2011 Heavy-Duty Hurt Locker: Eisenhower Pass Exhaust Brake Test

Sierra-eisenhower
Words by Mike Levine, Mark Williams and Kent Sundling, Photos by Ian Merritt

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Eisenhower Pass Exhaust Brake Test

We repeated the exhaust brake test on the westbound descent back to Dillon. Eisenhower averages a 2 percent steeper grade than Davis Dam, and that 2 percent made a big difference in slowing the 14-ton fully burdened rigs (trailer, truck and five adult males).

The finishing order changed, with the Sierra requiring five brake applies, the Ram 12 and the Ford 13.

What happened to the Ram? Before we explain, we’ll note that we had both a GM and a Ram engineer in the cab with us (two of the five passengers) during this testing.

If there’s a glaring weak spot with the High Output Ram, it’s the six-speed automatic transmission. The Cummins’ exhaust brake is willing and able to slow the truck, but it doesn’t seem to have the full cooperation and support of the gearbox. By contrast, this is where the GMC’s Duramax and Allison transmission form a formidable team.

Ram-eisenhower

By itself, the Duramax exhaust brake doesn’t feel as strong as the Ram’s, but to make up for this, the exhaust brake and transmission work extraordinarily well together. They’ve been engineered that way from the start. The Duramax and Allison downshifted to as low as 2nd gear while the engine stopped burning diesel and only pumped air at 4,150 rpm to slow the truck. The Ram, however, stubbornly stayed in 3rd gear, from 2,500 to 2,900 rpm, and picked up speed until it was forced to upshift to 4th gear to keep from over-revving. Fourth gear, for all the trucks, allowed speeds to increase over 60 mph, forcing us to apply the brakes. If the Ram could have downshifted to 2nd gear, like the Sierra, we think there would have been far fewer brake applies.

The Ford’s weak exhaust brake is its Achilles heel. It had minimal effect slowing the rig and keeping our driver from getting that “white knuckle” feeling you don’t want while barreling down I-70 at night. Although the Ford’s six-speed transmission did a nice job downshifting from 4th to 3rd gear after the foot brake was applied, the wheel brakes on both truck and trailer suffered as stopping power was turned into heat. At the end of every descent from Eisenhower Pass to Dillon, the F-350’s brakes were literally smoking.

Ford has excellent stopping power on flats, but it needs to step up its game in the mountains to stay even with GM and Ram.

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Comments

Congratulations G.M.C, again!

Yes ford does need a better exhaust brake system. But overall they do fairly well in downhill grades aside from this testing runs.

Exhaust brakes are like a reverse HP activity, and because the Duramax V8 can tolerate a lot more rpm than the Cummins I6, it can ultimately put down more retardation. I know guys who have spun the D-max up to 5400rpm downshifting the Allison. I wonder how the Cummins would have done with the 6speed manual. Then again, the TWR with the MT is WAY lower...

@Mrknowitall - valid point. I remember when exhaust brakes first came out "Jake brake" was the most common system I remember. There were guys that were blowing engines because of over revving.
The Cummins system does look like the best system but only if the tranny would of done its job.

It is interesting to note that these tests favor the Ford tranny over the Allison tranny.

Diesel engines have two redlines: one under combustion pressure, and another for overrun retardation.

Pickuptrucks.com should have found out what the limits are, and posted them. (Don't you guys have contacts at Ford/GM/Chrysler?)

Secondly, all of these engines have intake throttles-which control eEGR, why aren't they integrated into the whole engine brake package?
Are pickups that much less sophisticated than the rest of the automobile universe?

@George- under vacuume, tbe engine has less retardation than under pressure. That means the throttle is less effective than an exhaust brake.
@Lou- while the V8's can safely spin to over 5 grand, the I-6 just isn't built for that. Ultimately, that means less retardation, no matter what the transmission.

Everybody:: there are 4 types of braking systems, 1) is the brakes built into the wheels, 2) then there is the (exhaust) brake where the exhuast system is mechanicly blocked when you take you foot off the fuel pedal. 3) then there is the (Jake-brake), where inside the engine there are soliniods atached to some of the valves, this is where it can get compicated because there are diff. systems, the top of the line offers 3 of the 6 cly or 2 of the 6cly or1 of the cly to work for diff. levels of retardation, it is diff. on 12,8,6,4, cly engines all diff. types. and 4) Caterpilar has a very complicated sys. of oil passages in the fly-wheel, that creates resistance with a lever on the steering colume. The most common on p/u's thou are exhaust type brakes! and finaly some of these systems word with cruze control when you gain speed going down hill. Sandman 4X4

@Mr. Knowitall.
I didn't say that the intake throttles need to be fully closed, but closed so that there is negative work done on the intake stroked.(60 degrees or so)
This would help with the negative work done on the exhaust stroke from an exhaust restriction system.
Fully throttling would nullify the exhaust brake.

Yes, an exhaust brake has more possible negative power potential because you can only get 1 atmosphere of pressure differential from vacuum. But how much cool air (relative to the normal combustion exhaust gas temperature) can be pumped into the exhaust system before the catalytic converter cools below the critical temperature?

@George- I see what you're talking about. I'd have to check what we do here as far as using the throttle. For the catalytic cooling off, the energy that is being absorbed by the engine, is largely put into heating the expelled air (friction from being forced through the exhaust brake. This will keep the cats warm to some extent. worst case, it behaves like on cold start for a bit. Should warm right back up- what goes down, usually goes right back up.



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