MDS, CAFE, Emissions, and a 392 Powered RAM

[HEMIMANN]

As gas motors go, the 6.4L Hemi has about as good a low rpm torque curve as there is, and pretty flat too. I was looking for this when I bought without having to go diesel, where the expense has gotten out of control with all the aftertreatment crap. Not to mention HPCR piezo injectors. Getting diesels to zero emissions took a friggin' science fair.

Yer rocker arms sure are purty, @kurek !

 

[hemihustlin]

I don't think you guys are getting what I'm laying down here.

I'm not talking about just on the highway, I'm saying it should run in MDS nearly constantly once your accelerated to whatever speed you are driving at. Mine goes into MDS at all speeds when I let out of the pedal, meaning light load. If I am on level ground cruising at ANY speed, I'm in MDS mode and getting between 27 and 30+ MPG. The only time my truck drops out of MDS is when I get to a hill or want to accelerate. This is a function of load on the engine, which decreses as gearing is shortened. The truck will run in MDS mode MORE OFTEN with the 8 spd.

Since increased load is overcome by more diplacement, in theory MDS should stay engaged even longer (like when climbing a hill) with a 6.4 than it does with a 5.7 because a 3.2L four cylinder makes more power under a given set of conditions than a 2.8L four cylinder. So why didn't we see a 6.4L R/T? Was it the load that a federal emissions dyno puts on the truck during testing? Did it keep kicking out of MDS during testing, and thus not pass? I just can't imagine why a 6.4L/485hp/8HP70 RAM R/T wasn't a thing from 2013 to 2022.

Y'all need to quit TL: DRing, you might learn something.

If you don't think that MDS saves you gas then you don't understand how an engine works.

It feels like they changed the parameters for mds engagement with the 8 speed vs my old 545rfe
same 3.92 rear end stock tire size 4x4 hemi both trucks
the 2006 would happily and often go into mds while cruising on the highway at speeds as high as 140 kmh
the 2021 is really hard to get into mds above 120 kmh
that or the warlock is really not aerodynamic compared to a 2006 slt? that would seem to be the biggest difference I can see. they both went easily into mds around town.

 

[Wild one]

Yep! Gears, converter, shift kit, subframe connectors, shocks, bushings, and slicks are first, then a set of Twisted Wedge heads, 220/230ish roller cam, long tubes, and a Vic Jr aught to get it done. If you have to spin over 6500 to get close something is wrong.

Old motors are the best. For sure don't have to worry about ring ends butting together or spinning a rod bearing when you got huge blow-by and 10psi at idle on 20w50.

*but*

Why do you think RAM team never offered a 6.4L R/T model?

It's gonna take a 331 stroker to reach 400hp with a 220 cam.
I have no idea why they never offered a 6.4 1500,they built a couple as prototypes,but never released one to the general public.There were rumours floating around about a 6.4 R/T being released several years ago,but it never materialized.They would of sold every one they built if they had,lol

 

[White six four]

I thought there was a post on here awhile ago that the person posted a picture of his loggings and when the mds kicks in the other 4 injectors double their duty cycles. I've put over 50k on my 6.4 and it doesn't seem to matter whether I have it on or off as far as mileage goes.

 

[StickyLifter]

I thought there was a post on here awhile ago that the person posted a picture of his loggings and when the mds kicks in the other 4 injectors double their duty cycles. I've put over 50k on my 6.4 and it doesn't seem to matter whether I have it on or off as far as mileage goes.

If it doubled the injector duty cycle it would flood the cylinders and be too rich to ignite. If you double the fuel to an engine in steady cruise without opening the air door a lot more, it will cough and die.

 

[Jwithing]

 

[White six four]

If it doubled the injector duty cycle it would flood the cylinders and be too rich to ignite. If you double the fuel to an engine in steady cruise without opening the air door a lot more, it will cough and die.

Thats what is said on the "Poll do you like mds" thread on here

 

[Bandit1859]

So I don't know how you guy's trucks run, but my RCSB 2wd 5.7/65RFE/3.92/31" tire Tradesman runs in MDS a lot since I went to the 3.92:1 final. I like MDS because it saves gas and is non intrusive. It's a pretty slick setup and I think it's cool.

Assume that Chrysler's durability testing works abd that the "hemi tick" is really a "warped cast iron truck manifold tick" for most occurrences and has nothing to do with the valvetrain.

When I swap out the turdly 65RFE for an 8HP70, I'm expecting to be able to spend even more time in MDS mode, which you would think Chrysler would like for their CAFE and emissions compliance. So here is what I'm wondering:

If the factory had offered a 6.4L/485hp MDS version, wouldn't it have helped their CAFE? I don't see how it would fail emissions, and I'm pretty sure that the added power from the displacement increase would help the truck stay in MDS mode EVEN EVEN MORE. I could them offering it as an option on the R/T models that already have the 8spd and 3.92 final. I don't see how it would burn more gas at light load, and with 40 extra cubes it's bound to pull hills in MDS and stay in more often. How can a 3.2L running on four NOT get better mileage and emissions than a 5.7 running on all eight under those conditions?

Well this will be a non issue soon as 2024 hemi is gone. A straight 6 will replace it. And EV truck coming soon after. I hate that you will not have a V8 choice. But this is being craned down our throats. Just the way it is

 

[retired]

Well this will be a non issue soon as 2024 hemi is gone. A straight 6 will replace it. And EV truck coming soon after. I hate that you will not have a V8 choice. But this is being craned down our throats. Just the way it is

You will always have a choice When it comes to v8’s

 

[jerm53]

A lot of the people that don't like MDS blame it for "hemi-tick", or have modified exhausts. To be fair, I test drove a truck with a custom exhaust and it sounds ... weird when MDS kicks in.

I've had mine over four years now and I can tell when MDS kicks in sometimes, but not often. I've run the same section of I-95 both with it on and off and for my use it does save some gas, but only in specific situations. Running around town, it seems better with MDS off.

But for the most part, I like it.

i have a '19 warlock with a magnaflow muffler delete, when it drops into 4cyl the note kinda reminds me of a modified subaru does. i turn MDS off under 45/50 most of the time..

 

[pacofortacos]

Going to 3.92's doesn't increase the amount of time in MDS mode. I am in MDS very seldom at 70+ mph.
Now if we limit the speed to 40-55 mph, then the 3.92's will get great mpg at that speed on the level with light throttle.

I also thought having 3.92's would allow longer MDS operation, however, it doesn't appear to be so. Possibly the added losses due to the higher rpms puts more of an actual load on the motor and negates longer MDS operation.

Stock @ 74 mph the best mpg I could get was 17 mpg. Doing some aero mods has allowed me to hit 20 mpg if there isn't any wind. Any headwind coming from frontal sides kills my mpg - I have seen as low as 12.5 mpg @ 74 mph in the winter. A straight on frontal headwind doesn't affect it as bad as if the wind is at a 30-45 degree angle from straight on, but will still drop mpg up to 4 mpg vs. no wind.

 

[Ram Night]

I would make sure you have everything you will need to do the swap to 8 speed because the computer programming will be different.
Plus when I get in my truck, the first thing I do is shut the mds off as after I get up to speed. Because I do not like how the lifter system works in mds, plus the Magflow exhaust sounds like a old helicopter.

 

[StickyLifter]

Going to 3.92's doesn't increase the amount of time in MDS mode. I am in MDS very seldom at 70+ mph.
Now if we limit the speed to 40-55 mph, then the 3.92's will get great mpg at that speed on the level with light throttle.

I also thought having 3.92's would allow longer MDS operation, however, it doesn't appear to be so. Possibly the added losses due to the higher rpms puts more of an actual load on the motor and negates longer MDS operation.

Stock @ 74 mph the best mpg I could get was 17 mpg. Doing some aero mods has allowed me to hit 20 mpg if there isn't any wind. Any headwind coming from frontal sides kills my mpg - I have seen as low as 12.5 mpg @ 74 mph in the winter. A straight on frontal headwind doesn't affect it as bad as if the wind is at a 30-45 degree angle from straight on, but will still drop mpg up to 4 mpg vs. no wind.

Mine spends more time in MDS more often under all conditions since I went from 3.55:1 to 3.92:1. The difference was quite noticable. I get 21mpg on flat sections of the interstate and will activate more often at 80 than at 70.

 

[joesstripclub]

I have a 3.92 truck and MDS is very quick to turn off with the slightest throttle input. Could be the power loss at altitude causes it to be more sensitive. I used to turn it off all the time when I first got the truck so I cant really compare.

 

[ramffml]

If it doubled the injector duty cycle it would flood the cylinders and be too rich to ignite. If you double the fuel to an engine in steady cruise without opening the air door a lot more, it will cough and die.

That is actually how MDS works; pretty much all the fuel from the now shut off 4 cylinders is sent to the 4 remaining cylinders. The fuel efficiency gains is due to the engine being more efficient at WOT for the 4 cylinders, instead of 8 cylinders at normal throttle.

If you're asking "where's the fuel savings in that?" you aren't alone. The 20 percent improvement in fuel economy from cylinder deactivation isn't from four cylinders of fuel going away, it's actually from a far more modest reduction in pumping losses. In its simplest explanation, pumping loss refers to the power an engine spends sucking in fresh air and fuel past the throttle, then pumping the spent exhaust gases out. It's a mechanical tax, and it works like this: Imagine a big V8 running down the road at a cruise speed of 70 mph. At low load and low rpm, the throttle on that big motor is barely cracked open.

That sounds great, until you pluck that imaginary big-block from between the fenders and stick an in-line four in its place. If you could swap engines that quickly—in a heartbeat—you'd notice that to keep the same speed, you'd need to open the throttle much farther. That little four banger is getting better fuel economy, and because the throttle is nearly wide open, the engine doesn't have to work so hard to suck air past the throttle. You'll never think of WOT the same way again because WOT is actually when your engine is most efficient—not the least efficient.

What Is Hemi MDS?

We explain how Chrysler Hemi MDS cylinder deactivation works, what goes wrong, and how to fix it when it breaks.

www.motortrend.com

 

[ramffml]

I've wondered if it wouldn't get far better gains by downshifting 1 gear and then running MDS after the downshift. It would probably feel busy and horrid, but I suspect that it wouldn't be lugging nearly as much so the 4 cylinders would operate longer.

MDS would probably work best with a CVT. But that's not going to go over well with the truck crowd.

 

[pacofortacos]

Mine spends more time in MDS more often under all conditions since I went from 3.55:1 to 3.92:1. The difference was quite noticable. I get 21mpg on flat sections of the interstate and will activate more often at 80 than at 70.

Might be the PCM programming between different years.

Mine is like others, drops out of MDS very quickly. I don't think I have ever noticed it above 80 mph - just way too much wind resistance to run on 4 cylinders.

Yours is the rare exception, most only get 17 mpg or so with 3.92 gears Maybe you have an aerodynamically slippery truck vs. my brick.

 

[StickyLifter]

Might be the PCM programming between different years.

Mine is like others, drops out of MDS very quickly. I don't think I have ever noticed it above 80 mph - just way too much wind resistance to run on 4 cylinders.

Yours is the rare exception, most only get 17 mpg or so with 3.92 gears Maybe you have an aerodynamically slippery truck vs. my brick.

Regular cab short bed 2wd at stock ride height with 31" all season tires, 65rfe, and 3.92 final. Mech fan is gone and the exhaust has a smooth Y pipe, but that is about it. Aero will improve when I lower it and add a half cover to the back side of the bed like Gail Bank's diesel powered land speed record truck and some plastic inder the bottom of the chassis.

Aerodynamics - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

Aerodynamics - Interested in aerodynamics, or tweaking your vehicle's CdA for better fuel economy? Check in here.

ecomodder.com

 

[pacofortacos]

Ok, I have a crew cab 4X4, so a lot more weight.

 

[Wild one]

Regular cab short bed 2wd at stock ride height with 31" all season tires, 65rfe, and 3.92 final. Mech fan is gone and the exhaust has a smooth Y pipe, but that is about it. Aero will improve when I lower it and add a half cover to the back side of the bed like Gail Bank's diesel powered land speed record truck and some plastic inder the bottom of the chassis.

Aerodynamics - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

Aerodynamics - Interested in aerodynamics, or tweaking your vehicle's CdA for better fuel economy? Check in here.

ecomodder.com

There used to be a wind tunnel test on a 4th Gen,but i can't find it right at the moment.But i'd research the 1/2 cover a bit more on a 4th Gen,as FCA found a full cover to work better on their early 2013 High Efficiency trucks.Very rare trucks,but they were all regular cab shortboxes with the 8 speed and 3.21 gears.The early HE trucks could be had with the 5.7,the later ones were 3.6 trucks.Your 3.92 gears aren't the best for milege,otherwise the HE trucks would of had 3.92's

2019 Ram 1500 - Wind Tunnel Testing - video Dailymotion

Marc Babich, aerothermal manager 2019 Ram 1500, talks about the aerodynamic testing that went into the building of the all-new 2019 Ram 1500.

www.dailymotion.com

Wind Tunnel Pickup Truck w/ Shells

www.youtube.com