Force Truck to Run in 4 Cylinder Mode

[5thGenLoco]

I find it strange how upset people get at the idea someone might want to run in 4 cylinders and be more efficient with their truck - will never understand why others seem defensive about it and feel the need to question the validity of the question. My 1st ram had the V6 and it was a great truck that did everything I needed and used less fuel doing it. My last 2 have been Hemi trucks because I couldn't have a CC and 6'4" bed with a V6 and that combo was important to me. I would love a truck that could run on fewer cylinders in daily driving and more when I want fast starts or heavy towing like to OP is thinking. My truck is in Eco mode at least 50% of the time in both city and highway driving so it seems to work well. In the end I think the V6 is the perfect mix of mpg and power for 99.9% of truck users who don't tow more 5,000lbs.

Curious what your ave mpg is with yours running 50% in eco mode.

 

[millerbjm]

Curious what your ave mpg is with yours running 50% in eco mode.

Really hard to say as I've only had it a month and still under 2,000 miles. The computer says 17mpg average and I do about 50/50 driving. Seems to be a general belief engines don't get their best mpgs until they have a few more miles on them so we'll see.

 

[5thGenLoco]

Really hard to say as I've only had it a month and still under 2,000 miles. The computer says 17mpg average and I do about 50/50 driving. Seems to be a general belief engines don't get their best mpgs until they have a few more miles on them so we'll see.

Just trying to compare with mine. I always turn off the MDS except maybe for a long fwy drive, like driving out of state. I ave ~18.5 combined driving with 3.21 and a slightly larger tire, 53k miles currently. I have compared letting the MDS do it's thing and not running it at all and saw no difference in mpg.

 

[Dusty1948]

This isn't actually true - when you switch to MDS, those 4 cylinders will run at higher load. That load transition is why there's a "bump" - you're switching from 8 cylinders putting along to 4 running almost as hard as they can

If you think about it, a V8 running at 30% load can't just switch 4 cylinders off without suddenly slowing down and the driver noticing. It has to change throttle position and run the four at 60+% load to make up for the dead cylinders. The magic here is that cylinders at high load have a lower Brake Specific Fuel Consumption (BSFC), so it's more efficient overall.

That being said, the ECU has loads of tables it uses to tell when it's optimal to engage MDS - BSFC is not linear. I'm sure it's not *entirely* optimized for fuel consumption since they know it creates NVH; it's probably true that placing efficiency over comfort would yield positive results at the pump... but there are other areas where all 8 cylinders probably have a better overall efficiency. I'm honestly not sure if you can even idle properly with MDS engaged, for example.

Thank you for the thought provoking comments.

I’m not sure the explanation is completely valid since it applies a simple rule and leaves more questions than answers.

As I recall Brake Specific Fuel Consumption is used to compare the maximum power efficiency of engines to other engines using a fixed brake horsepower load on a test stand. The subject at hand for this discussion is a little different. Instead of comparing one engine design to another (two completely different engines), we are discussing one engine that switches off one-half of its cylinders on a common crankshaft. There’s an assumption that the BSFC of just four cylinders is in ratio to all eight when not in MDS. I don’t think this is true. Ignoring all other possible influences for the moment, when in MDS four cylinders are now supplying power but with twice the amount of internal frictional load, i.e.: it has to overcome the friction induced by the non-power producing cylinders.

“If you think about it, a V8 running at 30% load can’t just switch 4 cylinders off without suddenly slowing down without the driver noticing it. It has to change throttle position and run the four at 60+% load to make up for the dead cylinders.”

Except that does not appear to happen. On all three of my MDS engines the transition from eight to four cylinders (MDS mode) more often than not, is totally imperceptible. I have never observed a change in engine RPM so I don’t believe the throttle plate changes position. In my current MDS model with the 3.21 axle sometimes the slightest throttle pressure will kick it out of MDS.

“The magic here is that cylinders at high load have a lower Brake Specific Fuel Consumption, so it’s more efficient.”

Okay, but for the scenario in this discussion the torque requirements will vary considerably as the normally aspirated vehicle is driven in normal driving where there are launches from stops and power required on grades, not to mention the varying cargo and passenger loads. As you pointed out, BSFC is not linear. I am very certain that running on just four MDS cylinders in a zero grade, wind-less environment the MDS engine will indeed outshine with respect to fuel consumption. But I think in a purely practical usage and environment of day-to-day driving there will be times that all eight cylinders will be contributing to a more efficient use of fuel.

There may be somewhat of a demonstratable example. The Silverado can be equipped with a forced induction, four-cylinder engine. The EPA rating of 20/23 (2WD) and 19/22 (4x4) is really quite unexceptional, especially when one considers the Silverado is 300-400 pounds lighter than the current 5th gen Ram.

All the best,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 053971 miles.

 

[User_3336]

Buy a 2021 Ford Ranger or 2020 Maverick. (4 cyl 4x4 Ranger gets same mileage as Ram 5.7 ETorque!!)

Remarkably a 2021 Ford Ranger 4x2 got me 25.9 in the 1400 miles I had it!

 

[User_3336]

I find it strange how upset people get at the idea someone might want to run in 4 cylinders and be more efficient with their truck - will never understand why others seem defensive about it and feel the need to question the validity of the question.

Don't think anyone got "upset". Did seem an odd question by a new member and their first post.
When the OP returns to the discussion, I'll take back my accusation that we're being trolled!

 

[soSincerious]

I drove the Sierra with the 4 cyl 2.7 turbo, and was pretty damn impressed.

What doesn't impress me is how its only 2 mpg better in the city (same highway) and only 1 mpg better combined

 

[User_3336]

I drove the Sierra with the 4 cyl 2.7 turbo, and was pretty damn impressed.

What doesn't impress me is how its only 2 mpg better in the city (same highway) and only 1 mpg better combined

The Sierra/Silverado does pretty good with the 2.7 considering its weight. Engine is slightly noisy.

The Ranger does "ok" with its 2.3, but sluggish at slower speeds and doesn't need a 10 speed trans.

 

[HSKR R/T]

It will never run on 4 all the time. You'd be accelerating from stops like one of those 1980s Toyota motorhomes. To get the most MDS, a light foot is what it takes.

And be sure you have turned on the ECO light in the uConnect settings so you know when it's active.

When the eco light is on, doesn't necessarily mean MDS is active. The ECO light can be in, even without MDS active.

 

[HSKR R/T]

Dang, I wish I could “force“ my Ram to run on eight cylinders all the time,

You can, and pretty easily without any aftermarket products.

 

[jdmartin]

I find it strange how upset people get at the idea someone might want to run in 4 cylinders and be more efficient with their truck - will never understand why others seem defensive about it and feel the need to question the validity of the question. My 1st ram had the V6 and it was a great truck that did everything I needed and used less fuel doing it. My last 2 have been Hemi trucks because I couldn't have a CC and 6'4" bed with a V6 and that combo was important to me. I would love a truck that could run on fewer cylinders in daily driving and more when I want fast starts or heavy towing like to OP is thinking. My truck is in Eco mode at least 50% of the time in both city and highway driving so it seems to work well. In the end I think the V6 is the perfect mix of mpg and power for 99.9% of truck users who don't tow more 5,000lbs.

Hopefully I wasn't one of those people, because I certainly wasn't trying to insult the OP, but I still don't see what purpose this would serve. I demonstrated that the math makes no sense relative to the added complexity, so unless you're doing it for ecological reasons (which would be odd to be driving a big thirsty truck and worried about carbon emissions) I can't see the point. Surely if Ram thought it was a major fuel saver without risk to the engine or ridiculous cost or complexity they would make it a feature so they could claim maximum fuel economy figures.

 

[HeavyMetalHauler]

When the eco light is on, doesn't necessarily mean MDS is active. The ECO light can be in, even without MDS active.

On a 5th gen, the ECO light is directly related to MDS.

 

[mmcbeat]

You can, and pretty easily without any aftermarket products.

How? Everything thing I’ve read indicates you need a tuner, mopar installed high performance cam, or something like keep it in max 5th gear mode. I would like to have the ability to disable it in set up. Thanks.

 

[Drivinfast247]

How? Everything thing I’ve read indicates you need a tuner, mopar installed high performance cam, or something like keep it in max 5th gear mode. I would like to have the ability to disable it in set up. Thanks.

Use gear selector and set to -8.

Sent from my SM-N970U1 using Tapatalk

 

[Ram4Lou]

Use gear selector and set to -8.

Sent from my SM-N970U1 using Tapatalk

Yep, that’s what I do, unless on long highway trip. It has become 2nd nature to tap that button when I get in the truck now

 

[SpeedyV]

On a 5th gen, the ECO light is directly related to MDS.

Correct! It’s crazy how much misinformation keeps popping up.

 

[Trooper4]

Correct! It’s crazy how much misinformation keeps popping up.

Ever play the kids game of telegraph/telephone where each kid whispers something into the next kids ear, and on down the line, the last kid says what they heard? Kind of like that here sometimes. ""Well I heard that"" can really spread fast.

 

[Drewster]

Thank you for the thought provoking comments.

I’m not sure the explanation is completely valid since it applies a simple rule and leaves more questions than answers.

Like it or not, that's how it works. You don't get to postulate alternatives just because it's not what you originally had in your mind.

This article is on GM's implementation, but the physics - and notion of ECU's shooting for a targeted load rather than throttle position - is the same

All the Ways Automakers Squeeze Big Fuel Economy Out of Tiny Turbocharged Engines

Vehicles like the four-cylinder turbo Chevy Silverado perform a constant engine-management dance to give you the power you want—while burning as little fuel as possible.

www.roadandtrack.com

Cylinder deactivation is a boon because gasoline engines are typically most efficient at or near full load. As a result, powertrain computers tend to favor cruising in top gear with the throttle either nearly or fully open. Shutting down cylinders reduces an engine's available power, making the active cylinders work harder and bringing them closer to their maximum output

 

[HSKR R/T]

On a 5th gen, the ECO light is directly related to MDS.

Not saying you are wrong, but is there any documentation to support this? Seriously want to know so I'm not spreading false info. Anyone can say what someone else says is wrong, but without documentation to support this claims who is really right and who is wrong

 

[SpeedyV]

Not saying you are wrong, but is there any documentation to support this? Seriously want to know so I'm not spreading false info. Anyone can say what someone else says is wrong, but without documentation to support this claims who is really right and who is wrong

Empirical evidence over 3 years of daily driving. The ECO light has always, without exception, turned on when the truck goes into MDS (easily heard) and turned off when MDS is disengaged. Again, not one exception.

To my knowledge, this isn’t documented (aside from a great many posts in very old threads on this forum and others).

Note: The misinformation almost always comes from previous or current 4th-gen owners who assume (incorrectly) that 5th-gens work the same way.