rpm when towing

A point here is that DI and turbocharging has made the old adage that the gasser needs to rev like crazy to produce power no longer holds true. The 3.0 Audi gas example that was given has an extremely flat torque curve, one any diesel would be proud of. It produces that 364 lb ft from 1370 RPM all the way to 4500 RPM.

BTW that 3.0 diesel in European S trim is now putting out 340 HP and 516 lb ft but we know it won't meet NA emission standards. Wish it did cause it must be a hoot.

Some good points explained clearly here. Modern diesels have come so far that alot of preconceptions and myths are still out there about them. So many people make the blanket statement, "diesels cost so much more to maintain..." When this isn't always the case. It boggles my mind why people still tow 7k+++lb trailers with gas trucks. I love gasoline engines. I have raced several of them. My wife's last two daily driven cars have had over 350whp and run high 11s in the quarter. But that being said, I would never choose to tow with a gasoline vehicle. Once you buy a reliable(Cummins) modern diesel and live with it for several years you learn to appreciate them.

ShinerBock wrote:



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.

The DI gasoline engines are no longer regulated to a 14.7 air/fuel ratio nor is the compression ratio limited. Preignition can't happen without fuel being injected.

Hopefully emission equipment for the diesels will become simpler and less taxing on fuel economy but until they can get rid of the DPF and the DEF there is an efficiency tax that the diesel has to pay.

ShinerBock wrote:
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

That is a good comparison.

I believe the potential is there for making higher torque with a gasoline engine but it becomes kind of futile. For instance say Ford decides they want to make a gasoline engine to put in a F350. They will consider how fast it needs to be able to pull a 20,000 lb trailer up the Ike and determine the truck needs to make say 450 HP. The operating rpm range of a gasoline engine easily allows for that HP to be made at 5000 rpm. Therefore the size of the engine and level of boost is determined to be much smaller than a turbo charged diesel designed for the same use. If they were to decide to make a 6.7 liter turbocharged gasoline engine with a peak torque of 900 lbft of torque at 1700 rpm they would have a 900 HP monster that would require a radiator larger than the one on our Kenworth to keep things cool. So in other words because the nature of gasoline engines so easily allows for them to make HP at a higher rpm it is unlikely that the manufacturers will ever build the gasoline engines to produce the same gobs of torque they design into the diesel.

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.

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.

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

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.

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

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.