In an exclusive comparison, PickupTrucks.com took a 2011 Ford F-450 King Ranch Super Duty and 2011 GMC Sierra Denali 3500HD and pushed them to their limits on the twisting high altitude roads of the Colorado Rockies. We also tested them head-to-head on a dyno to measure their real-world power ratings a mile above sea level.
First, some background about two of the most luxurious and capable pickups you can buy.
Separate comprehensive road tests are coming for each of these rigs, but for this first test, we visited the folks at ATS Diesel in Denver.
ATS invents, designs, manufactures and distributes diesel truck performance products worldwide for Cummins, Duramax and Power Stroke owners. They also have a one-stop full-service shop for repairs and upgrades.
We put the F-450 and Sierra 3500 on a chassis dynamometer at ATS to measure their power output at the rear wheels.
Like the Ford F-350 we dynoed recently at Gale Banks Engineering, the F-450 was running with Ford’s updated “Job 2” engine and transmission control software and calibrations. It boosts the 6.7-liter Power Stroke V-8’s power rating to 400 horsepower (@2,800 rpm) and 800 pounds-feet of torque (@1,600 rpm) – an increase of 10 hp and 65 pounds-feet over the initial "Job 1" version of the engine (announced in February) that made 390 hp (at 2,800 rpm) and 735 pounds-feet of torque (at 1,600 rpm). The Sierra Denali’s 6.6-liter Duramax V-8 is rated at 397 hp (at 3,000 rpm) and 765 pounds-feet of torque (at 1,600 rpm).
Before someone points it out, the F-450 and Sierra 3500 are technically not the same class of truck. The F-450 is considered a one-and-a-half-ton while the 3500 is a one-ton. But for 2011, they’re closer to each other than ever before because of recent mechanical changes to the F-450. Ford made major revisions to lighten the 2011 F-450’s frame and running gear by 600 pounds for improved fuel economy and a higher top speed, including using more chassis components from the F-350 and reducing wheel and tire size from 19 inches to 17 inches.
The F-450 is only available with a 4.30 rear axle, while the Sierra Denali 3500 has a 3.73 rear axle. To compensate for this major difference, we ran the F-450 in 5th gear and the Sierra in 4th on the dyno because when you multiply the driveline ratios (transmission gear ratio times rear axle ratio) for both trucks in those gears, they are incredibly close to each other. The F-450 is 3.70 (0.86 times 4.30) and the Sierra is 3.73 (1.00 times 3.73).
Rear axles are rated with a number to describe how many rotations the driveshaft must make to turn the rear wheels. For example, the F-450’s rear axle turns once every 4.30 driveshaft rotations. Generally, the higher the ratio, the faster the driveshaft turns, and the sooner the driveshaft can transfer peak horsepower and torque from the engine to the rear wheels. The result, generally, is faster acceleration and higher towing capacity than a rear axle with a lower ratio. The drawback is lower fuel economy and top speed.
We were able to measure power output in the Ford about 600 rpm sooner than in the Denali because of its manually locked-up torque converter.
Tire size between both trucks was almost identical, so this didn’t play a significant role by acting as a kind of third reduction gear, because tire height can also affect how quickly power is transferred to the road. Generally, the taller the tire, the more effort needed to reach peak horsepower and torque.
The dyno results were surprising.
Using a new manual-shift mode that’s unique to Ford’s all-new 6R140 six-speed automatic transmission, we could select and hold fifth gear down to 1,400 rpm with the torque converter locked up, acting like a virtual manual transmission.
The Denali’s six-speed automatic Allison transmission only runs in full automatic shift mode – though you can lock out the top gears to firmly control the gearbox’s range – with the torque converter automatically disengaging at around 1,950 rpm. A torque converter is used to transmit engine power to the truck’s automatic transmission. At high engine speeds, it’s locked for the best fuel economy. At low engine speeds, it’s unlocked to help the driveline manage engine power and to prevent the truck from stalling at a full stop.
You’ll see the difference between the two trucks in the dyno chart because we could reliably measure power output in the Ford about 600 rpm sooner than in the Denali because of its early locked-up torque converter.
For torque, the Denali came out on top, but just barely. It was rated at 700 pounds-feet versus 697 pounds-feet in the King Ranch. But the Super Duty’s Power Stroke had a much broader and flatter peak torque curve than the GMC’s Duramax, with near-peak torque starting around 1,800 rpm and stretching to about 2,800 rpm. Altitude seemed to play a role, since Denver is about 5,500 feet above sea level. It pushed the peak numbers up the rpm range versus their sea-level-rated measurements. The Duramax was especially high up in the revs, with peak torque at 2,380 rpm instead of 1,600 rpm.
For horsepower, the Power Stroke beat the Duramax, 365 hp (at 2,800 rpm) to 348 hp (at 2,870 rpm). In contrast to torque, peak horsepower presented itself in both trucks at rpms close to where the truck-makers’ maximum claimed numbers are.
Again, on the chassis dyno, both measurements were made at the rear wheels instead of at the crankshaft. A 15 to 20 percent power loss from the crank to the rear wheels due to friction and rotational parasitic forces is a fair number to use, gauging the relative difference between claimed and dynoed numbers.
We noticed a slight dip in the Power Stroke’s ratings across the wide rpm range. We’re not sure why. It could be from the exhaust gas recirculation system, which helps reduce nitrogen oxide emissions by cooling combustion temperatures.
But dyno results can only tell us so much. So we took both trucks and hooked them up to a Dutchmen Colorado fifth wheel travel trailer that weighed about 10,700 pounds and tested them at high altitude on Interstate 70.
We drove both trucks on the infamous 7 percent eastbound hill climb that leads up to the Eisenhower Tunnel at about 11,000 feet above sea level in the Rocky Mountains. It’s a steady grade that’s about seven miles long. It starts at around 8,500 feet elevation. At these altitudes, an engine has noticeably less oxygen to work with than at sea level, so it must work harder to do the same amount of work. Engines can compensate to a certain degree by using different performance calibrations that change combustion and fueling strategies. But with a lack of oxygen, heavy load and steep ascent, you can run a truck at wide open throttle, and it will still gradually lose speed until engine performance and atmospheric challenges find a balance point.
Starting at wide open throttle running the trucks up the grade from Dillon, Colo., to the tunnel entrance, we measured the Sierra Denali 3500HD at an average speed of 66 mph and the Ford F-450 at an average speed of 51.87 mph.
The Sierra Denali had a curb weight of 8,100 pounds. The F-450’s curb weight was 8,760 pounds – 210 pounds more than an F-350 we tested during the HD Shootout.
The Sierra Denali 3500HD's performance advantage over the Super Duty is about as stark as the difference we saw in the power curves on the dyno. We believe it’s because the Sierra Denali’s calibrations at altitude are dialed in almost perfectly to match engine output. However, it seems that Ford still has plenty of room in the future to tune the 6.7 to tap its full potential at high elevations.
The rear axle ratios also played a role, but it wasn’t many years ago that a 3.73 rear axle on the Eisenhower grade would have been considered a major handicap versus having a 4.30 back end. It seems that the recent power levels both engines have attained are helping to make up for mechanical workarounds in the driveline.
We’ll have more details about these two trucks in each of their upcoming road test stories.
Special thanks to Kent Sundling, MrTruck.com, Dutchmen Travel Trailers, Cimarron Horse Trailers, Popup Hitch and the fine folks at Transwest Truck Trailer RV for all of their help and support with this test.