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Old 12-28-2014, 06:26 PM   #1
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Engine Braking

Tried searching, but the AS forum is vast and this topic is well hidden in threads. If you point me to this topic, much appreciated. So, my questions are....how does engine braking work? Can ones engine really survive the RPM's of being pushed down hill by a TT? What is the best approach to truck brake, engine brake, and trailer brake combo?
Came down the Big Horn Pass, WY this summer and was shocked at how much gravity influences the whole rig. I mostly kept it in control. At the bottom ran into another RV'er who proclaimed his awe and said he was turning around and going back to Maryland after that ordeal. I rode my rig down in 2nd gear, avoiding 1st in fear I would explode the engine. Alternated truck and trailer brakes, but avoided all out foot stomping. I have a 2013 Ford F150 8 cyl. auto, 2013 International Signature 23'. Any recommendations about how to safely handle steep downhill grades without burning out the brakes? thanks!
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Old 12-28-2014, 06:36 PM   #2
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I don't have a heavy duty diesel with exhaust brake, but I was interested to learn how it works. This is what I found (and posted in another thread):

http://www.airforums.com/forums/f42/...ml#post1559470
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Old 12-28-2014, 06:38 PM   #3
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It really depends on what sort of engine you have. A standard gasoline engine, will provide some degree of braking when the throttle plate is closed. This creates a vacuum which causes some drag. Also the engine is a big air pump at this point compressing air. You want to select a gear that won't allow the engine to over rev.

Diesel engines without some sort of exhaust brake don't have as much drag as a gas engine because they don't have a throttle plate. To create more drag they use an exhaust brake or Jake brake.

In general, use engine braking to slow down as much as possible and intermittent use of the truck and trailer brakes. You want the brakes to have time to cool down between applications. Worst case is excessive wear, warped rotors, and loss of braking as a result of boiling brake fluid. Heat might also contribute to tire failure.

What is a Jake Brake? (with picture)

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Old 12-28-2014, 06:43 PM   #4
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Engine or exhaust braking is a function of diesel engines. It uses back pressure of the engine's exhaust to slow the vehicle. On the big trucks, these are called Jake Brakes.

The late model diesel pick-ups come with an exhaust bake function. Our 2011 Silverado 3500 Duramax has this exhaust brake function. We have found that this system works beautifully to maintain a safe and comfortable speed on a significant downgrade while towing the Airstream. We no longer experience that "runaway train" feeling that we used to get when towing on these downgrades with the 3/4 ton gasoline Suburban.

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Old 12-28-2014, 06:44 PM   #5
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Here's some information from US Gear regarding exhaust braking. This is the next major modification I want to do to my PowerStroke before I contemplate any mountain towing.

D-Celerator Diesel Exhaust Brake
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Old 12-28-2014, 06:47 PM   #6
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Actually, the 5.7L Hemi Ram features a form of engine braking, basically the computer tells the transmission to down shift automatically when in tow/haul mode when the brake is applied. It works incredibly well.

Since most folks ignore the USE LOW GEAR signs, it basically does it for you, thus slowing the vehicle. But only when tow/haul is engaged. It will do it without apply the brake too, I'm not sure how it senses it. But coming over a hill today, I took my foot off the gas and the engine brake engaged while going down the hill.
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Old 12-28-2014, 07:29 PM   #7
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Originally Posted by sandrade View Post
Any recommendations about how to safely handle steep downhill grades without burning out the brakes? thanks!
Engine braking will supplement your brakes but is of little use unless you are in a low enough gear. Easy with a manual tranny. Varies on an automatic. On an automatic, engine braking is even less effective if there is not a mechanism for the torque converter to lock up on deceleration. Without lockup there is too much slippage between the tranny and engine to get any real brake effect. Some vehicles will lock the torque converter during deceleration in tow/haul mode.

The best thing to do is anticipate the steep downhill. Even knowing where and when you will find them before setting out for the day is a big help. When the warning signs appear, it is time to start downshifting - before the dropoff. Be at the speed you want and in the lowest gear you can, before you start down the steep slope.

When in tow/haul mode, the software in my F250 uses a sensor to sense the downhill slope and starts downshifting when I hit the brake pedal. It also locks up the torque converter to get maximum engine braking.

If your truck does not have a system like that, maybe you can downshift by some other means.
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Old 12-28-2014, 07:50 PM   #8
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Reading your Fords owners manual would be a good place to start to see what is available to you.😉
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Old 12-28-2014, 07:52 PM   #9
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I don't know which way you were going, east toward buffalo or west toward Tensleep, either way you want to go down slow,if you let it roll you are going to get the brakes hot and end up with none ,which is very bad..You did fine, slow and you got down....
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Old 12-29-2014, 08:34 AM   #10
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I don't have one on my Cummins diesel. Wish I did. I was thinking of having one installed and after speaking with a diesel tech ( shop in Fla) he advised against the aftermarket ones. He did recommend those with factory installations. Been down a few steep passes in Colorado, Wyom. & Montana and find I can manage them OK by simply going SLOW. Rt. 70 west of Denver takes the trucks down to 25mph
( eastbound). I found I could manage them nicely at 35mph. The other inclines vary but SLOW is the key. So much easier to go up them.
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Old 12-29-2014, 09:36 AM   #11
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Quote:
Originally Posted by sandrade View Post
I rode my rig down in 2nd gear, avoiding 1st in fear I would explode the engine. Alternated truck and trailer brakes, but avoided all out foot stomping. I have a 2013 Ford F150 8 cyl. auto, 2013 International Signature 23'. Any recommendations about how to safely handle steep downhill grades without burning out the brakes? thanks!
I think you did the best you can given what you have (I have a 2012 F150 V8 so essentially the same rig).

This is one reason I hesitated against the smaller ecoboost engine in the F150. I know it will pull more, but I suspect the engine braking will be even worse? And now they are adding 2.7 in addition to the 3.5.
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Old 12-29-2014, 09:45 AM   #12
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This is the rare occasion where my non-AS trailer comes in handy......My trailer is SO un-aerodynamic, I hardly need to touch my brakes! Hahahahaha!
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Old 12-29-2014, 09:52 AM   #13
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For an F150.....

OP asked about engine braking on an F150. My F150 (2006 5.4L V8, AT) will not downshift into a gear unless it is safe to do so. If I put the selector in 1 at 45 mph it will only downshift into 2nd.

The best way to determine what is a reasonable shift point is to do it while upshifting. Put the selector in 1 and accelerate until you reach 3500 rpm or so and note the speed. This is the maximum speed you can go downhill in 1st gear. Then shift into 2nd and do the same.

When you crest a hill, slow to the speed you noted for 1st or 2nd gear and then shift into that gear. Select the gear based on the slope of the hill you are descending. Then use your brakes to keep the speed to one that is reasonable for the engine. If you start out in 2nd and are still accelerating, brake to your 1st gear speed and shift into 1st. Serious grades usually have truck speed limits. One 8% I am familiar with has a truck limit of 35 mph. If your grade has such a limit, use that speed or less.

I have always heard that you should not "ride" the brakes. Brake firmly to your desired speed or lower and then release the brakes. Control the speed with periodic application of the brakes rather than staying on them all the time.

This is what I do and it works for me. My trailer GVWR is 6300#. I have never felt like I had insufficient braking. The trailer brakes seem to take care of the trailer and the truck takes care of itself. That said, the worst grade I have been on is about 6% for 4 or 5 miles.

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Old 12-29-2014, 09:54 AM   #14
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I had just posted this on another thread but it is likely more appropriate here. You won't hurt your engine using it for engine braking. People who live in the mountains do it every day.

Diesels without an exhaust brake produce very little engine braking because there are no throttle plates in a diesel motor to restrict the airflow.

Gasoline engines naturally have engine braking when the throttle is lifted. However to get sufficient engine braking with a gas engine you need the RPM higher than would seem normal. Most gas engines today need to be run down hill between 3500 and 5000 RPM. As a rule of thumb if you are using your brakes to control speed going down a hill then you are in too tall a gear.

Not long ago when all we had to tow with were 4 speed transmissions you had some fairly large gaps in the speed ranges you could descend at. For example on a 7% interstate grade second gear would hold you at 50-55 MPH but if the traffic was slow or the road was too twisty for 55 they you had to apply brakes and reduce the speed to 25 MPH and then shift into low gear and then likely feed it gas to maintain the lower speed. Today with 6 & 8 speed transmissions you have a much larger selection and you can almost always find the right gear for the speed you want to descend at.

On two lane roads in serious mountains remember that brake temperatures can sneak up on you. For example you may be descending in low gear using engine braking but every 1/2 mile you're putting the brakes on to slow for a switchback. Do this repeatedly and you can still overheat the brakes.

What I do in these situations is use the trailer brakes manually to slow for switchbacks etc. Then if I feel the trailer brakes start to fade I know I have a completely fresh set in the tow vehicle to pull over and stop with and then allow the trailer brakes to cool. I have only had to do this once in all the trips I have taken. That was the sea to sky highway in BC where you go down 11 kilometers of 10% grade followed by 5 kilometers of 15%.

I hope this helps.

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Old 12-30-2014, 12:47 PM   #15
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I know the Big Horn Mountains well. Some decent trailer camping off the highway at elevation, making it a wonderful breather... until going down.

I keep my speed with the Tundra 5.7L at 50 to 60mph and in 3rd. Sometimes if I am using the trucks brakes more than I want, I will manually apply the trailer's brakes... but they are minimal in effect. The Big Horn Mountains of Wyoming are a challenge. Just stay at a speed you are comfortable. After this practice... most other highways with big drops in a short distance will seem easy.

Buffalo, Wyoming... home of Sheriff Longmire, Wyoming County 24 plates and home of the Author that Longmire is based upon. By the way... there are only 23 counties in Wyoming. And the "Rez" has to be the Wind River Indian Reservation way WEST which has two small Casinos. Close Encounters of the Third Kind also used the Wyoming County 24 plate when the Aliens arrive at Devil's Tower... that looks nothing like the movie's version... but only those from Wyoming know.
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Old 12-30-2014, 07:23 PM   #16
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All internal combustion engines have some type of "throttle plate" you have to regulate the intake of air into the engine. More air = more fuel = more power. A diesel without a "exhaust brake" will still use the engines compression to resist acceleration. Diesel engines are a more effective engine brake because of the higher compression (2x or more than most gas engines)

Always followed a simple rule that whatever gear I needed to climb a hill was the gear I used to descend the other side. Both climbing and descending are fighting gravity, in theory the work load to maintain a given speed should be similar.
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Old 12-30-2014, 07:39 PM   #17
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Quote:
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A diesel without a "exhaust brake" will still use the engines compression to resist acceleration. Diesel engines are a more effective engine brake because of the higher compression (2x or more than most gas engines)
Not sure what diesel engine(s) you are referring to, but I disagree if you are including my 1995 PowerStroke in that comment. I don't have an exhaust brake yet and there is hardly any engine braking. Wayyyyy less than any carburated vehicle I've ever driven. In gear, heading downhill with my foot off the pedal, the engine is basically just acting as an air pump..... breathing in air, passing it straight through the engine. At a certain point, the injectors are not even firing, which is real strange because of how quiet it is as the vehicle continues to pick up more and more speed. An exhaust brake is needed to create some back-pressure to help slow things down, otherwise you need to use the brakes to slow down.

Edit to add: my truck has an automatic transmission. Otherwise, with a standard transmission, I could use the transmission to help slow things down, to an extent.
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Old 12-30-2014, 08:10 PM   #18
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All internal combustion engines have some type of "throttle plate" you have to regulate the intake of air into the engine. More air = more fuel = more power. A diesel without a "exhaust brake" will still use the engines compression to resist acceleration. Diesel engines are a more effective engine brake because of the higher compression (2x or more than most gas engines)
All spark ignited engines have some form of throttle plate. Not true for compression ignition engines. Thus, diesels have no significant retardation effect unless fitted with an auxiliary brake such as a Jake or other design. Compression ratio doesn't enter into it as the work to compress air on the up stroke is returned on the piston down stroke.
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Old 12-31-2014, 02:10 AM   #19
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Link to explanation of how a diesel engine "Jake brake" works:

Compression release engine brake - Wikipedia, the free encyclopedia
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Old 12-31-2014, 07:39 AM   #20
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We have had an F150 Eco Boost as well as F250 6.7 Diesel and now F350 6.7 Diesel. The F150 did an admirable job towing, a fair to poor job braking on long steep down hills, as any gas powered vehicle would do towing 9,000 pounds. The F250 and F350 are identical except in load carrying capacity and therefore their engine braking and braking in general are identical as well. Both the diesels can slow our rig(s) down on most hills with only an occasional use of the actual brakes. The engine gets up in RPMs, but have never reached the danger level.

As to methods, the CDL drivers test and recommendations for commercial drivers is to slow down at the top of the hill to the speed limit posted OR a little under so one is starting the decent at a reasonable speed. Then using all braking available, engine, jake and foot activated brakes keep to the posted limit and when the speed increases by five miles per hour slow down by using the foot brakes to the speed limit or slightly under. Both my wife and I have CDL licenses and we use this method with our truck and AS and it works, now with our gas powered SOB motorhome out west it's a whole different story, white knuckle even on I70 going towards East getting to Moab was an interesting experience to say the least. But joyous day the MH has been sold and we are awaiting pick up by new owner.

To summarize, the F350 Diesel with engine brake towing the 30' AS is a joy to drive, down hills are not really a big issue with this set up (A 3/4 Ton would give the same results, except with added payload and 20" tires we get a little less bed sway on tight turns going down hill).

Happy New Year to all.
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