rpm when towing

[bbaker2001]

When towing with my ram diesel how do I tell when I should downshift.
it seems to tow so easy, but slows when going uphill
at 60 what should my rpm show.

[ShinerBock]

4x4ord wrote:


Torque comes from the amount of energy in the way of fuel that can be burned in an engine per revolution. Larger displacement yields more torque. Higher boost pressure ... more torque. Diesel needs more air to properly burn than does gasoline so more energy can be burned into a set volume of air if gasoline is used as the fuel as opposed to diesel.... therefore gasoline has a higher potential for making torque than does diesel. You are right that the use of an engine has a lot to do with its design. It won't surprise me if the newer gasoline DI/turbo engine technology and the fact that gasoline requires less restrictive measure to be used to meet emission requirements causes engine builders to turn toward designing gasoline engines to be favoured over the diesels for towing.

What each fuel does outside of the engine is one thing. How each engine uses that fuel is another. Gasoline engines are regulated by air and add a certain amount fuel to stay around a stoichiometric air/fuel ratio (14.7:1) while diesel engines are regulated by fuel and uses all the air available to burn lean most of the time. The more air you add to a diesel engine, the more efficient it becomes. The more air you add to a gasoline engine, the more fuel it consumes to stay around stoich. This is one of many reasons why diesel are way more efficient than gas engines under load and are used for towing applications.

Yes, with the same compression ratio gasoline engines would be more efficient, but I don't live in unicorn would-a could-a should-a world so I go by what the real world has. Diesel fuel itself allows for much higher compression ratios making the diesel compression ignition more efficient at utilizing more out of a drop of fuel than spark ignition.

I disagree on the emissions thing. As technology increases, I see less emissions devices on future diesels. Cummins has already deleted the EGR on their current ISL engines and only utilizing SCR making the engines more efficient and better performance. they already have plans to do the same for other engines on their line up including the ISB 6.7L.

[4x4ord]

ShinerBock wrote:

Some of those aren't facts.

Diesel torque stems from its more explosive combustion which pushes the piston down with more force the fact that diesels generally have longer strokes than gasoline engines which creates more low end torque, but also limits max engine speed. Turbos only make the output greater just like they do with gasoline engines.

Comparing the the torque output of a Honda 1.5L to Cummins pickup engine is just plain silly. They are built and rated for two different things. The Cummins is de-rated significantly to keep exhaust and other temps in check for towing while the Honda is not. If you put that Honda engine in a tow vehicle application then I can guarantee that the torque rating will go down. The Cummins also has more restrictive emissions equipment limiting that power a long with turbos and cams made for two different applications.

If you want to make both engines that you used an example for a more even playing field, then you would use the QSB 6.7L which is not de-rated for towing and has about the same emissions equipment as the Honda. The QSB 6.7L has a max output of 542 hp @ 2,900-3,300 rpm and 1,250 lb-ft @ 2,000 rpm. It is down to 890 lb-ft by the time it hits max hp at 2,900 rpm. That would put it at 187 lb-ft per liter at peak torque and 132 lb-ft per liter when it hits max hp at 2,900 rpm.

Torque comes from the amount of energy in the way of fuel that can be burned in an engine per revolution. Larger displacement yields more torque. Higher boost pressure ... more torque. Diesel needs more air to properly burn than does gasoline so more energy can be burned into a set volume of air if gasoline is used as the fuel as opposed to diesel.... therefore gasoline has a higher potential for making torque than does diesel. You are right that the use of an engine has a lot to do with its design. It won't surprise me if the newer gasoline DI/turbo engine technology and the fact that gasoline requires less restrictive measure to be used to meet emission requirements causes engine builders to turn toward designing gasoline engines to be favoured over the diesels for towing.

[ShinerBock]

If you want to compare a diesel and gas engines with similar displacement, bore/stroke, duty cycle, and technology then you can compare the Audi 3.0L turbocharged diesel and gas engine.

Audi 3.0L Diesel
240 hp
428 lb-ft


Audi 3.0L Gas DI
354 hp
369 lb-ft

[4x4ord]

wilber1 wrote:
All these numbers are at WOT so it is just as important to have a gear which allows you to maintain WOT without either over reving or losing speed.

My Cummins 650 has a flat torque curve, it peaks at 1500 RPM and stays flat until 2900 RPM. At 1500 RPM at WOT it can make only 186 HP. At 2900 RPM it can make 350 HP.

Another graphic example of what a turbo DI gas engine can do is the new Audi RS3 engine. It is a performance engine but it also comes with a four year warranty.

2.5L 5 cylinder
HP 400 hp at 5850 RPM 160 hp per litre
Torque 354 lb ft from 1700 to 5850 RPM

That is 141 lb ft of torque per litre at 1700 RPM compared to the Cummins HO's 134 lb ft per litre at 1700 RPM.

Those are impressive numbers. There might be a day in the near future that the most desirable tow vehicles will be powered by gasoline.

[ShinerBock]

4x4ord wrote:


Here is some facts for you:

Gasoline has more energy per pound than does diesel.

Diesel engines need more air to burn their fuel than do gasoline engines. So when comparing a gasoline engine to a diesel of the same displacement the gasoline engine will make more torque and because it can be designed to rev much higher it will make much more Hp than the diesel.

The designer of the diesel engine (Rudolph Diesel) predicted that due to the slow burning of the fuel, the diesel engine would never be able to maintain operating speeds above 700 rpm. High pressure fuel injection has changed this.

Unlike gasoline engines, Diesel engines have been well suited for turbocharging for decades. The high torque of the diesel stems from turbo charging.

The reletvely recent development in direct injection gasoline engines is allowing gasoline engines to be effectively turbocharged. For instance the Honda 1.5 liter turbocharged gasoline engines make 108 lbft of torque per liter from 1700 rpm all the way to 5500 rpm. The HO Cummins makes 134 lbft of torque per liter at 1700 rpm but by 2800 rpm it is down to the same 108 lbft that the Honda makes. Peak HP of the Honda is 116 HP per liter. The HO Cummins is capable of only 54 HP per liter.

Although Diesel fuel has less energy than gasoline per pound it has more energy per gallon than does gasoline. This and the high compression ratio of the diesel engine contribute to its better fuel economy.

A high HP gasoline engine will beat a lower horsepower diesel up the hill everytime if the gasoline engine has the necessary gears to keep it in its peak hp rpm range.

Some of those aren't facts.

Diesel torque stems from its more explosive combustion which pushes the piston down with more force and the fact that diesels generally have longer strokes than gasoline engines which creates more low end torque, but also limits max engine speed. Turbos only make the output greater just like they do with gasoline engines.

You also can't use final displacement as a power comparison. You can have a long stroke 6.0L engine and it will have completely different characteristics than that of a short stroke 6.0L engine. Cams and what duty cycle the engine is tuned for will also make a big difference of how much output and engine has and when.

Comparing the the torque output of a Honda 1.5L to Cummins pickup engine is just plain silly. They are built and rated for two different things. The Cummins is de-rated significantly to keep exhaust and other temps in check for towing while the Honda is not. If you put that Honda engine in a tow vehicle application then I can guarantee that the torque rating will go down. The Cummins also has more restrictive emissions equipment limiting that power a long with turbos and cams made for a towing duty cycle.

If you want to make both engines that you used an example for a more even playing field, then you would use the QSB 6.7L which is not de-rated for towing and has about the same emissions equipment as the Honda. The QSB 6.7L has a max output of 542 hp @ 2,900-3,300 rpm and 1,250 lb-ft @ 2,000 rpm. It is down to 890 lb-ft by the time it hits max hp at 2,900 rpm. That would put it at 187 lb-ft per liter at peak torque and 132 lb-ft per liter when it hits max hp at 2,900 rpm.


Cummins QSB 6.7L Spec Sheet

[Frostbitte]

bbaker2001 wrote:
When towing with my ram diesel how do I tell when I should downshift.
it seems to tow so easy, but slows when going uphill
at 60 what should my rpm show.

Depends...I know when mine gets bogged down so I usually just put it in T/H mode and let it do its thing when towing. On some bigger highway hills, I'll let off the gas and stomp it to force a down shift to keep speed. On some really big hills, I'll use the shift button to downshift as necessary. I like to keep the RPM's up on the hills to prevent lugging and get the engine to produce power. Like most said, 2200 to 2500 roughly on the hills. When cruising the flats or light hills I'm usually at 1750 RPM at 100 KM/h.
I have 4.10's for rear gears and the 68rfe transmission.

[wilber1]

All these numbers are at WOT so it is just as important to have a gear which allows you to maintain WOT without either over reving or losing speed.

My Cummins 650 has a flat torque curve, it peaks at 1500 RPM and stays flat until 2900 RPM. At 1500 RPM at WOT it can make only 186 HP. At 2900 RPM it can make 350 HP.

Another graphic example of what a turbo DI gas engine can do is the new Audi RS3 engine. It is a performance engine but it also comes with a four year warranty.

2.5L 5 cylinder
HP 400 hp at 5850 RPM 160 hp per litre
Torque 354 lb ft from 1700 to 5850 RPM

That is 141 lb ft of torque per litre at 1700 RPM compared to the Cummins HO's 134 lb ft per litre at 1700 RPM.

[4x4ord]

rhagfo wrote:

4x4ord wrote:

Wild Card wrote:


Thats cause the Cummins is most at home at 2100rpm. Peak torque. Everyone gets caught up in the HP craze. Horsepower doesnt do crapola for pullimg grades. Gear it for peak torque and let it pull.

Comments like this are what confuse people. Here is the math:

If you have a 2015 Cummins/Aisin it makes 865 lb ft of peak torque at 1600 rpm. If it is run through gears to slow the rear axle down to 507 rpm (5th gear with a 4.10 rear end) the torque on the rear axle will be 1600/507 x 865 or 2768 lb ft of torque.

Now rev that engine up to 2800 where it makes 385 HP and the torque drops down to 722 lb ft. Drop down to 3rd gear. Your rear axle will be turning 509 rpm. The powertrain will be able to deliver 2800/509 x 722 or 3972 lb ft of torque to the rear axle.

So in 5th it can put 2768 lb ft to the rear axle and in 3rd 3972 lb ft to the rear axle. HP is king ...... if you have HP, torque can be generated with gears.


These numbers assume no power loss through the gears and so are to demonstrate the physics rather than be absolute values.

Yep, that is why high HP gas engines, can't tow what a diesel. More energy per gallon. I will take a Low HP/High torque over a High HP/low torque gasser any day. Diesels don't need to spin fast to move a load.

Here is some facts for you:

Gasoline has more energy per pound than does diesel.

Diesel engines need more air to burn their fuel than do gasoline engines. So when comparing a gasoline engine to a diesel of the same displacement the gasoline engine will make more torque and because it can be designed to rev much higher it will make much more Hp than the diesel.

The designer of the diesel engine (Rudolph Diesel) predicted that due to the slow burning of the fuel, the diesel engine would never be able to maintain operating speeds above 700 rpm. High pressure fuel injection has changed this.

Unlike gasoline engines, Diesel engines have been well suited for turbocharging for decades. The high torque of the diesel stems from turbo charging.

The reletvely recent development in direct injection gasoline engines is allowing gasoline engines to be effectively turbocharged. For instance the Honda 1.5 liter turbocharged gasoline engines make 108 lbft of torque per liter from 1700 rpm all the way to 5500 rpm. The HO Cummins makes 134 lbft of torque per liter at 1700 rpm but by 2800 rpm it is down to the same 108 lbft that the Honda makes. Peak HP of the Honda is 116 HP per liter. The HO Cummins is capable of only 54 HP per liter.

Although Diesel fuel has less energy than gasoline per pound it has more energy per gallon than does gasoline. This and the high compression ratio of the diesel engine contribute to its better fuel economy.

A high HP gasoline engine will beat a lower horsepower diesel up the hill everytime if the gasoline engine has the necessary gears to keep it in its peak hp rpm range.

[wilber1]

rhagfo wrote:

4x4ord wrote:

Wild Card wrote:


Thats cause the Cummins is most at home at 2100rpm. Peak torque. Everyone gets caught up in the HP craze. Horsepower doesnt do crapola for pullimg grades. Gear it for peak torque and let it pull.

Comments like this are what confuse people. Here is the math:

If you have a 2015 Cummins/Aisin it makes 865 lb ft of peak torque at 1600 rpm. If it is run through gears to slow the rear axle down to 507 rpm (5th gear with a 4.10 rear end) the torque on the rear axle will be 1600/507 x 865 or 2768 lb ft of torque.

Now rev that engine up to 2800 where it makes 385 HP and the torque drops down to 722 lb ft. Drop down to 3rd gear. Your rear axle will be turning 509 rpm. The powertrain will be able to deliver 2800/509 x 722 or 3972 lb ft of torque to the rear axle.

So in 5th it can put 2768 lb ft to the rear axle and in 3rd 3972 lb ft to the rear axle. HP is king ...... if you have HP, torque can be generated with gears.


These numbers assume no power loss through the gears and so are to demonstrate the physics rather than be absolute values.

Yep, that is why high HP gas engines, can't tow what a diesel. More energy per gallon. I will take a Low HP/High torque over a High HP/low torque gasser any day. Diesels don't need to spin fast to move a load.

Maybe not but it moves it a lot slower. Boosted gas engines put out peak torque at some pretty low RPM's as well. I have one that hits peak torque at just over 1500 RPM.

[wilber1]

Wild Card wrote:

rhagfo wrote:

4x4ord wrote:

VoodooMedicineMan wrote:
Next time I am going to try that. 3300 seems awful high.

There is nothing really wrong with what your doing it just means that you will go slower up the hill. Anyway the way to maximize your speed up a long hill is to start by anticipating the hill. By that I mean downshift before the truck begins to slow down.

So if you are going 60 mph in high gear with 3.73 gears your engine will be turning about 1713 rpm. Drop two gears and you should be travelling 60 mph and revving 3117 rpm. If the truck is pulling a long steep hill it can slow all the way down to 42 mph before you can shift again. At 42 mph your engine will be revving 2183 before the shift and 3500 after the shift.

Well don't know the listed red line on the Cornbinder 6.0. but 3,300 rpm would be 100 rpm over my CTD 5.9's red line.
I don't like taking it much above 2,500 as that seems to be the point power flattens out.

Thats cause the Cummins is most at home at 2100rpm. Peak torque. Everyone gets caught up in the HP craze. Horsepower doesnt do crapola for pullimg grades. Gear it for peak torque and let it pull.

Wrong

[STANG23L]

I'm sure its been stated before. But TOW HAUL mode is your friend.

Computers and the people that tune these trucks are pretty smart and have thousands of hours in testing. Let it do its job and just put your foot in it!

[rhagfo]

4x4ord wrote:

Wild Card wrote:


Thats cause the Cummins is most at home at 2100rpm. Peak torque. Everyone gets caught up in the HP craze. Horsepower doesnt do crapola for pullimg grades. Gear it for peak torque and let it pull.

Comments like this are what confuse people. Here is the math:

If you have a 2015 Cummins/Aisin it makes 865 lb ft of peak torque at 1600 rpm. If it is run through gears to slow the rear axle down to 507 rpm (5th gear with a 4.10 rear end) the torque on the rear axle will be 1600/507 x 865 or 2768 lb ft of torque.

Now rev that engine up to 2800 where it makes 385 HP and the torque drops down to 722 lb ft. Drop down to 3rd gear. Your rear axle will be turning 509 rpm. The powertrain will be able to deliver 2800/509 x 722 or 3972 lb ft of torque to the rear axle.

So in 5th it can put 2768 lb ft to the rear axle and in 3rd 3972 lb ft to the rear axle. HP is king ...... if you have HP, torque can be generated with gears.


These numbers assume no power loss through the gears and so are to demonstrate the physics rather than be absolute values.

Yep, that is why high HP gas engines, can't tow what a diesel. More energy per gallon. I will take a Low HP/High torque over a High HP/low torque gasser any day. Diesels don't need to spin fast to move a load.

[4x4ord]

Wild Card wrote:


Thats cause the Cummins is most at home at 2100rpm. Peak torque. Everyone gets caught up in the HP craze. Horsepower doesnt do crapola for pullimg grades. Gear it for peak torque and let it pull.

Comments like this are what confuse people. Here is the math:

If you have a 2015 Cummins/Aisin it makes 865 lb ft of peak torque at 1600 rpm. If it is run through gears to slow the rear axle down to 507 rpm (5th gear with a 4.10 rear end) the torque on the rear axle will be 1600/507 x 865 or 2768 lb ft of torque.

Now rev that engine up to 2800 where it makes 385 HP and the torque drops down to 722 lb ft. Drop down to 3rd gear. Your rear axle will be turning 509 rpm. The powertrain will be able to deliver 2800/509 x 722 or 3972 lb ft of torque to the rear axle.

So in 5th it can put 2768 lb ft to the rear axle and in 3rd 3972 lb ft to the rear axle. HP is king ...... if you have HP, torque can be generated with gears.


These numbers assume no power loss through the gears and so are to demonstrate the physics rather than be absolute values.

[RCMAN46]

OK I will take a Cummins that is geared so that I am at 2800 rpm full throttle going up a 12 percent grade with 20,000 in tow.

You take the same exact Cummins that is geared to run at 1700 rpm full throttle up the same grade and same load.

I will be at the top having lunch waiting for you.

In case you are not aware I will be putting about 100 more horsepower to the ground than you.

[djgodden]

We live in Southern California and in the last years have gone to Oklahoma City twice, southern Colorado once, and Yellowstone once. Our 2012 Ram 2500 w/6.7 CTD and a 68 tranny does magnificently towing our 32ft Montana (not the lightest of beasts). Peak HP and torque is as most folks are saying 1800-2000 RPM. I have an Edge Evolution tuner and have a economy/towing power setting loaded. We drive along like there's nothing behind us at all. The most shifting I do is manually dropping to 5th on some climbs just so the tranny doesn't shift back and forth, as they're prone to do at times. I have noticed my pre-turbine exhaust temp edging up to 1000F deg when flat and cruising at 65mph in 6th, manually dropping to 5th causes the temp to drop into the normal 700-800 range.