upgrades allowed GM to bumped up the cylinder pressure 20 percent higher than the LML.
Another upgrade that has a significant performance impact is the LP5's use of aluminum pistons that have bowl rims "remelted," which is a strengthening process similar to what's used to harden steel. Stanczak said GM is the first automaker to use this technology in a full-size pickup engine, which should help limit or prevent "thermal-cycle fatigue," a failure commonly seen as hairline cracks radiating out from the center of the piston after heavy use, caused by the ever-changing temperatures inside the combustion chamber under repeated high-load, high-stress, high exhaust gas temperature situations such as towing.
Change in Fuel Delivery
The LP5 received another big change: It now uses solenoid-activated injectors instead of the earlier piezo type. Tom Dye, the lead development engineer for GM HD pickups, said going to solenoid-activated injectors allowed engineers to fine-tune the LP5's overall performance in many different situations.
Dye said the computer controlling the injection system can activate as many as "seven distinct injections in a cylinder during a single combustion event, allowing fuel to be delivered at precise timing and amounts depending on the engine load and operating conditions."
There have been changes getting the fuel to the high-pressure fuel pump and how the turbo is used. The new engine now employs a low-pressure electric lift pump to prime the fuel system faster, and the BorgWarner turbo has a newly refined variable-vane system to help increase engine exhaust braking in different stages. The turbo system can now deliver 75 to 180 brake hp between 1,600 and 2,800 rpm, according to Dye.
The LP5 also employs a dedicated turbo oil circuit separate from that of the engine, which is specifically designed to keep the bearings cooler, improving long-term durability. On the downstream side to better reduce emissions, the diesel oxidizer catalyst has been relocated from the truck's exhaust that runs under the floor so that it now connects at the turbo. Dye said this change was done to provide the maximum heat in the shortest amount of time to the catalytic converter, firing off the catalysts, which lowers nitrogen oxide and hydrocarbon emissions, making the LP5 the cleanest and most environmentally friendly diesel GM has ever produced.
Speaking of emissions, GM said the 2017 HDs' selective catalytic reduction system has been improved, reducing the number of regens per tank. It now uses a 7-gallon diesel exhaust fluid tank, replacing the smaller 5.3-gallon tank and giving owners more miles between fill-ups. Unfortunately, GM continues to locate the DEF tank under the floor of the front passengers, making it quite exposed to underbody damage. Thankfully, it remains well insulated.
More Downstream Upgrades
In addition to the engine changes, the Allison 1000 transmission received a few changes in its solenoid control pack so it would continue to respond seamlessly and handle the extra power and performance increases from the new engine. For example, the six-speed automatic now provides the driver with three separate modes of grade braking: Manual, Tow/Haul and Cruise Control, each of which has a unique set of parameters and capabilities.
We saw the direct results of those improvements during our drive event, with near instant cold starts and the strong exhaust braking coming down several steeper grades (one as much as 8 percent) where we never had to touch the brakes during our descent. Both the transmission and software upgrades are big improvements over the 2016 Duramax powertrain.
Bumping up torque by 145 pounds-feet and hp by 48 also necessitated a couple of upgrades in the drivetrain to ensure long-term reliability under demanding conditions. Those upgrades include stronger and thicker drive shafts for both the 2500 and 3500 models, and the use of bigger Dana Spicer 1485 series universal joints to hold it all together. Model-year 2016s used the Spicer 1410s.
From new hood technology to stronger U-joints, the 6.6-liter V-8 LP5 Duramax/Allison 1000 combo in the 2017 GM HD pickups will make them a force to reckon with in the towing world. Moreover, the luxury level of the GMC Denali trim wraps all the new-found power and performance in refined elegance. While we know pricing for these trucks pushes $70,000, the upgrades and improvements make a strong value case at this level — especially if you've got a big trailer and a lot of miles in your future.
We'll have more to say about this new Duramax in May when we roll out our 2017 3/4-Ton Premium Truck Challenge. For that competition we brought together four crew-cab 4x4 luxury-level diesel pickups for a head-to-head competition in scores of categories. We'll have more to tell you in the coming weeks.
Cars.com photos by Bruce W. Smith
The 2017 GMC Sierra 3500 Denali dually is rated to tow up to 23,200 pounds when equipped with a gooseneck/fifth-wheel hitch. GMC only offers one axle ratio with the 6.6-liter Duramax V-8 turbodiesel: a 3.73:1 gear with an auto-locking limited-slip rear differential.
General Motors no longer requires the use of a weight-distributing/weight-equalizing hitch when towing trailers "on-the-ball" up to the 2017 HDs' maximum 14,500-pound weight-carrying capacity. We were told that's because of the way the receiver hitch is attached to the frame; the front receiver attachment point on the frame is almost to the rear axle, allowing it to achieve similar results to what a typical weight-distributing hitch can accomplish.
GM's LP5 turbo-diesel has new heads, valve train, rods, pistons, camshafts, crank, turbo and injectors, all combined to give this latest-generation Duramax 19 percent more torque (910 pounds-feet versus 765) and 12 percent more horsepower (445 versus 397) than the LML it replaces. The new engine delivers 90 percent of its peak torque by 1,500 rpm.