Duramax vs Power Stroke Fuel Economy Tow Test and Drag Race

[FishOnOne]

Enjoy...

Fuel Economy Test:
Power Stroke: 7.8mpg
Duramax: 6.9mpg

Link

[4x4ord]

ShinerBock wrote:

4x4ord wrote:


It would be interesting to see the brake specific fuel consumption graphs for these engines. You'd expect the transmissions to shift according to fuel usage.At 1800 rpm these engines can produce over double the power that is needed to tow a typical 5ver down the highway. I'm surprised lower rpm wouldn't save a little fuel. A general rule is that higher cylinder pressure produces greater efficiency. If you're running higher rpm than is needed the engine is operating at a lower cylinder pressure than it could be.

On flat ground under low load the fuel economy is better in 6th, but on hilly roads, a lot of wind resistance, or in other situations that put more load on the engine it is is not in my experiences. In 6th and under higher loads, it feels like the engine has to add more fuel/air to compensate for the lack of torque from a taller gear ratio and the lack of horsepower from being at a lower rpm. In 5th, it feels like it does not have to add that much fuel(fuel pressure is lower) and boost to do the same work. There is less load on the engine in 5th even though it is at a higher rpm. There is also the factor that EGT's are lower by about 200F as well.

Under part throttle at a constant speed the power at the rear wheels is the same regardless of the engine rpm so the difference in the amount of fuel burned per time will be close. (it will vary by the difference in efficiency which usually favours the lower rpm). However, the fuel burned per power stroke will be higher with the engine running slower, so higher exhaust temperatures makes perfect sense. (roughly the same amount of fuel but more air at higher rpm) When the truck is working very hard it makes sense that it would get its best fuel economy at 1800 rpm but, if it's working hard at 1800 rpm it would be getting absolutely terrible fuel economy.


Edit: I hope I'm not coming across as being argumentative, I'm just thinking about what you're saying and trying to understand the physics of it.

[Cummins12V98]

ShinerBock wrote:

Cummins12V98 wrote:
"I would bet that you would not want to start off in second gear."

Did you notice I said "start in second while unloaded" ?

Yes I noticed that and as I stated you would probably not care about that in the 68RFE because it has a taller first gear. Loaded, the Aisin is better at take off.

True. Misread what you said.

[ShinerBock]

4x4ord wrote:


It would be interesting to see the brake specific fuel consumption graphs for these engines. You'd expect the transmissions to shift according to fuel usage.At 1800 rpm these engines can produce over double the power that is needed to tow a typical 5ver down the highway. I'm surprised lower rpm wouldn't save a little fuel. A general rule is that higher cylinder pressure produces greater efficiency. If you're running higher rpm than is needed the engine is operating at a lower cylinder pressure than it could be.

On flat ground under low load the fuel economy is better in 6th, but on hilly roads, a lot of wind resistance, or in other situations that put more load on the engine it is is not in my experiences. In 6th and under higher loads, it feels like the engine has to add more fuel/air to compensate for the lack of torque from a taller gear ratio and the lack of horsepower from being at a lower rpm. In 5th, it feels like it does not have to add that much fuel(fuel pressure is lower) and boost to do the same work. There is less load on the engine in 5th even though it is at a higher rpm. There is also the factor that EGT's are lower by about 200F as well.

[4x4ord]

ShinerBock wrote:

4x4ord wrote:
So I guess by locking out 6th and running 1800 rpm you gain a little responsiveness and have less gear hunting but it's going to cost something on fuel.

Not exactly. I have tested it out multiple times towing my RV 160 miles down to the coast and other trailer like my cattle trailer. I did it 6th a few times and then in 5th. I averaged about 1-2 mpg better towing in 5th than I did 6th. I noticed on my gauges that the engine did not have to add as much fuel/air to make torque/power in 5th like it had to in 6th when going up hills.

We also noticed the same when I worked for Cummins. The "sweet spot" for the engines best efficiency was not static and changed depending on load. Unloaded it was between 1,300 and 1,500 rpm and loaded it was between 1,800 and 2,100 rpm depending on load. This is why Cummins' PowerSpec application recommends these rpms when loaded for best efficiency out of the Cummins.

Cummins PowerSpec

It would be interesting to see the brake specific fuel consumption graphs for these engines. You'd expect the transmissions to shift according to fuel usage.At 1800 rpm these engines can produce over double the power that is needed to tow a typical 5ver down the highway. I'm surprised lower rpm wouldn't save a little fuel. A general rule is that higher cylinder pressure produces greater efficiency. If you're running higher rpm than is needed the engine is operating at a lower cylinder pressure than it could be.

[ShinerBock]

Cummins12V98 wrote:
"I should have reworded that bnecause you can't actually shift manually in a Ram"

Am I missing something? I can shift mine with the toggle on the shift column.

No, you can actually shift manually in the Ford. I could start off in 2nd or 3rd if I wanted to. I could also force it to upshift as well by selecting the next gear. You can't do this in a Ram. You can only lock out gears meaning that you have to start out if 1st gear no matter what and you cannot force it to go to the next gear. Ford has a manual mode(M on the gear selector) and gear lockout while Ram just has gear lockout.

[ShinerBock]

Cummins12V98 wrote:
"I would bet that you would not want to start off in second gear."

Did you notice I said "start in second while unloaded" ?

Yes I noticed that and as I stated you would probably not care about that in the 68RFE because it has a taller first gear. Loaded, the Aisin is better at take off.

[Cummins12V98]

"I should have reworded that bnecause you can't actually shift manually in a Ram"

Am I missing something? I can shift mine with the toggle on the shift column.

[Cummins12V98]

"I no longer fill with at a metered pump."

HMMMMMMMM

[Cummins12V98]

"I would bet that you would not want to start off in second gear."

Did you notice I said "start in second while unloaded" ?

[ShinerBock]

4x4ord wrote:
So I guess by locking out 6th and running 1800 rpm you gain a little responsiveness and have less gear hunting but it's going to cost something on fuel.

Not exactly. I have tested it out multiple times towing my RV 160 miles down to the coast and other trailer like my cattle trailer. I did it 6th a few times and then in 5th. I averaged about 1-2 mpg better towing in 5th than I did 6th. I noticed on my gauges that the engine did not have to add as much fuel/air to make torque/power in 5th like it had to in 6th when going up hills.

We also noticed the same when I worked for Cummins. The "sweet spot" for the engines best efficiency was not static and changed depending on load. Unloaded it was between 1,300 and 1,500 rpm and loaded it was between 1,800 and 2,100 rpm depending on load. This is why Cummins' PowerSpec application recommends these rpms when loaded for best efficiency out of the Cummins.

Cummins PowerSpec

[4x4ord]

So I guess by locking out 6th and running 1800 rpm you gain a little responsiveness and have less gear hunting but it's going to cost something on fuel.

[ShinerBock]

4x4ord wrote:

ShinerBock wrote:

Cummins12V98 wrote:
"I think the 3.42 rear end on the GM might have had an effect on mpg here especially if they let the truck do all the shifting."

But But, I thought the magical 10 speed was supposed to keep the engine in the perfect rpm???

This is not due to it being a 10-speed but rather the trans tuning. Shifting manually or having a shorter final gear would alleviate this. To be fair, I had to do the same in my 68RFE when it had stock trans tuning and so do my friends and family who have Aisin's so Ram's stock trans tuning is not perfect either.

When do you manually shift a transmission with stock tuning? What are you trying to accomplish by manually shifting?

I should have reworded that bnecause you can't actually shift manually in a Ram like you can in a Ford which is one thing I missed when I went from my F150 to my 2500 because there certain situations where it was helpful like starting the truck in second gear in slippery conditions. You only have the ability to lock out gears in the Ram and in this case I would lock out 6th to keep the truck from going into second over drive.

Both 5th and 6th are overdrive gears on the 68RFE as it is with most 6 speed transmissions. With a 3.42 rear end and 34 inch tires, 6th gear puts me in a great rpm unloaded(1,500 @ 75 mph) and 5th puts me in in the prefect rpm loaded(1,800 @ 65 mph).

[4x4ord]

ShinerBock wrote:

Cummins12V98 wrote:
"I think the 3.42 rear end on the GM might have had an effect on mpg here especially if they let the truck do all the shifting."

But But, I thought the magical 10 speed was supposed to keep the engine in the perfect rpm???

This is not due to it being a 10-speed but rather the trans tuning. Shifting manually or having a shorter final gear would alleviate this. To be fair, I had to do the same in my 68RFE when it had stock trans tuning and so do my friends and family who have Aisin's so Ram's stock trans tuning is not perfect either.

When do you manually shift a transmission with stock tuning? What are you trying to accomplish by manually shifting?

[4x4ord]

The computer displayed mpg was almost exactly accurate on my 2011 Powerstroke. My 2016 lied and it was not always consistant in how much it lied by. Generally hand calculated proved to be about 8% better than the computer said. I used to check every tank but can't remember checking my 2017. I no longer fill with at a metered pump.