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RE: Why diesels are most efficient around 1,800 rpm

What mileage do you get in 5th vs 6th towing your 13 k trailer? :S I don't know why your truck's fuel economy is such a secret but anyway here's a map giving you all the benefit I can give you. The yellow and red dots represent 8 mpg. https://i.imgur.com/In45NBTl.png According to your data a 6.7 Cummins pulling about as hard as it possibly can in 5th puts it at 832 Nm of torque output at 65 mph and 1375 rpm. The red dot is at the exact same power output. This power output represents 8mpg at 65 mph. Notice that according to the graph both gears are getting similar fuel economy with a slight favour possibly going to 5th gear. As the hills get steeper 6th does not offer enough torque and the transmission absolutely has to shift to 5th so the green line represents shared output during heavier parts of the pull. As the load gets lighter the bottom of the yellow line corresponds to the same power output as the bottom of the red line... Notice how the 6th gear is clearly getting better fuel economy during these stages of the journey. There is absolutely no way that this BSFC map supports any claim that 5th gear will produce better fuel economy towing a 13k trailer than 6th gear. Even if you slow down and increase the load by going up a steeper incline so that 5th gear is in the dark blue sweet spot a corresponding power output @ the same mph for 6th gear does not exist on the BSFC map. IE under heavy loads the transmission absolutely must downshift for the truck to maintain speed.
4x4ord 10/28/20 09:57pm Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

^^^^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. Again, take it up with Ram and Ford. Not sure the current Cummins or Powerstroke has to do with my truck getting better fuel mileage in 5th rather than 6th when towing my 13k trailer at 65 mph. Again, take it up with Ram, Ford and the other makes if you think you can tune a transmission better than they can. What mileage do you get in 5th vs 6th towing your 13 k trailer?
4x4ord 10/28/20 06:28pm Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

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. I'm not accusing them of being clueless you are. I'll bet you could even wire your truck to lock out 6th automatically every time you select tow haul. Do you think a Ram engineer can't even come up with that?
4x4ord 10/28/20 05:42pm Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

^^^^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.
4x4ord 10/28/20 05:34pm Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

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?
4x4ord 10/28/20 05:01pm Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

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.
4x4ord 10/28/20 10:03am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

Here is a prefect article showing power ratings and fuel consumption rates of various diesel engines with different characteristics. . Notice what happens to the fuel lb/hp-hr after 1,800 rpm for most of them. It starts to drop off significantly and most cases the lower rpm where the engines first reach peak torque is not the most efficient in regards to hp-hr. It also points out why engine manufacturers make different power ratings for the same engine and the different of short intermitted power output versus sustained(continues) power output. Having more power than needed will reduce fuel economy and reduce engine life (depending on how far you go past what the engine components can support) while too low of power for the application will reduce life due to being at higher loads more often. The higher power units will not require as high rpm to do the same work, but will see lower engine life if that added power is always utilized at higher rpms or too low of rpms. Making Sense of Diesel Engine Specs I didn’t read the article yet. I’ll try to later. I’m sure we both already agree with what you are saying regarding more hphrs per gallon at higher rpm under heavy loads. We are talking about light loads here.
4x4ord 10/28/20 09:25am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

The detail that would add some clarity to your BSFC map would be to mark the average Nm output of your engine. Then focus our attention on the part of the map a little above and a little below. If the load becomes moderate to high in 5th we already agree that a downshift is in order. Our difference is that I believe the engineers who design the trucks know when this downshift should take place better than we do and design the trucks to make the downshift at the appropriate time. You seem to think the rule is that if tow haul is needed 6th should be locked out. If conditions are such that the transmission is annoyingly shifting back and forth I would say that is one time I’d lock out 6th.
4x4ord 10/28/20 09:08am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

^^^^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.
4x4ord 10/28/20 08:53am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

I think there are various principles at work. One principle is the idea that an engine is most efficient at the rpm where it makes peak torque. Another principle is that throttling down and gearing up under light load conditions conserves fuel. Shiner is pointing out that another principle is that small diesel engines run most efficient around 1800 rpm under medium to heavy loads. I have trouble believing that it is complicated for engineers to write the shifting software for our trucks’ transmissions to ensure that the engines run at the most fuel efficient rpm under light load conditions. (our engines shouldn’t be lugging in 6th at 1100 rpm when they are coupled to an automatic transmission) Maybe the questions that need to be answered are: what is a light load? Is it different for a 300 hp 6.7 litre engine vs a 475 hp 6.7 engine vs a 525 hp custom tuned 6.7 with a stock turbo?
4x4ord 10/28/20 04:32am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

This is the torque curve for the PACCAR MX 13 that’s Peterbilt uses in their medium duty truck Looks like PACCAR is unaware of the 1800 rpm fuel saving rule. I think you are a little confused as to what that 1,800 rpm rule is. It is the rpm at which diesel burns most efficiently and completely and there for will generally have the highest thermal efficiency for the amount of power it is creating around that rpm. Basically, it is the point at which you get the most out of a drop of diesel fuel. This is not to be mistaken for the spot at which an engine is using the least amount fuel or the different rpm ranges to make power for different applications, industry needs, or rpm limitations due to engine size. I might be a little confused. This quote from your original post in this thread: Knowing all this, manufacturers of diesel engines, turbochargers, fuel injection systems and engine control systems have all directed their efforts at producing engines that make the most of this 1,800-rpm phenomenon. So, drive your diesel at or near to 1,800 rpm if you’re looking for the very best in both engine torque and fuel economy." has me understanding that engine manufacturers would be designing their diesel engines to be used at 1800 rpm to make best use of the “rule”. The PACCAR 13 litre totally misses out as it won’t even rev to 1800. Would you expect the PACCAR 13 litre to be more fuel efficient at 1700 than 1100 rpm? Re-read the sentence above especially the part in red. In order to create more horsepower within reliability and emissions specs, you have to go big. The bigger you go, the more you are limited on rpm. I believe you are your own worst enemy here. That’s not helping. We have 2017 emissions compliant 12.5 litre Cat engines that work all day long at a constant speed of 2100 rpm. Our application requires high power being transmitted with belts and pulleys so higher rpm is desirable. PACCAR has designed their 13 litre to be used in medium duty truck applications and I would have thought fuel economy would be a high priority.
4x4ord 10/27/20 01:53pm Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

This is the torque curve for the PACCAR MX 13 that’s Peterbilt uses in their medium duty truck Looks like PACCAR is unaware of the 1800 rpm fuel saving rule. I think you are a little confused as to what that 1,800 rpm rule is. It is the rpm at which diesel burns most efficiently and completely and there for will generally have the highest thermal efficiency for the amount of power it is creating around that rpm. Basically, it is the point at which you get the most out of a drop of diesel fuel. This is not to be mistaken for the spot at which an engine is using the least amount fuel or the different rpm ranges to make power for different applications, industry needs, or rpm limitations due to engine size. I might be a little confused. This quote from your original post in this thread: Knowing all this, manufacturers of diesel engines, turbochargers, fuel injection systems and engine control systems have all directed their efforts at producing engines that make the most of this 1,800-rpm phenomenon. So, drive your diesel at or near to 1,800 rpm if you’re looking for the very best in both engine torque and fuel economy." has me understanding that engine manufacturers would be designing their diesel engines to be used at 1800 rpm to make best use of the “rule”. The PACCAR 13 litre totally misses out as it won’t even rev to 1800. Would you expect the PACCAR 13 litre to be more fuel efficient at 1700 than 1100 rpm?
4x4ord 10/27/20 11:24am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

This is the torque curve for the PACCAR MX 13 that’s Peterbilt uses in their medium duty truck https://i.imgur.com/lXtfY6Yl.jpg Looks like PACCAR is unaware of the 1800 rpm fuel saving rule.
4x4ord 10/27/20 06:12am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

Otrfun is correct. The lower the rear axle ratio, the more stress it puts on the driveline component between the engine and rear axle. This is regardless of trans gear. Downspeeding has become a big deal in the medium/heavy duty industry in the past ten years due to the new fuel economy regulations for these class of trucks. What people thought they would save in fuel economy, they ended up spending on driveline components because the low rpm and gearing put a lot of torque stress on the driveline. They finally started making drivelines that can handle it, and many handled it great within their respective ratings. However, the drivelines of the taller rear gear still could not handle as much weight as the shorter geared trucks due to this driveline stress. I have no qualms with using my 3.42 axle to tow 15k, but I would started looking at getting a truck with shorter gears if I towed more than that especially if it was on a regular basis. Ive got very close to one million miles on my ‘07 Pete with a 15 litre Cat engine. It has fairly tall rear gears. I broke all 4 axles within about 20000 miles of each other which I thought might be more than coincidental. Anyway that is the only driveline components that have been touched on the truck. It is usually pulling foot to the floor about 12 to 1600 rpm. I think the newer heavy duties might be set up to run around 1000 to 1400 rpm under load. When you say the medium duty industry is running lower rpms to conserve fuel what engines and rpm are they running?
4x4ord 10/26/20 10:41pm Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

Shiner have you checked out the TPS % between 5th and 6th? If not please do so and report back. I bet for what you're doing 5th will be lower. Yes, the TPS % is lower in 5th and so is engine load % according to my Edge CTS2 which reads from my ECU. Boom. That's all anybody needs to know. TPS % lower= less fuel. TPS higher= more fuel. IMHO I also think your correct about people applying gasoline engine knowledge to diesel engines. You could just say MPG lower more fuel; MPG higher less fuel.
4x4ord 10/26/20 02:46pm Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

Here's an interesting Cummins video for the bigger engines. click
4x4ord 10/26/20 11:49am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

I think we are closer in our opinions than we are coming across in this thread. My 2011-2017 Powerstrokes have all had 3.55 gears and always held 6th gear on level ground while towing in the 9-10 mpg range. Under these conditions I sometimes find the engine might even rev higher than I like when the tow/haul is selected and so I might not even bother selecting tow/haul. When towing in long hills I will often manually lock out gears on the downhills to utilize the exhaust brake and occasionally I might drop a gear anticipating a climb. When towing heavier (I sometimes tow close to 40,000 lbs combined) my automatic starts using 3rd, 4th, 5th and 6th and I might lock out 6th to limit the back and forth shifting but it is rare that I feel the need to screw with it.... if I'm climbing I'm in 3rd or 4th and going down a bit of a grade I might be in 6th.
4x4ord 10/26/20 11:40am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

^^^ Shiner, if on average you are towing up a 1% grade with wind and at 65 mph you would be getting more like 6.5 or 7 mpg. At that kind of power output I would expect the transmission of a factory tuned truck to select 5th gear.
4x4ord 10/26/20 10:01am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

You will find a blank map in this thread. Make a snip of it and apply the plots in paint yourself. I already have the date of multiple runs towing my trailer in 5th and 6th to show that I get better fuel economy in 5th. So does my brother doing the same windy and hilly 160 mile commute to the coast that I tow mine in. And soon my father in law might, but he has a different engine and the terrain he will be towing is much flatter with less wind. Your calculation are based on only towing on flat terrain which is not the case. It is also very windy half way through the 160 mile drive. So windy in fact that you pass through a giant wind farm for about 30-50 miles. And I already posted where I will be at with different grades and speeds earlier. My grade is more likely to change over my speed which is generally 65 mph or 70 mph on rare occasions. My calculations are entirely based on the data that you posted based on an average fuel economy of 10 mpg. If your fuel economy is significantly different than that then my calculations are wrong ..... that is why I have been asking you over and over for your actual fuel economy.
4x4ord 10/26/20 09:27am Tow Vehicles
RE: Why diesels are most efficient around 1,800 rpm

I’ll assume you get 10 mpg towing at 62 mph. So your truck Is burning 6.2 gallons per hour. ... 19683 grams per hour. According to the BSFC map you posted your engine will be running at about 205 grams per kwhr and producing 96 kW of power. This corresponds to 535 Nm and 1715 rpm ... can you plot that on your map. If it takes 96 kW to tow your trailer at 62 mph it will take about 129 kW to tow it at 70 mph.... 636 Nm at 1936 rpm. Can you plot that. Then plot 510 Nm at 1660. If you connect the dots that line will represent your average engine output and fuel consumption running in 5th gear from 60 to 70 mph. For 6th gear plot 1270 rpm and 667 Nm; 1312 rpm and 699Nm; and finally 1482 rpm and 831 Nm. Join the dots and this line will represent your average engine output and fuel consumption towing in 6th gear from 60 to 70 mph.
4x4ord 10/26/20 08:37am Tow Vehicles
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