Exhaust brakes and downshifting

[TBSarfari28]

Check out the Jacobs Vehicle Systems website: http://www.jacobsvehiclesystems.com
There are videos explaining compression brakes (Jake Brake) and exhaust brakes, exhaust braking on newer diesel engines is accomplished by using the variable geometry turbocharger to restrict exhaust flow out the cylinders to retard vehicle speed. Some late model diesel engines for vehicle applications use both compression and exhaust braking to supplement vehicle braking.

[Rich Jenkins]

Originally Posted by Cowbell

With the Dodge 2500, it has a two position switch. Hit the button once for "high" EB and then hit the button again for "automatic" EB. When in the "automatic" mode, you slow the truck to your desired speed with your pedal brake and the let go. The EB engages and feathers on and off to maintain that speed. It's a great system and a major reason to buy diesel, in my opinion.

Good summary. I can't add much, just to say on my 2016 Ram 2500 with the Cummins, this works astonishingly well on mountain grades. We've towed down interstate grades at 6% as well as down into Death Valley (10%) and all I do is get to the right speed and let the EB do the work.

One other point, the owner's manual Diesel supplement for my truck does indicate the transmission downshifts as well; the statement is 'more agressively in tow/haul mode'. I've never actually felt any downshift, to be honest. It may be I'm fairly circumspect on tow speed going up the mountain; I tend to crest the top at about the speed I want to descend. I'm not in a race. I typically don't use the brakes, at all.

My usual mode of driving is with tow/haul engaged, and the EB with 'auto' (green) selected, even in the flat lands.

Heck, sometimes I leave the EB on unhitched, it is just such a cool sound.

[tjdonahoe]

With my dodge, 6.7 ,and 6 speed auto, I don't use the tow haul mode as it always wanted to down shift, usually I go down the grade in the gear I go up and let the engine brake do it's thing, long grade with no curves , usually let the brake hold back part way then let it go, just like running a big truck with a Jake brake, after a while it comes naturally, on the KW with a E model cat , the Jake will come on stronger as needed, all done in the computer...(no variable pitch turbo)..

[SSquared]

Originally Posted by Alluminati

......

Sounds like at the end of the day, a diesel with EB is not better off than a gas engine with someone willing to downshift properly. Do I have this correct?

......

I haven't driven a diesel since about '92, but I do think the modern diesels have an advantage with their EB.

I tow with a 400HP gasser with a six speed auto transmission. As an example, the last big descent I did was Rabbit Ear's pass. The speed limit is 50. I prefer 40 to 45. I have three choices on the way down:

1) Third gear or higher would mean heavy brake use all the way down.
2) Second gear will result in the speed creeping up to 50 and beyond, so I have to use the brakes occasionally.
3) First gear means no brakes needed, but speed will be around or under 30.

Descending at 40 or 45 without using the brakes is impossible.

With a modern EB diesel, it can hold whatever speed you select.

[Mike1997]

After reading all the posts with great info, I suppose I am the ONLY person with an Airstream Landyacht diesel pusher and a Jeep Grand Cherokee diesel. The AS is on a freightliner frame with a Cummins diesel engine. It has a Allison 6 speed electronic transmission with a jake brake. The jake break has a toggle switch on the dash that is manually operated. When switched on it will activate when the accelator petal is released and automatically makes the transmission start downshifting and you can feel the engine rev higher. Its a great feature to have in the mountains and eliminated any need for touching brakes even while towing the Jeep.

The Jeep is a 2007 model with a Mercedes 6cyl diesel and 5speed transmission. No jake brake but downshifts are easily made with the shifter by pushing it left or right to upshift or down shift as needed. Only time I recall using that feature to slow down was when I pulled a loaded double axle Uhaul trailer from TX to NC without trailer brakes.....not by choice I might add!

Both diesels get great mileage considering their size, weight, and ability. Airstream averages about 10mpg towing the Jeep and 12mpg without. Jeep averages 19 mpg in town and 24mpg on the highway and its AWD on demand.

[Vitaver]

What am I missing? Engine braking by any means does not apppear to actuate the trailer's brakes, or do they? Cruise control does but it is discouraged on snow/since. We reduce/control the speed of the TV but could the trailer then either push the TV or jackknife? Newbie questions of course. BTW I have F3350 Diesel 2017 with three settings for engine braking (including 'off', yet have to understand how they operate differently), TH mode, 4HL than can be 'blocked' so all wheels have engine traction. No wonder the Manual is some 700 pages long in small print (done with half so far).

[dznf0g]

Originally Posted by Vitaver

What am I missing? Engine braking by any means does not apppear to actuate the trailer's brakes, or do they? Cruise control does but it is discouraged on snow/since. We reduce/control the speed of the TV but could the trailer then either push the TV or jackknife? Newbie questions of course. BTW I have F3350 Diesel 2017 with three settings for engine braking (including 'off', yet have to understand how they operate differently), TH mode, 4HL than can be 'blocked' so all wheels have engine traction. No wonder the Manual is some 700 pages long in small print (done with half so far).

I don't think you're missing anything. EB would not be a good thing on very slippery surfaces. No, they don't apply the trailer brakes. There have been times (solo) in my business travels, when black ice was so bad (rain on top of very smooth ice) that I actually will shift into neutral anytime my foot comes off the throttle. I trust ABS, and Stabilitrac and my "smart" braking foot much more than the uncontrolled loss of drive wheel traction due to deceleration. This is especially true with rear drives and/or manual transmissions. Worst case for me was a drive from Peoria to the Quad Cities in a Trans Am w/stick. Think about that one for awhile.

[Echelon73]

I have a RAM 2500 with the Cummins diesel. I always activate the tow/haul mode and the exhaust brake when towing. I never have to manually downshift on steep grades. The truck downshifts as needed which allows the exhaust brake to be extremely effective down to speeds much lower than the 45mph (perhaps because I use tow/haul mode???). Having moved from a large gas engine, I can definitely say my diesel holds speed much better than my gas truck did. In fact, on steep inclines I could not hold my speed in any gear without heavy brake usage. That was just my own personal experience. Others seem to have good results with engine braking/lower gears in vehicles with a gas engine.

[Louie Dee]

I totally agree with Russingr. Living in Colorado & pulling a 2000 34' A/S with a 2015 GMC Durmax down some very steep grades, I often find myself using the EB to maintain a set decent speed with the Tow Haul speed set with the cruise control & never having to use normal braking. Brakes should last a very longtime! The biggest plus is the security of knowing that I will avoid a run away situation or over stress the braking system.

[Dexterpix]

I have a 2009 Dodge w/6.7 Cummins I run the ENGINE BRAKE on all the time. I run it with or without the trailer. I have 125k miles on the truck and I have not had to replace the brakes yet. The Engine Brake lets the engine do the work not the brakes. If you just ease of the throttle the engine will begin to slow the truck. As an example, the amount of braking it takes to get your truck and trailer slowed from 65mph to a safe off ramp speed in the Northeast of 30mph for most clover leafs, I do only with the engine brake and need no mechanical brake assist. I believe that not having to hit the mechanical brakes on the TV when towing has the added advantage of not heating up the TT brakes and rim which I believe can contribute to tire failure. Race cars run cooling fans on the brakes to keep excess heat not only out of the brakes but also out of the rim/tire. If you are running a long decline and are constantly on the TV brakes, you are also constantly on the TT brakes. The heat from the TT brakes goes directly to the rim and bead are of the tire. Engine Braking in a modern diesel is light years ahead of the old "Jake Brake" both in terms of effectiveness and negative effect on the engine it self. The old "jakes" (which is a company name that made exhaust brakes) would build up massive heat loads on the engine. It only restricted the exhaust from exiting, without retarding the engine from firing and adding hot exhaust, by holding it back it pushed EGT to the stratosphere and you had to monitor your EGT gauge and give it a rest to lower temps or you tended to fry the oil seals in the turbo. I find the engine brake to be great around town when you are only slowing from 40 - 20 and back to speed for traffic. Great tool to have.

Joe D

[Alluminati]

So this is what I’ve learned (please correct me if I misunderstand): On a diesel, exhaust braking creates a compressor action to apply resistance to the driveline. Downshifting a gas engine creates vacuum. Compression is a more effective force than vacuum.

For the average user, having a button on the dashboard that says “EB” is probably more inviting than simply downshifting. Any “Pull Me” button will get more use, particularly in the hands of the unskilled. EB works at lower RPMs, and arguably cooler sound than the screaming of a gas engine in low gear, especially when downshifting delivers its jolt when done improperly.

You say EB, TH, and cruise control work together seamlessly. I don’t use CC when towing. My trailer puts my truck at about 95% capacity, so my transmission tends to hunt, though it does return better fuel economy. Still,.... So I have not tried TH and CC together. Perhaps they were meant to work together too. I’ll have to give it a try. I know that bobtail without TH, CC will let it fly downhill. Maybe TH will make CC downshift.

Wow. What an education! Yous guys are the bestest.

[dznf0g]

Originally Posted by Alluminati

So this is what I’ve learned (please correct me if I misunderstand): On a diesel, exhaust braking creates a compressor action to apply resistance to the driveline. Downshifting a gas engine creates vacuum. Compression is a more effective force than vacuum.

For the average user, having a button on the dashboard that says “EB” is probably more inviting than simply downshifting. Any “Pull Me” button will get more use, particularly in the hands of the unskilled. EB works at lower RPMs, and arguably cooler sound than the screaming of a gas engine in low gear, especially when downshifting delivers its jolt when done improperly.

You say EB, TH, and cruise control work together seamlessly. I don’t use CC when towing. My trailer puts my truck at about 95% capacity, so my transmission tends to hunt, though it does return better fuel economy. Still,.... So I have not tried TH and CC together. Perhaps they were meant to work together too. I’ll have to give it a try. I know that bobtail without TH, CC will let it fly downhill. Maybe TH will make CC downshift.

Wow. What an education! Yous guys are the bestest.

Pretty much accurate:

In addition to the differences in "compression" of exhaust gasses and vacuum, understand that a diesel has upwards of 25:1 compression ratio vs. gas at somewhere between 9.0 - 10.0:1 ratio.....so the "resistance" of a diesel of the gasses INSIDE the combustion chamber are greater, let alone in the exhaust piping. Also, generally speaking, the stroke of a diesel in many cases is longer, thus compressing a higher volume of air at a higher pressure. This makes for a much more effective physical environment for engine braking.

[Alluminati]

Originally Posted by Vitaver

What am I missing? Engine braking by any means does not apppear to actuate the trailer's brakes, or do they? Cruise control does but it is discouraged on snow/since. We reduce/control the speed of the TV but could the trailer then either push the TV or jackknife? Newbie questions of course. BTW I have F3350 Diesel 2017 with three settings for engine braking (including 'off', yet have to understand how they operate differently), TH mode, 4HL than can be 'blocked' so all wheels have engine traction. No wonder the Manual is some 700 pages long in small print (done with half so far).

Engine braking does not engage trailer brakes. Under normal conditions, properly using engine braking will never cause loss of traction. Likewise, properly using acceleration will not cause traction loss. Think of engine braking as counter-acceleration, which is very different from braking. Just as you accelerate carefully on slippery roads, likewise engine braking should be used very cautiously.

Engine braking works exceptionally well when descending steep grades. You don’t want to stop, but you want to keep your speed under control without undue stress on your brakes. The proper technique for engine braking with a gas engine is as follows: BEFORE beginning descent, used your brakes to reduce your speed to what will be safe during the descent. Then shift to a gear that will hold your speed down. If your vehicle speeds up too fast during descent, use your brakes again to bring it down to a safe speed, then downshift further. You will be in the right gear when your vehicle only gains speed slowly. If 45 mph is safe, then hit the brakes smoothly and firmly until you slow to 40 mph. Then let your truck slowly gain speed up to 50, before firmly braking back to 40. If your incidence of braking is too frequent, downshift further.

It may seem strange to let your engine race when using engine braking, but it is totally harmless as long as your tach does not stray above your redline. Most likely it will remain 25% below, but it will still seem strange if you’re not used to it. You are doing far less stress to your engine than you would be doing to your brakes.

You should always drive with heightened awareness when in hazardous conditions, such as steep downgrades, high winds, slippery roads, but especially so when pulling a trailer. You should be ready to engage the manual trailer brake lever. On a recent trip out of the Rockies, I was blessed with a powerful tailwind. I let my speed creep up to 70 while the wind pushed me along. But the road eased to the right, and shortly I took a severe blow (literally) to my right side. It blew my trailer over the edge of my lane. Without reducing truck speed, I hit the manual trailer brake lever. It pulled the trailer back inline. Then I eased up on the throttle until I was safely below 55. I still kept my hand at the ready.

[phann]

Originally Posted by Echelon73

I have a RAM 2500 with the Cummins diesel. I always activate the tow/haul mode and the exhaust brake when towing. I never have to manually downshift on steep grades. The truck downshifts as needed which allows the exhaust brake to be extremely effective down to speeds much lower than the 45mph (perhaps because I use tow/haul mode???). Having moved from a large gas engine, I can definitely say my diesel holds speed much better than my gas truck did. In fact, on steep inclines I could not hold my speed in any gear without heavy brake usage. That was just my own personal experience. Others seem to have good results with engine braking/lower gears in vehicles with a gas engine.

You're right Echelon73. I was not factoring in the use of Tow-Haul mode. With TH and automatic EB both engaged, the EB kicks in without having to manually downshift, even at speeds well below 45.

[tjdonahoe]

The be works better at higher engine speed,that I why the down shift..

[rmatkovich]

Originally Posted by Alluminati

Please enlighten those of us who have never used exhaust brakes.

I blindly suppose the exhaust brake increases engine compression by restricting exhaust flow (closing exhaust valves?), which would reduce RPMs. I suppose that would only work if the transmission speed was locked to the engine. The result would be resistance to the drive axle.

Wouldn't this give the same effect as downshifting? The difference would be the exhaust brake slows the engine, while downshifting drives RPMs up. But the net result is the same. Please correct me if I'm wrong.

I have a 6-speed transmission which gives me 6 choices when descending a hill. How does one control speed when using exhaust brakes?

Been towing for over 40 + years and been through almost every mountain range in the US plus on the pan American HWY in central America to the panama canal and never had an exhaust brake and never warped my front rotors. If U know how to down shift and do not run over the rpm on UR teach U will not have a problem and the TV will last for many miles.


1997 F-250 349,000 miles same 7.3 4.0 trans 355 diff. 2000 34’ limited #12,500 going strong.

[slowmover]

The short answer is that a diesel with an EB isn't superior to a typical gas rig.

The trailer lash-up is in compression and this creates the most vulnerable condition the rig encounters as to NOT being able to deal with crosswinds.

The correct speed and gear plus readiness to manually activate trailer brakes AND instantly accelerate downhill are what matter. Thus, following distance.

[2012FB]

🙄🤐

Originally Posted by slowmover

The short answer is that a diesel with an EB isn't superior to a typical gas rig.

The trailer lash-up is in compression and this creates the most vulnerable condition the rig encounters as to NOT being able to deal with crosswinds.

The correct speed and gear plus readiness to manually activate trailer brakes AND instantly accelerate downhill are what matter. Thus, following distance.

Sent from my iPhone using Airstream Forums

[Vitaver]

Originally Posted by dznf0g

I don't think you're missing anything. EB would not be a good thing on very slippery surfaces. No, they don't apply the trailer brakes. There have been times (solo) in my business travels, when black ice was so bad (rain on top of very smooth ice) that I actually will shift into neutral anytime my foot comes off the throttle. I trust ABS, and Stabilitrac and my "smart" braking foot much more than the uncontrolled loss of drive wheel traction due to deceleration. This is especially true with rear drives and/or manual transmissions. Worst case for me was a drive from Peoria to the Quad Cities in a Trans Am w/stick. Think about that one for awhile.

Will ABS work while on neutral?

[dznf0g]

Originally Posted by Vitaver

Will ABS work while on neutral?

Yes