Ultimate One-Ton HD Challenge: Davis Dam Exhaust Brake Test

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We knew that the winning truck in this contest wasn't going to be just the fastest. As all serious truckers know, how a vehicle handles and controls its load on downhill runs is probably more important than any top-to-bottom speed run. Given that all three of the one-ton heavy-haulers have redone their exhaust brake capabilities, we knew this aspect was important.



The first truck we tested was the Ford F-350, pulling close to 21,000 pounds behind it. Not long after the crest of the summit, the first brake event allowed the grade-shifting transmission to downshift two gears to 3rd. In combination with the newly separate, albeit somewhat hidden, exhaust brake switch, the truck continued to slow until we had to touch the accelerator, shifting the truck up to 4th. From there, we never had to touch the brake on this run again. In the end, the F-350's new exhaust brake was a huge help keeping all the weight under control between our 50- and 60-mph targets.

The GMC Sierra 3500 was a little different. The new, more aggressive exhaust brake system is supposed to be smarter and better able to adjust to more variables than before, but we didn't find that to be true. It was clear the truck and trailer combination, and our target speeds, were not within the GMC Sierra's comfort zone, at least when not using cruise control. We discovered that GM engineers have directly interfaced the computer controls of the cruise control with the exhaust brake controller for the 2015 Chevrolet and GMC HD pickups to better control the trucks on downhill runs.

Sure enough, without the cruise control set to a prescribed speed, we found our Sierra had quite a bit of trouble controlling the fluctuating downhill speeds of our truck-and-trailer combo. In fact, after our first brake event after cresting the downhill summit, we had six alternating brake events and eight acceleration events, for a total of 15 touches. Each time we touched the throttle, it would jump a gear or two. Each time we held the brake to 55, it would drop one gear too much and slow too much (we found it quite frustrating not having a real-time gear indicator; both of its direct competitors have one).

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We did try the Sierra's cruise control, and it kept a constant speed between 55 and 57 mph for the duration of the downhill run after the first braking event, but since our procedures were set, we did not think it was fair to modify the exhaust test (however, we did allow the cruise-control system to be used in our later Eisenhower Pass grade exhaust test).

The Ram 3500 tester came with an upgraded smart exhaust brake that has three separate settings (Off, Auto and Full) and a smarter computer controller. The new system allows for more flexibility and is intended to give customers the right amount of exhaust brake control as the terrain and situations change. We made different runs with the Ram Cummins exhaust brake in both Full and Auto, and found we had fewer brake touches in the Auto position. It seemed like there was a type of learning going on with the Auto setting, because each time we had to brake (with the smart exhaust brake engaged), it would hold gearing and speed for a longer amount of time. In the end, the Ram needed three accelerations and three brake touches. Additionally, the engine's very loud fan kicked in several times.

How We Did the Testing

We completed our brake testing after each of our uphill timed runs and employed the same procedures we've used in the past. At the top of the Davis Dam grade, right at the Union Pass summit sign, we started our brake test by slowing the truck and trailer to 55 mph, with each truck in Tow/Haul and with the exhaust brake button on. The range we allowed before a brake or acceleration touch was allowed was between 50 and 60 mph. If our vehicle speed fell to 50 mph, we'd throttle to 55 mph; if vehicle speed went above 60 mph, we'd slow back down to 55 mph.

We kept track of both acceleration touches needed as well as braking events, and we scored each "touch" the same. The winner was the truck that needed the least number of touches (accels or brakes) to keep our preferred speed.

Cars.com photos by Evan Sears


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Overview | Milan Dragway | Fuel Economy | Milford Hill Climb | Milford Braking | Davis Dam | Davis Braking | Eisenhower Pass | Eisenhower Braking | Results


The Ford exceeded your 60 mph target speed. The others did not. How many touches would it have taken to keep the Ford under 60 mph.

Your writing says the Ford stayed within limits. BUT THE DATA SHOWS THAT IT DID NOT.

The ford speed down the hill apx 1 minute and 15 seconds faster than the Dodge and apx 1 minute faster than the GMC.

It doesn't appear that the new turbo has helped the Ford that much.

Good catch. You won't get answer from PUTC though.

@buddylam and zviera:

The graphs threw me at first, it looks like they are having some issues with formatting, its the same on the Eisenhower results page, the time and speed graph is for the ascent runs, they duplicated them for the climbing and braking sections (not sure why).

You guys failed horribly at driving the ram, you shouldn't have had to touche the brakes once if you manually shifted with the exhaust brake.

I think there is a little confusion with the braking test speeds posted here. I think they are the same max speeds and times achieved on the way up the hill. It looks as if the braking data was needlessly included on the climb test and the max speed/time data needlessly included on the braking test.

I'm not sure I like the GM idea of having to use the cruise to get more consistent speeds. It is okay for those who don't spend much time on tight winding roads or where traction can be questionable.
I have never liked cruise control for anything other than reasonably flat ground. It doesn't allow you to think ahead but then again most drivers don't think ahead either. Not a good combination when towing these kinds of weights.

Interesting tests. As I said in previous posts, I have had a lot of opportunity to use my '14 Ram 3500 SRW towing in the mountains of Colorado over the last month. With 15,000# in tow, I found that on the long steep hills (Red Mountain Pass, Vail Pass), I liked the control of manually shifting the Aisin with the exhaust brake in FULL mode. Steep downhill descents probably should not be attempted with cruise control anyway. Milder grades seem to be handled easily with the auto mode for the exhaust brake and tow/haul engaged with or without cruise control.

Um, isn't there a sign at the top of that hill that says, "Trucks 45 mph"? If you weigh over 26,000 # you're legally in 'truck' territory. Next time test the trucks as to how they can hold to the speed limit.

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