First Look: GM HD Diesels

We Drive The 2015 Silverado/Sierra HD Diesels

While diesel enthusiasts patiently wait for General Motors to pull the drapes off the 2.8L Duramax in the 2016 Colorado, the company released more powerful versions of its stalwart 6.6L Duramax. Mechanically, the diesel V-8 and Allison 1000 six-speed automatic remain the same as the last model year, but an updated body allowed GM to do some cooling tricks to bump up power and workability specs for the 2015 models.

The 2015 Silverado/Sierra HDs are basically a combination of the previous rolling chasses with the redesigned 2014 ½-ton trucks’ sheet metal and doghouses. But adding the brawnier grille in front of the LLM Duramax allowed GM engineers to do some wind-tunnel tweaks. Coefficient of drag isn’t highest on the priority list for wide, tall trucks, so GM focused on cooling and noise.

Gaps in the engine compartment were sealed, and incoming air was routed through the slotted grille and bumper ducts. The radiator/trans cooler and air filter/intake were priorities. The intake was isolated from underhood heat in the engine compartment.

Under-vehicle airflow also affects efficiency. Controlling the undercarriage turbulence improves powertrain cooling. This translates to fewer DPF soot regeneration cycles, requiring less fuel. GM’s estimated regen interval is 700 miles. (The EPA doesn’t test fuel economy for HD trucks. Published numbers for 2011-14 GM Duramax-powered HDs fluctuate between about 13 and 17 mpg.)

Wind noise was also prioritized. Here, the 2015 HDs use the 2014 1500’s cab configurations. The doors are inlaid under the roof, cancelling a significant amount of road noise. Triple sealing further isolates the office from the outdoors and engine sound. The roof and tailgate were also sculpted for optimal wind slicing.

In the real world, in-cab conversation with backseat passengers didn’t require drivers with high-mileage ears to turn their heads rearward. Most noteworthy were the conditions: hauling a 3,500-pound payload up a 2,900-foot elevation change in a Sierra 3500 4×4 dually. Voices didn’t have to be raised to project over the Duramax drone.

A stouter cab contributes to this interior solitude. Compared to the previous HD trucks, GM uses higher-carbon steel in the majority of the new cab and bed. Plus, suicide-door Extended Cab models are replaced by the front-hinged Double Cab. Adding B-pillars makes the cab more structurally sound and accommodates larger double doors for better ingress/ egress. Hydraulic rear cab mounts also improve interior isolation and NVH (noise,  vibration and harshness).

Updated software accounts for many of the other 2015 changes. Standard autopilot programming includes StabiliTrak traction control and Trailer Sway Control, which applies truck and trailer brakes as needed (individually if necessary) to straighten trailer shimmy. An integral dash-mounted Trailer Brake Controller is available.

Hill Start Control is also standard. If the truck is stopped on a five percent or steeper grade, this feature holds the brakes for up to 1.5 seconds so the foot can move to the fat pedal without a heavy trailer dragging the truck down.

Drivers who want to abdicate their thrones even further can get the Driver Alert package. This includes front and rear Park Assist and Lane Departure Warning. Bumping up to the Safety Alert Seat gives a tactile warning when the truck’s windshield-mounted camera sees the tires veering beyond the lane stripes. The seat sensation is similar to the old Magic Fingers coin-operated motel bed massagers.

GM is hoping that people who tow and haul but want all the modern sedan amenities will kick down more than $50K for a four-door 2015 HD. Granted, MSRP for the base Regular Cab Work Truck model (cloth interior, bench seat, also available as a cab/chassis) begins at more than $31,000. The Duramax/Allison option adds $8,395 to the sticker, and other options such as 4WD and navigation make the $50,000 threshold easy to exceed. A loaded Sierra 4×4 dually we hauled around in had a sticker of $65,520. Choosing every option on the online GMC Sierra Denali 3500 configurator cranks the MSRP to more than $70K.

So, what do you get for the money? Foremost, durability—more than 1.5 million Duramax/Allison GM HD trucks have been built since 2000. GM rates the engine for 200,000 “rough duty” miles before major overhaul. Powertrain warranty is five years/100,000 miles, and routine maintenance is covered for the first two years/24,000 miles. This includes oil changes, tire rotations and urea refills.

Although GM can’t claim the overall brute-strength towing title, the 2015 HDs are rated best-in-class in two categories: payload (7,374 pounds—6.0L gas models; up from 7,222 last year) and conventional hitch/ball towing (19,600 pounds—Duramax 4×4 dually; was 18,000 in 2014). They might not be the most powerful HDs available, but the available power is ready for work. DW

Side-By-Side Tow-Off

Although the Duramax’s SAE output numbers aren’t industry bests, improved cooling in the 2015 HD models allows greater powertrain programming flexibility. GM engineers didn’t share many details, but the tuning feels more aggressive: mainly higher shift points in the venerable Allison 1000 transmission (which has two PTO ports for operating snowplows and other accessories), possibly more allowable boost from the variable-vane turbocharger.

For testing, uphill rolling-start drag races among 3/4-ton crew cabs towing 10,000 pounds were staged on the Rye Grade near Payson, Arizona. The Silverado convincingly pulled away from the others, increasing the gap up the grade. The caveat here is that the Silverado’s axle gears were about eight percent lower (numerically higher) than the others. Also, there was no independent verification that all trucks were configured in on-the-dealer-lot condition.

Downhill control was even more impressive. GM’s Auto Grade Braking combines cruise control with Tow/Haul and Diesel Exhaust Braking. It kept the Silverado at a constant speed down the six-percent grade. The competitors motored by as their trailers and momentum required braking, overriding their cruise controls. GM uses the variable-vane turbo to provide aggressive backpressure when necessary. The Allison transmission is in the loop, smoothly shifting to maximize compression braking. Brake pads should live longer as a result.