Words by Mike Levine, Mark Williams and Kent Sundling, Photos by Ian Merritt
Unloaded Acceleration Tests
The only time our trucks were unloaded was during the quarter-mile testing at Chrysler’s proving grounds.
Free of its burden, the Sierra ran the fastest sprint, finishing with a best time of 17.22 seconds at 84.97 mph – about 8.5 seconds and almost 30 mph faster than when it was towing. The Ford F-350 was right behind it, finishing in 17.56 seconds at 83.03 mph.
Again, the Ram was close behind, running the quarter-mile in 18.02 seconds at 79.94 mph.
All three one-tons were within 2 mph and less than a second of the times we measured for similar one-ton trucks in the 2010 Heavy-Duty Shotoout.
Of course, the trucks ran cooler without the Titan trailers hanging off their backs.
Coolant temperatures for the F-350 dropped from 219 degrees to 206 degrees, and the transmission hovered around 203. The F-350 finished each run in 5th gear, and because we stayed at wide open throttle for several hundred feet after the quarter-mile mark to ensure clean data capture, the engine revved as high as 3,100 rpm, and fuel cutoff occurred at just over 90 mph. In one run, we felt the Ford’s traction control intervene at the start of the sprint and cut engine power.
Temperatures didn’t vary much for the Sierra after we unhitched the trailer. Coolant remained around 210 degrees, and transmission temps stayed around 183. Without the gooseneck, the GMC was able to shift all the way to 6th gear (with the wind at its back) and hit more than 90 mph past the quarter-mile marker. During all the wide-open-throttle runs, shifts from the Allison gearbox were smoother, though we noticed some torque converter slip between 2nd and 3rd gear.
We’ll note that we encountered a terrible buzzing/vibrating noise coming from the Sierra’s engine compartment during hard pulls on the highway and at the proving grounds when the engine was running between 2,300 rpm and 2,800 rpm. It turned out to be a relatively common problem for 2011 Duramax owners, caused by the position of the heater hose that runs from the exhaust gas recirculation cooler to the heater core. After we repositioned the hose with a zip tie, the noise immediately disappeared. GM dealers are aware of the problem, and a production fix will be implemented soon to prevent it in new trucks, GMC says.
The Ram ran much cooler without the trailer. Coolant temperature never exceeded 206 degrees, and transmission temperatures stayed below 200. With tow/haul off, the shifts seemed smoother, and they became closely spaced. The Ram shifted all the way to 6th gear in the quarter-mile. The 2-3-4 upshifts were particularly smooth and well-cadenced. Each new gear started at 2,500 rpm and upshifted at 3,000 rpm like clockwork. There was some torque converter slip that caused a slight rpm drop between 4th and 5th gears.