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Discussion Starter #1
I have never driven a truck with an exhaust brake so I have no idea what one would sound like or feel like, but from my understanding diesels cannot naturally provide engine braking by design, so downshifting should not provide compression braking with out an exhaust brake correct?

When I have towed with a heavy load and tow mode engaged the moment I touch the brake it downshifts and provides considerable compression braking - Has anyone else experienced this?

From the development paper of the Holset M2 turbo it shows a flow diagram of the rotary valve stages. One of those stages is exhaust throttling. I am not a turbo designer and have limited working knowledge of them, but is this not the same thing as an exhaust brake? The image appears to show the rotary valve working as a restriction to exhaust flow in this mode.

For those that are more knowledgeable could you chime in here? Thanks.
 

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I have never driven a truck with an exhaust brake so I have no idea what one would sound like or feel like, but from my understanding diesels cannot naturally provide engine braking by design, so downshifting should not provide compression braking with out an exhaust brake correct?

When I have towed with a heavy load and tow mode engaged the moment I touch the brake it downshifts and provides considerable compression braking - Has anyone else experienced this?

From the development paper of the Holset M2 turbo it shows a flow diagram of the rotary valve stages. One of those stages is exhaust throttling. I am not a turbo designer and have limited working knowledge of them, but is this not the same thing as an exhaust brake? The image appears to show the rotary valve working as a restriction to exhaust flow in this mode.

For those that are more knowledgeable could you chime in here? Thanks.
This has been discussed on here since well before the truck was released. I'm not sure what the exact answer is because all articles/reviews I have read/watched specifically say there is no exhaust braking. (ok maybe not ALL of them, but all of them that mention it say there is no exhaust brake)

http://www.titanxdforum.com/forum/490-5-0l-cummins-v8-turbo-diesel/6786-tow-haul-mode-downhill-speed-control.html
 

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Discussion Starter #3
@digitalbliss - Thanks. I should have searched my bad. So the thread conclusion was maybe? lol.
 

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Discussion Starter #5
No exhaust brake, just transmission speed control.
For myself and those that may be looking for similar information - Could you please explain the difference and how transmission speed control works? Thanks.
 

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Discussion Starter #6
So after reading this https://en.wikipedia.org/wiki/Retarder_(mechanical_engineering) and seeing the rotary vane operation on the Holset M2 turbo the Cummins XD most certainly has an exhaust brake.
@AZPRO4X I did a little reading on a transmission retarder and there is no provision for this on the XD. No fluid pump or cooler that I can find. And simply downshifting the XD Diesel will provide zero brake effect due to the lack of intake throttling from what I understand.

So it seems that whether or not it was advertised it is an included feature based of the rotary valves modes of operation.
 

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Loaded with 4 persons, ~500lbs in the bed, towing 2500lbs and going downhill on a 16% grade for 2km, the truck held 100km/h in tow mode in 3rd gear.

Call it what you will, it's good enough for me! ;)
 

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So after reading this https://en.wikipedia.org/wiki/Retarder_(mechanical_engineering) and seeing the rotary vane operation on the Holset M2 turbo the Cummins XD most certainly has an exhaust brake.
@AZPRO4X I did a little reading on a transmission retarder and there is no provision for this on the XD. No fluid pump or cooler that I can find. And simply downshifting the XD Diesel will provide zero brake effect due to the lack of intake throttling from what I understand.

So it seems that whether or not it was advertised it is an included feature based of the rotary valves modes of operation.
I have read in cummins publications it's in the system
 

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Loaded with 4 persons, ~500lbs in the bed, towing 2500lbs and going downhill on a 16% grade for 2km, the truck held 100km/h in tow mode in 3rd gear.

Call it what you will, it's good enough for me! ;)
2km 7% grade 9000lbs in tow. 80km/hr. didn't have to use brakes except to engage the grade shifting mode. Truck grade brakes just fine in tow mode.
 

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Discussion Starter #11
Why in the world would you call a feature that every person understands as an exhaust brake - downhill speed control? :S-A-Smack:
 

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Discussion Starter #12
From my understanding, it would be a result of torque converter lock up.
I could be wrong, but I don't believe locking up the torque converter will enhance the braking ability due to the fact that a diesel engine cannot be throttled. It would be effective in a gas engine because you are reducing the drivetrain slip during deceleration against a load (the vacuum of a gas engine) and increasing the braking power.
 

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So after reading this https://en.wikipedia.org/wiki/Retarder_(mechanical_engineering) and seeing the rotary vane operation on the Holset M2 turbo the Cummins XD most certainly has an exhaust brake.
@AZPRO4X I did a little reading on a transmission retarder and there is no provision for this on the XD. No fluid pump or cooler that I can find. And simply downshifting the XD Diesel will provide zero brake effect due to the lack of intake throttling from what I understand.

So it seems that whether or not it was advertised it is an included feature based of the rotary valves modes of operation.
If the transmission downshifts the vehicle is going to slow down. Semi's use the same gear to go down the hill as they used to go up it for that reason. Yes, they have a Jake brake but they are dealing with a lot more weight. A Jake brake is not needed for what the XD can handle, I didn't need one while dragging around 23k worth of total weight with my 06 3500 on 8% grades.
Brains, brakes & gears are all that's needed. Heck I was pulling a boat one time with a old Cherokee & I completely lost my brakes, so all I had was my brains & gears, it turned out fine.
 

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Discussion Starter #15
Well I'm telling you that there has to be something in addition to the standard brakes. Transmission braking in the standard sense is not possible with a diesel by design.

A diesel cannot just downshift and experience compression braking. If you read the article I linked to it explains transmission braking application for a diesel and I highly doubt the XD has this. If that were the case it wouldn't be referred to as "just" transmission braking since a transmission retarder is superior to an exhaust brake.

If I turn on tow haul with out a load or trailer attached and just tap the brakes it stops VERY aggressively on its own. I wish I was more knowledgeable to better explain, or someone more knowledgeable could provide an actual explanation for how a diesel could just downshift and experience compression braking.
@digitalbliss - tone doesn't come through very well on the internet. I'm not bashing you are getting worked up here. Just asking questions and looking for a technical explanation for what I am experiencing that says it isn't an exhaust brake I.E. how does transmission braking work on a diesel.
 

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Well I'm telling you that there has to be something in addition to the standard brakes. Transmission braking in the standard sense is not possible with a diesel by design.

A diesel cannot just downshift and experience compression braking. If you read the article I linked to it explains transmission braking application for a diesel and I highly doubt the XD has this. If that were the case it wouldn't be referred to as "just" transmission braking since a transmission retarder is superior to an exhaust brake.

If I turn on tow haul with out a load or trailer attached and just tap the brakes it stops VERY aggressively on its own. I wish I was more knowledgeable to better explain, or someone more knowledgeable could provide an actual explanation for how a diesel could just downshift and experience compression braking.
@digitalbliss - tone doesn't come through very well on the internet. I'm not bashing you are getting worked up here. Just asking questions and looking for a technical explanation for what I am experiencing that says it isn't an exhaust brake I.E. how does transmission braking work on a diesel.
Huh? All I did was post a video where the Nissan guy clearly says there is no exhaust brake.
 

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OK I'll bite. Quickest way to end this discussion is simple. Mount a Go-Pro with a view of the RTC actuator linkage and monitor it's position. If it transitions beyond the degree need for average driving on decel then they have an exhaust retarder/brake algorithm built into the programming. Which makes sense as the exhaust system uses this restriction (aka exhaust retarder) to get the catalyst up to temperature in minutes after startup for emissions, and I agree with Kadoi, torque converter lockup is not going to do anything for engine braking except reduce the revs lost in a freewheeling fluid clutch. The higher ratio gear changes are going to be minimal increases with efficient modern transmissions these days. If it was just the transmission doing the work, then mileage would severely suffer do to parasitic losses within the geartrain during routine driving. Diesels by nature, as we all know don't utilize a throttle valve, therefore pumping loses are minimal and no vacuum to negate that, certainly not enough for adequate engine braking. Having driven various over the road trucks, some modern units even employ a strategy where they close down the variable vanes on the turbine to induce back pressure before even touching the jake brake, conversely, they use it to also keep the turbo online to reduce lag during on and off power transitions. Without looking at the ECU programming, I can't say for sure, but the key elements are there. Anyway, food for thought;)

BigRed
 

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Discussion Starter #18
OK I'll bite. Quickest way to end this discussion is simple. Mount a Go-Pro with a view of the RTC actuator linkage and monitor it's position. If it transitions beyond the degree need for average driving on decel then they have an exhaust retarder/brake algorithm built into the programming. Which makes sense as the exhaust system uses this restriction (aka exhaust retarder) to get the catalyst up to temperature in minutes after startup for emissions, and I agree with Kadoi, torque converter lockup is not going to do anything for engine braking except reduce the revs lost in a freewheeling fluid clutch. The higher ratio gear changes are going to be minimal increases with efficient modern transmissions these days. If it was just the transmission doing the work, then mileage would severely suffer do to parasitic losses within the geartrain during routine driving. Diesels by nature, as we all know don't utilize a throttle valve, therefore pumping loses are minimal and no vacuum to negate that, certainly not enough for adequate engine braking. Having driven various over the road trucks, some modern units even employ a strategy where they close down the variable vanes on the turbine to induce back pressure before even touching the jake brake, conversely, they use it to also keep the turbo online to reduce lag during on and off power transitions. Without looking at the ECU programming, I can't say for sure, but the key elements are there. Anyway, food for thought;)

BigRed
Thanks big red. always informative. That's a good idea about the go pro. I have one laying around I'll see if I can get it rigged up right.

Sent from my XT1565 using Tapatalk
 

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OK I'll bite. Quickest way to end this discussion is simple. Mount a Go-Pro with a view of the RTC actuator linkage and monitor it's position. If it transitions beyond the degree need for average driving on decel then they have an exhaust retarder/brake algorithm built into the programming. Which makes sense as the exhaust system uses this restriction (aka exhaust retarder) to get the catalyst up to temperature in minutes after startup for emissions, and I agree with Kadoi, torque converter lockup is not going to do anything for engine braking except reduce the revs lost in a freewheeling fluid clutch. The higher ratio gear changes are going to be minimal increases with efficient modern transmissions these days. If it was just the transmission doing the work, then mileage would severely suffer do to parasitic losses within the geartrain during routine driving. Diesels by nature, as we all know don't utilize a throttle valve, therefore pumping loses are minimal and no vacuum to negate that, certainly not enough for adequate engine braking. Having driven various over the road trucks, some modern units even employ a strategy where they close down the variable vanes on the turbine to induce back pressure before even touching the jake brake, conversely, they use it to also keep the turbo online to reduce lag during on and off power transitions. Without looking at the ECU programming, I can't say for sure, but the key elements are there. Anyway, food for thought;)

BigRed
I don't know, I'm inclined to believe what the Nissan rep said when Mr. Truck specifically asked if the truck had an exhaust brake. If anyone would know, Nissan would. Right?
 

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Well, I believe some kind of compression braking is going on here but maybe not in the classical sense of an after market exhaust brake such as the Banks Brake on my Powerstroke. I don't know how it's happening but I can tell you that with a 6000 lb trailer I can bring this truck to practically a complete stop in pretty short order by manually downshifting 1 gear at a time into 1st.

On a recent trip towing a trailer to Brian Head (10,000+ ft country) from Northern AZ, I relied almost exclusively on manual downshifts throughout the mountain roads while only covering the brake to apply it if needed. It works so well that I trusted it completely and rarely had to apply the brakes except for the occasional deer. In addition, have not yet been in a situation yet where even on an 8% downhill grade, the XD would overrun its gearing. I don't know how well it will do with a heavier trailer. When I bought the XD, I had no intention of towing at its rated capacity. But now I'm curious and would hook up a heavy trailer just to see how well it would do!

Tapping on the brake in my stock 99 Powerstroke would UNLOCK the torque converter and leave me practically freewheeling with my 5th wheel down a mountain grade. To relock the torque converter you had to give it some throttle! The most nonsensical arrangement for a truck rated for 11,400 lb 5th wheel towing that I ever heard of. That is the way it was designed! It also shot transmission temps into oblivion in desert operating temps. Ford later rethought that with the 6.0 and Torq-shift tranny. The Banks Brake with the setup to automatically lock the torque converter with no throttle fixed all of that nonsense. Leaving the BB activated on a downhill grade would actually require throttle to keep up with traffic!
 
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