Diesel Exhaust Brake Performance
One of the most significant new features back in 2007 that caught both GM and Ford by surprise was Chrysler's introduction of a segment-exclusive factory exhaust brake in its 6.7-liter Cummins diesel.
An exhaust brake saves on brake and transmission wear by creating back pressure to engine brake the truck. It also reduces the potential for brake fade during long descents, increasing both downhill safety while towing as well as overall wheel brake life.
GM and Ford have both added exhaust brakes for the 2011 model year, but they operate differently. GM’s Duramax diesel exhaust brake works like the Cummins. It’s activated with the push of a button in the center console. It has four operational modes: tow/haul mode engaged, tow/haul mode disengaged, cruise control on and cruise control off.
Ford’s Power Stroke exhaust brake works automatically when tow/haul mode is enabled. Unlike the Duramax and Cummins, it can’t be turned off by the driver.
We spent time evaluating each truck’s exhaust brake on the backside of the 16 percent grade, following a winding 6 percent half-mile downhill path back to the base.
Each truck started downhill at 40 mph with the exhaust brake and tow/haul mode engaged. We wanted to see which trucks required the least amount of wheel brake application.
In the Ford diesels, we found it difficult to perceive when the exhaust brake was working. It wasn’t noisy like the other trucks’ exhaust brakes, which isn’t necessarily a positive trait, depending on your personal preferences. We also had to pump the foot brakes more in the Super Dutys than the Ram and GM trucks.
GM’s exhaust brake works seamlessly with the transmission. We consider it to be the best integrated; it wasn’t as loud as the Cummins, but we could tell when it was working and when it was turned off.
We thought the exhaust brake in the Ram trucks performed the best, but it required a bit of training. The Cummins’ exhaust brake and six-speed automatic transmission aren’t integrated like they are in the GM pickups. When the engine was above 1,600 rpm, only the exhaust brake worked. Grade shifts only occur below this threshold to prevent the engine from redlining on a descent. When engine speed dropped below that mark, the transmission grade downshifted to slow the rig further.
Gas Engine Grade-Shifting Performance
Though they lack exhaust brakes, the gas trucks benefited from the same sophisticated tow/haul systems that help reduce wheel brake wear by automatically downshifting when it can be safely done without redlining the engine. We grade-braked the trucks on the same descent as we did with the diesels.
Driving the Ford F-250, we toggled the automatic shifter into manual mode, which kept the truck in full automatic as we descended the grade but allowed us to see what numeric gear the truck was in – very useful when dropping down over long grades. The Super Duty consistently grade-shifted (down) twice and seemed to have the strongest engine braking of the three gassers.
The Chevy grade-shifted once, and its engine braking didn’t help as much in slowing the truck as the Ford’s did, requiring additional brake pedal pressure from the driver. The Ram 2500 performed similarly to the Silverado.
60-to-Zero MPH Brake Test
We added a new 60-to-zero mph brake test this year because performance testing isn’t just about how fast trucks can accelerate or how fuel efficient they are. When you’re on public roads, it’s just as important to know how quickly your truck can stop in a panic brake situation.
We tested stopping distances from 60 mph with the trucks empty and with one ton of ballast in their cargo box.
Each truck was tested three times loaded and unloaded with the same driver behind the wheel. Tow/haul mode was on when the trucks were loaded and off when they were empty. The exhaust brakes on both the Ram and GM trucks were also enabled when testing loaded. The Ford’s exhaust brake is automatically enabled when the truck is in tow/haul mode.
All the brake stops were performed on GM’s 65-acre “black lake” asphalt surface, where GM tests the handling attributes of its latest cars and trucks. Two of the trucks we tested have new or improved wheel brakes since our last HD Shootout.
In 2009, the Ram HD’s disc brakes were reengineered for additional stopping power, better wear and improved fuel economy. The front and rear brake rotors grew 2 percent to 14.1 inches, while the front brake calipers increased 7 percent with twin 2.4-inch pistons for grip. The brake pads were made 14 percent thicker, to 0.5 inches, and gained a 77 percent larger surface area of 15.3 inches. Those improvements also reduced passive brake drag, boosting gas mileage by up to 5 percent, according to Chrysler.
The Silverado’s wheel brakes increased from 12.8 inches to 14 inches in diameter and were widened from 1.5 inches to 1.57 inches. They feature a larger swept area for better stopping power, and the operating pressures have been changed to provide a firmer feel during application with less pedal travel required. The bigger disc brakes are a necessary improvement to reach higher gross combined weight ratings across the line.
Ford’s F-250 and F-350 pickups continue to use the same 13.7-inch discs up front and 13.4-inch discs in back as on the 2008-10 Super Duty.
It’s worth mentioning that even though we didn’t do brake testing with trailers, all of the trucks we tested come with integrated trailer brake controllers, something the Dodge Ram didn’t have in 2007.
The GM and Ford trucks also include trailer sway control, to help shut down any unwanted yaw from a squirrelly trailer to prevent loss of control. On the GM trucks, it’s only available for the single-rear-wheel pickups, but it’s available for both single- and dual-rear-wheel Fords. Ford also is the first to offer a full-size pickup truck with a factory-installed trailer brake controller that’s compatible with most aftermarket electric-over-hydraulic trailer brake systems, which use an electric signal to actuate the hydraulic brakes in the trailer.
We noticed that the Rams’ wheels were covered in the most brake dust after testing.
Three-Quarter-Ton Gas Trucks
Unloaded and loaded, all of the gas trucks stopped within two feet of each other, with the Ram having slightly better braking over the Chevy and Ford trucks in both scenarios.
Three-Quarter-Ton Diesel Trucks
The Ram pickups stopped in the shortest distances in both loaded and unloaded testing. Unloaded, the Ram’s stopping distance was six feet shorter than the Silverado and two feet shorter than the Ford.
In loaded testing, the Silverado improved its braking distance relative to the Ram and Super Duty, but the Ram still came to a halt almost a foot sooner. It outperformed the Ford by just over a foot.
One-Ton Diesel Trucks
The big-dog duallys came close to repeating the brake test results of the three-quarter-ton trucks with one significant difference: The GMC Sierra Denali 3500HD stopped sooner than either the Ram or Ford trucks – by nearly eight feet. Not only that, when loaded, the Sierra stopped even sooner than it did when it was empty – in two out of three trials. We don’t have a reason for why it happened, though we suspect the one-ton changed the proportionality of its brakes when it was loaded.