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 > Why diesels are most efficient around 1,800 rpm

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TomG2

Central Illinois

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Posted: 10/28/20 09:37am Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

....snip....
Manufacturers do not make different trans tunes for different rear gear ratios. YOU are suppose to pick the rear gear ratio that has the best blend of of efficiency/performance based on how you use your truck.


Drivers can create their own transmission tune with a little click when the situation requires it. Selecting 7 instead of 8 when approaching a hill has the same effect as changing rear axle ratios and a whole lot cheaper and easier.

4x4ord

Alberta

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Posted: 10/28/20 10:03am Link  |  Quote  |  Print  |  Notify Moderator

I would guess that towing 13k down the highway at 65 mph takes about 140 hp. If an engine is designed to put out 300 hp at 1700 rpm and 400 hp at 2800 I’m thinking it is lightly loaded towing a 13k trailer. The best rpm to tow at for fuel economy is not mystical. If a transmission control module is loaded with a BSFC map it is very easy for it to “know” exactly and instantaneously where it is on the map at all times. The decision to shift or not should be very simple for a computer ..... “I’m at 600 Nm at 1270 rpm and after the shift I’ll be at 1700 rpm and 450 Nm. My BSFC is better after the shift hmmm I think I’ll shift.” We have only a seat of the pants feel for where our trucks are running in relation to their BSFC map ..... especially when we have never even seen a map for our engine.

* This post was edited 10/28/20 10:23am by 4x4ord *


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Grit dog

Black Diamond, WA

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Posted: 10/28/20 10:36am Link  |  Quote  |  Print  |  Notify Moderator

Oh, here we go again with the “theoretical” hp required to move x load at x speed....
Which in the absence of a flat road in a vacuum essentially does not exist.


"Yes Sir, Oct 10 1888, Those poor school children froze to death in their tracks. They did not even find them until Spring. Especially hard hit were the ones who had to trek uphill to school both ways, with no shoes." -Bert A.

ShinerBock

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Posted: 10/28/20 12:44pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

I would guess that towing 13k down the highway at 65 mph takes about 140 hp. If an engine is designed to put out 300 hp at 1700 rpm and 400 hp at 2800 I’m thinking it is lightly loaded towing a 13k trailer. The best rpm to tow at for fuel economy is not mystical. If a transmission control module is loaded with a BSFC map it is very easy for it to “know” exactly and instantaneously where it is on the map at all times. The decision to shift or not should be very simple for a computer ..... “I’m at 600 Nm at 1270 rpm and after the shift I’ll be at 1700 rpm and 450 Nm. My BSFC is better after the shift hmmm I think I’ll shift.” We have only a seat of the pants feel for where our trucks are running in relation to their BSFC map ..... especially when we have never even seen a map for our engine.


1) Your 140 hp assumption is based on flat land and normal wind drag. I have told you multiple times that my route to the cost is not flat and the part that is flat has very high wind(next to a wind farm). The terrain is a constant up down with up to 3% grades. I am not sure why you keep neglecting this part. My truck will pull most hills in 6th, but I have to put my foot more to the floor to do so. 5th requires considerably less throttle.

2)Transmission control is not loaded with a BSFC maps. It is multiple charts that basically says at X rpm, Y load, and Z throttle position upshift or downshift(there are a few other parameters depending on the make). The way you think it works and the way it actually works are two different things my friend. All it knows is to shift when the preset parameters are met. If it was able to be tuned like you think it is, then there would be no need for tow/haul mode or select shift.


I tell you what, why don't you contact Ram and tell them your brilliant idea and how they should be tuning their transmissions from now on. Let us know what they tell you.

* This post was edited 10/28/20 01:03pm by ShinerBock *

ShinerBock

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Posted: 10/28/20 12:49pm Link  |  Quote  |  Print  |  Notify Moderator

While you are at it. Contact Ford as well because my father in laws truck does the same thing.

noteven

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Posted: 10/28/20 01:13pm Link  |  Quote  |  Print  |  Notify Moderator

Some dummy spec’d my old Dodge 6 speed IShift so it’s towing cruise rpm is 1/2 way between peak torque and peak power in overdrive. When you start up a grade a downshift to ... direct (who knew?) has the engine closer to peak power ...

4x4ord

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Posted: 10/28/20 05:01pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

4x4ord wrote:

I would guess that towing 13k down the highway at 65 mph takes about 140 hp. If an engine is designed to put out 300 hp at 1700 rpm and 400 hp at 2800 I’m thinking it is lightly loaded towing a 13k trailer. The best rpm to tow at for fuel economy is not mystical. If a transmission control module is loaded with a BSFC map it is very easy for it to “know” exactly and instantaneously where it is on the map at all times. The decision to shift or not should be very simple for a computer ..... “I’m at 600 Nm at 1270 rpm and after the shift I’ll be at 1700 rpm and 450 Nm. My BSFC is better after the shift hmmm I think I’ll shift.” We have only a seat of the pants feel for where our trucks are running in relation to their BSFC map ..... especially when we have never even seen a map for our engine.


1) Your 140 hp assumption is based on flat land and normal wind drag. I have told you multiple times that my route to the cost is not flat and the part that is flat has very high wind(next to a wind farm). The terrain is a constant up down with up to 3% grades. I am not sure why you keep neglecting this part. My truck will pull most hills in 6th, but I have to put my foot more to the floor to do so. 5th requires considerably less throttle.






Your truck has been tuned so it could be that even though it can make big torque at low rpm it might not get enough air to do it efficiently... I don't know. We agree that a stock Cummins 6.7 towing up a 3% grade should be in 5th. If conditions are such that the transmission is constantly and annoyingly shifting back and forth, lock out 6th. We agree on that as well. I'm pretty sure my Ford will downshift if it's pulling hard enough to be down at 8 mpg and 1400 rpm but when the road levels out it will upshift back to 6th .... it's an automatic.



2)Transmission control is not loaded with a BSFC maps. It is multiple charts that basically says at X rpm, Y load, and Z throttle position upshift or downshift(there are a few other parameters depending on the make). The way you think it works and the way it actually works are two different things my friend. All it knows is to shift when the preset parameters are met. If it was able to be tuned like you think it is, then there would be no need for tow/haul mode or select shift.

I tell you what, why don't you contact Ram and tell them your brilliant idea and how they should be tuning their transmissions from now on. Let us know what they tell you.


That sounds pretty simple to me .... x rpm, y load and z throttle position .... they don't need any more info than that. They can just take a look at a BSFC map to see what those parameters should be to initiate the shift points and program it in. Come on, it doesn't take a rocket scientist to understand this stuff.... If you really think the Ram engineers are that clued out don't you think they could at least wire the transmission so that when you click on the tow haul mode, it automatically locks out 6th... that would at least satisfy what you're saying?

ShinerBock

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Posted: 10/28/20 05:20pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

ShinerBock wrote:

4x4ord wrote:

I would guess that towing 13k down the highway at 65 mph takes about 140 hp. If an engine is designed to put out 300 hp at 1700 rpm and 400 hp at 2800 I’m thinking it is lightly loaded towing a 13k trailer. The best rpm to tow at for fuel economy is not mystical. If a transmission control module is loaded with a BSFC map it is very easy for it to “know” exactly and instantaneously where it is on the map at all times. The decision to shift or not should be very simple for a computer ..... “I’m at 600 Nm at 1270 rpm and after the shift I’ll be at 1700 rpm and 450 Nm. My BSFC is better after the shift hmmm I think I’ll shift.” We have only a seat of the pants feel for where our trucks are running in relation to their BSFC map ..... especially when we have never even seen a map for our engine.


1) Your 140 hp assumption is based on flat land and normal wind drag. I have told you multiple times that my route to the cost is not flat and the part that is flat has very high wind(next to a wind farm). The terrain is a constant up down with up to 3% grades. I am not sure why you keep neglecting this part. My truck will pull most hills in 6th, but I have to put my foot more to the floor to do so. 5th requires considerably less throttle.






Your truck has been tuned so it could be that even though it can make big torque at low rpm it might not get enough air to do it efficiently... I don't know. We agree that a stock Cummins 6.7 towing up a 3% grade should be in 5th. If conditions are such that the transmission is constantly and annoyingly shifting back and forth, lock out 6th. We agree on that as well. I'm pretty sure my Ford will downshift if it's pulling hard enough to be down at 8 mpg and 1400 rpm but when the road levels out it will upshift back to 6th .... it's an automatic.



2)Transmission control is not loaded with a BSFC maps. It is multiple charts that basically says at X rpm, Y load, and Z throttle position upshift or downshift(there are a few other parameters depending on the make). The way you think it works and the way it actually works are two different things my friend. All it knows is to shift when the preset parameters are met. If it was able to be tuned like you think it is, then there would be no need for tow/haul mode or select shift.

I tell you what, why don't you contact Ram and tell them your brilliant idea and how they should be tuning their transmissions from now on. Let us know what they tell you.


That sounds pretty simple to me .... x rpm, y load and z throttle position .... they don't need any more info than that. They can just take a look at a BSFC map to see what those parameters should be to initiate the shift points and program it in. Come on, it doesn't take a rocket scientist to understand this stuff.... If you really think the Ram engineers are that clued out don't you think they could at least wire the transmission so that when you click on the tow haul mode, it automatically locks out 6th... that would at least satisfy what you're saying?


Then why are you speaking to me about it? Go tell Ram and Ford what you think they should do. Then you can go tell ZF and BMW as well because there are times my car holds on to gears longer than it should too. Heck, why stop there. You might as well go enlighten all the makes about this awesome new discovery.

The only one you probably won't have to tell is Eaton although their way doing this is to spend years doing research with the engine manufacturer and deciding that using GPS and preloaded maps in the now very expensive TCM to tell the trans when to shift up hills. Maybe if you tell them how stupid they were and they could have just used engine BSFC maps that they already have available then they will give you some type of medal or post your picture on the employee of the month wall.

ShinerBock

SATX

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Posted: 10/28/20 05:32pm Link  |  Quote  |  Print  |  Notify Moderator

And no, my father in laws truck will try to hold 6th for as long as possible with the foot closer to the floor as well. The worst is when it starts to loose momentum and then decides to shift causing him to loose even more momentum and the he has to floor it even more when it jump to 5th to gain back speed. There are certain routes, like the Texas hill country, where he takes out 6th because of this. He didn't start doing it for fuel reasons(although he is about to test that), but rather to keep it from losing momentum and switching gears. I know he did it the last time we went to Fredericksburg hauling cattle for the stock show because I was in the passenger seat.

4x4ord

Alberta

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Posted: 10/28/20 05:34pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

4x4ord wrote:

^^^^I don’t think I’m confused. I’m just asking some questions. If an engine is designed to produce 600 lb ft of torque at 1700 rpm and it is running at 1700 rpm putting out 375 lb ft of torque I think we agree the engine is not lightly loaded. If an engine is designed to produce 1000 lbft of torque and is putting out 375 lbft I would say it is lightly loaded. If an engine is designed for to put out 850 lb ft and is tuned to make 1300 lbft I‘m not sure how to determine how it is loaded? Maybe measure it’s fuel economy? If it’s burning more fuel in 6th than 5th towing 65 mph than it likely indicates it is not lightly loaded.


It is not about the load of the engine at peak, but rather the load at the rpm it is at due to its gearing. There is a difference between using 5% to 75% of an engines output at an rpm you are forced to be at due to gearing, and using a percentage of the peak hp of the engine. I am forced to be at a different rpm in 6th than a 4.10 geared truck with the same trans ratio. If we all drove CVT's then I can see your point, but we don't so you have to look at things differently.


Yes. The current model Cummins makes 343 HP at 1800 rpm ..... At what HP or torque level would you say this engine is lightly loaded while turning 1800 rpm? What about the Powerstroke ... it makes about 360 HP at 1800 rpm. I might say under 50% load but could we agree that usually under 40% load at a given rpm would constitute a light load? That percentage would be significantly lower for many aftermarket tuned engines. Bottom line is we can only guess what gear our truck should be in at any given load and rpm. The engineers who design these trucks could very easily program them to pick the right gear for fuel economy .... If they don't take fuel economy into account when they program the shifting strategy I certainly don't know why they don't but it's not because it is too complicated for them.

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