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View Full Version : How long before MDS in the Cummins?


BigBlueDodge
02-16-2007, 06:08 PM
I'm sure everyone is aware of Dodge's MDS technology. At a high level, what the engineers discovered is that it only takes approx 40-50 hp to keep a car moving when at highway speeds. So, in the 5.7L Hemi, they effectively shut down 4 of the 8 cyclinders to conserve fuel. The technology does work. I have a buddy that had a R/T Magnum, and he was averaging 20+ on the highway.

It made me start to think. Yes, CTD are built to tow. However, I would dare say that 80% of the owners who buy one don't tow, or tow very lightly with it. For that user base, they don't need 325hp and 610 tq just to cruise 70mph. As fuel costs go up, and our pocketbooks start feeling the pressure, I'm sure that Cummins will begin to look at ways to increase the mpg in now towing situations. The MDS technology is proven to work. Is it just me, or would anyone like to see this technology move into the diesel engines. I think we could genuinely start seeing 25+ mpg again.

Let me hear your thoughts.

steelblitzkrieg
02-16-2007, 06:28 PM
I would hope that he gets that kind of mileage, I get 23-25mpg in my GT highway and 17-18mpg city, granted it's smaller and a bit lighter, but I'm not easy on the go pedal by any means. I go through Z rated tires every 10-12k miles.

P.S. She has 150k miles on original engine too (1qt of redline every 1500 miles)

signature600
02-16-2007, 06:30 PM
That's the nice thing about a diesel...it only uses the fuel/power it needs;)

I wouldn't think MDS would save that much fuel in a Cummins!

Chris

98.5POS
02-16-2007, 06:34 PM
A diesel needs the oil in the fuel for the pistons. How would that work if say 3 holes had no fuel? Just a thought.

RustyJC
02-16-2007, 06:35 PM
MDS works in a gasser because the fewer active cylinders work at a higher BMEP and require a larger throttle plate opening. This means these active cylinders operate at a higher load - closer to their best BSFC point, and the lower manifold vacuum reduces pumping losses.

There would be no pumping loss benefit in a diesel since it doesn't have a throttle plate.

Rusty

CCM591
02-22-2007, 04:56 AM
According to EPA, diesel engines benefit almost as much from "variable displacement" as gasoline engines. See:

http://www.epa.gov/otaq/reports/adv-tech/420r04002.pdf (caution - large .pdf file)

Presumably, this is the same technology as, or at least similar to, MDS.

Totallyrad
02-22-2007, 07:18 AM
Just think, if they did that by holding the exhaust valves open it wouldn't be a big jump to hold the intake valves closed. Then you could have added fuel economy and a real "Jake brake".

magram
02-22-2007, 08:27 AM
It's all to do with the cam in the dodge trucks and the cars. I had an mag rt before this truck and it was unblieveable the amount of fuel a guy could save. I know some guys on lx forums[tapdshut] swapped cams out on there cars, and bye bye MDS.
http://i98.photobucket.com/albums/l260/magram/2006_0904Image0131.jpg

own1bigtruck
02-22-2007, 11:25 AM
Just think, if they did that by holding the exhaust valves open it wouldn't be a big jump to hold the intake valves closed. Then you could have added fuel economy and a real "Jake brake".

and think of how much brake power you would gain, not to mention how cool it would sound through str8 pipes! Im all for a real jake brake!!

Spooler
02-22-2007, 12:57 PM
Who cares about MDS on a diesel. I bought this truck to work. Period. Cummins fuel mileage is still very good in my opinion for what I do with the truck. It is not going to do me any good when pulling my horse trailer and that is what I bought it for.

jkitterman
02-22-2007, 03:30 PM
The MDS system on the diesel and the auto for the EPA paper were multi-crankshaft types. This would be like having 2 3cyl engines and running only one for economy. Turning off 3 cyl on the inline 6 wouldn't really help

boostjunkie1
02-22-2007, 03:53 PM
There is to much weight to be pushing around for the deactivation to work! THat is why you cannot get the MDS in the 2500 Rams with the HEMI.

Sailpipes
02-22-2007, 05:16 PM
As signature600 said, diesels only make the power they need. Gasoline engines use a constant air:fuel ratio. Diesels vary it depending on throttle position. As opposed to shutting off to 3 of your 6 cylinders, you're just injecting less fuel.

BLACKBIRD8200
02-22-2007, 05:44 PM
It's all to do with the cam in the dodge trucks and the cars. I had an mag rt before this truck and it was unblieveable the amount of fuel a guy could save. I know some guys on lx forums[tapdshut] swapped cams out on there cars, and bye bye MDS.

I understand that if you swap cams the feature no longer works but I don't think the cam is the reason. because the cam is a solid peace nothing moves on the cam aside from the whole cam spinning.
I think it has more to do with the lifters.
I think how it works is it disengages the lifters from the 4cycl that shut downthat way the valves won't open/close (no air/fuel).

magram
02-22-2007, 06:55 PM
I understand that if you swap cams the feature no longer works but I don't think the cam is the reason. because the cam is a solid peace nothing moves on the cam aside from the whole cam spinning.
I think it has more to do with the lifters.
I think how it works is it disengages the lifters from the 4cycl that shut downthat way the valves won't open/close (no air/fuel).

The system was designed around what is called a "lost motion device." DaimlerChrysler engineers designed the Hemi with a separate oil circuit controlled by four solenoids that apply oil pressure to eight specially configured hydraulic roller lifters. When the computer decides to deactivate the cylinders, the solenoids are electronically engaged and oil pressure is sent to the lifters via a separate oil passage in the block that pushes on a small pin that allows the internal piston in the hydraulic lifter to disengage from the lifter. The lifter body still rises and falls with the cam lobe, but the piston that is linked to the pushrod remains stationary. DaimlerChrysler engineers state that the system can deactivate four cylinders within 40 milliseconds (0.040 second). Both the intake and exhaust lifters for each cylinder are deactivated. Every other cylinder is deactivated so the transition from eight to four and four to eight cylinders is seamless. We've driven both the new Dodge Magnum and the Chrysler 300-C, and it is impossible to tell when the engine is running on four cylinders and when it doubles back to eight

BLACKBIRD8200
02-22-2007, 07:02 PM
The system was designed around what is called a "lost motion device." DaimlerChrysler engineers designed the Hemi with a separate oil circuit controlled by four solenoids that apply oil pressure to eight specially configured hydraulic roller lifters. When the computer decides to deactivate the cylinders, the solenoids are electronically engaged and oil pressure is sent to the lifters via a separate oil passage in the block that pushes on a small pin that allows the internal piston in the hydraulic lifter to disengage from the lifter. The lifter body still rises and falls with the cam lobe, but the piston that is linked to the pushrod remains stationary. DaimlerChrysler engineers state that the system can deactivate four cylinders within 40 milliseconds (0.040 second). Both the intake and exhaust lifters for each cylinder are deactivated. Every other cylinder is deactivated so the transition from eight to four and four to eight cylinders is seamless. We've driven both the new Dodge Magnum and the Chrysler 300-C, and it is impossible to tell when the engine is running on four cylinders and when it doubles back to eight

Thanks I always like to learn new stuff.

Road Traveler
03-02-2007, 10:25 AM
and think of how much brake power you would gain, not to mention how cool it would sound through str8 pipes! Im all for a real jake brake!!


Well at the SEMA show this year I was talking to a guy about that. He seems to think it isnt that far off to see a real Jake in smaller trucks!! Oh the fun that will be!!![guitar]

12PACK
03-02-2007, 07:33 PM
In January a friend of mine bought a 1500 Hemi and also said the switch from 8 to 4 is seemless. He's yet to notice a delay in getting power when he needed it on the highway. This is one of the reasons he's stayed with Dodge. This 07 replaced his 00 which replaced his 96, all gassers. He was going to go back to Toyota but figured he'd get more truck for the $$ and the mileage would be about the same.