Why does the Hemi with VVT still need MDS?

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Jebb

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I was surprised to find out that my 2017 Hemi QC has "MDS". I thought they got rid of that when they went to variable valve timing.

I have been experimenting with MDS off (8th gear trick) and can really tell the difference in sound and feel when MDS is turned on/off.

Surprisingly, I am seeing NO DIFFERENCE in mpg with it turned off. In fact in my truck (3.21 gears and 35" tires) it seems MDS struggles - it's too low of gear and too much tire for a "4 cylinder". Sounds like it's bogging when on. Works OK at 70mph on flat ground but kicks out then causes downshift on the slightest grade. Runs better overall with it off.

Why does an engine with VVT even NEED MDS?
 

Smokey613

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I was surprised to find out that my 2017 Hemi QC has "MDS". I thought they got rid of that when they went to variable valve timing.

I have been experimenting with MDS off (8th gear trick) and can really tell the difference in sound and feel when MDS is turned on/off.

Surprisingly, I am seeing NO DIFFERENCE in mpg with it turned off. In fact in my truck (3.21 gears and 35" tires) it seems MDS struggles - it's too low of gear and too much tire for a "4 cylinder". Sounds like it's bogging when on. Works OK at 70mph on flat ground but kicks out then causes downshift on the slightest grade. Runs better overall with it off.

Why does an engine with VVT even NEED MDS?

On my 2011 300C with the 5.7 I disabled MDS with my Diablo Sport 3 years ago. In town I lost maybe 1 mpg and about 2.5mpg on highway. The plus side it performs much smoother all around. I will say the MDS on the wife's 2014 Laramie 5.7 operates much smoother than my car and is virtually undectable in it's operation.
 

gypsy400

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Mine is so smooth I'm not even sure if it's working.. when the indicator comes on I see no difference in fuel mileage.. and it doesn't stay on long enough to make a difference anyhow.

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ChargerX3

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35s yea no brainer. Put some normal tires back on and then see the improvement
 

Ratket

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321 gears... 35inch tires... 8 speed tranny..
mds is only going to work for you at sea lvl and flat ground.
I mean mds will activate in 5th,6th,7th gear.. but to maximize the potential on mds you either need to down size your tire size or do a gear swap-
It's the same thing as the mds on my 3/4ton 6.4 with 373"s...
 

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I was surprised to find out that my 2017 Hemi QC has "MDS". I thought they got rid of that when they went to variable valve timing.



I have been experimenting with MDS off (8th gear trick) and can really tell the difference in sound and feel when MDS is turned on/off.



Surprisingly, I am seeing NO DIFFERENCE in mpg with it turned off. In fact in my truck (3.21 gears and 35" tires) it seems MDS struggles - it's too low of gear and too much tire for a "4 cylinder". Sounds like it's bogging when on. Works OK at 70mph on flat ground but kicks out then causes downshift on the slightest grade. Runs better overall with it off.



Why does an engine with VVT even NEED MDS?



Nope they added VVT in 2009 and kept MDS too. I have mine turned off with tuner. I didn't find it to help with mileage and I think the MDS is probably why many have troubles with lifters and stuff but that is just my guess.
 
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Nick@GotExhaust

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They use both in an effort to get every last mpg they can out of these trucks to trend toward meeting the insane CAFE standards. Plus the VVT gets a cleaner/better combustion which not only helps with efficiency but also helps in the power dept.
 

huntergreen

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Mine is so smooth I'm not even sure if it's working.. when the indicator comes on I see no difference in fuel mileage.. and it doesn't stay on long enough to make a difference anyhow.

Sent from my SM-G903W using Tapatalk

What indicator do you you have that let's you know you are in mds mode ?
 
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Jebb

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I have mine turned off with tuner. I didn't find it to help with mileage and I think the MDS is probably why many have troubles with lifters and stuff but that is just my guess.
I can't back this up with any facts but it seems to me MDS would produce uneven wear and eventually F-up the engine.

I understand the concept but unless those 4 cylinders are constantly being rotated among all 8 (which I don't think happens) it seems a bad idea for engine life.
 

tonys ram

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I agree Jebb. Did you turn yours off? I'm thinking of getting a tuner to disable it. But too expensive right now.
 

smiley

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I can't back this up with any facts but it seems to me MDS would produce uneven wear and eventually F-up the engine.



I understand the concept but unless those 4 cylinders are constantly being rotated among all 8 (which I don't think happens) it seems a bad idea for engine life.



Exactly the same 4 deactivate every time for balance. With that being said the argument is that the cylinders are stills working just no gas to fire so they still would wear the same. It is hard to know for sure what I dislike about MDS is it makes the vehicle feel like it has to downshift both the engine and the transmission to accelerate and that lag bugs me.
 

Firebird

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I would imagine the irritation of MDS may depend on where you live? Here in Florida, it's flat, so MDS is really a minimal bother. However, I can imagine it would be irritating in hilly/mountainous areas.
 

Casper

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MDS is only annoying for me on certain roads with hills. I have to mash the pedal to force cylinder restart and/or down shift a couple of times each trip on certain roads.

I find myself spending a lot more time in a numbered gear nowadays, rather than putting up with that lugging sensation when MDS steals half the power.

As someone stated earlier its all about squeezing every last ounce of gas mileage out of the Hemi to make CAFE standards. It can't be good for emissions always lugging and having to stomp the skinny pedal to gittyup though.
 

tsielski

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I turn MDS off every time I use my truck (using the paddle shifter method). My highway mileage actually has improved with MDS turned off; I've gained from 1 to 1.5 MPG improvement, on the same 340 mile trip with it off vice leaving it engaged. I understand why the time and $$ was spent to engineer it (CAFE), but in real world driving, MDS has proven to be useless, based on my experience over at least 10K miles so far in my truck.
 

electricyeti

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@30K in my 16RCSB2wd and after I started shutting of MDS ever time I get in my truck my mileage went up almost the same 1.5. ( plus with the Outlaw it sounds like ****). I think with the MDS on it spends a lot of time searching for gears. Once I get in the highway it never shifts up or down with it off. With it on it shifts constantly, it's not flat here.

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huntergreen

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i can't tell when mine is on or off.
 

WilliamS

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2014 2014 ram will bog a little before kicking out of mds of a slight elevation. It will down shift shortly after that. Other than that flat out i cant even hear it, where on my 2012 300c I could hear it when i changed to 4cyl
 
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Jebb

Jebb

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Exactly the same 4 deactivate every time for balance. With that being said the argument is that the cylinders are stills working just no gas to fire so they still would wear the same.
The 4 cylinders that fire see a LOT higher loads than the 4 that aren't even moving air in/out. No way they wear the same.

BTW - I have the Mopar Cat back and CAI on my truck. When MDS is on it bogs and sounds like a Jake Brake!
 

smiley

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The 4 cylinders that fire see a LOT higher loads than the 4 that aren't even moving air in/out. No way they wear the same.

BTW - I have the Mopar Cat back and CAI on my truck. When MDS is on it bogs and sounds like a Jake Brake!



It is pretty terrible I guess whomever thought it was seamless was deaf and could not feel the difference either. I am sure they only did it to make CAFE people happy anyway.
 

Hemi395

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Question for everyone who has MDS turned off, how is your mpg compared to MDS being on? I know tsielski is getting better mpg I'm just trying to see how common it is to gain or maintain mpg with MDS off.
 
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