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Discussion Starter #1
Looking at the theory behind MDS just doesn't add up to me.

If it takes X amount of energy to move a 4,000 pound car along at 70 mph what difference does it make if that energy is generated by 8 or 4 cylinders? Just seems to me that 8 cylinders would only have to consume half as much gasoline to generate the same amount of energy as 4 cylinders consuming twice as much gasoline.

If energy is generated by combusting fuel how can four cylinders generate as much energy as 8 with less fuel?

School me. :read:
 

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The theory is it that it takes less power at cruising speed than under accel. 100% of 4 cylinders is still less gas then say 50% of 8. You cant control the gas flow to each cylinder separately. It also shouldn’t take 100% of 4 cylinders to maintain cruising speed. Throttle is most like still only at 20% or so. 4 cylinders on the 5.7L is still almost a 3.0L engine.
 

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four cylinders cant generate as much energy as 8, but 8 cyls are not required to move the car along at cruising speed, thats why mds only comes on for light throttle situations and kicks off when accelerating.
 

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Discussion Starter #4
I appreciate the responses but...

Consuming fuel generates energy. Energy is what moves the car. It takes X amount of energy to do so. How do four cylinders generate the same amount of energy as eight using less fuel?

Ignore acceleration, I'm ONLY referring to cruising.
 

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I can't give you a technical reason as to why it works...but it does. A lot of smarter and richer people than me have figured that out. The only reason they do MDS is to meet the retarded CAFE standards across the line so as to bring us such cool things as vipers, 6.4 Challengers, etc...

I had it in my 2012 RAM 5.7. I didn't like the way it felt around town so I turned it off in the city. But on the highway it was less annoying and did work. I ran 3 separate experiments with it on the highway. In every case the results were the same. With MDS off.. 17 mpg. With it on... 22 mpg.
 

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Newtons law. An object will stay in motion unless disrupted by an outside force. So in theory, once you get the 4000lb beast going it's going to keep going unless something interferes with it. Aka gravity. All you need is enough energy (4 cylinders) to counteract with gravity. You can actually probably get away with 1 cylinder once you are cruising but you have to factor in road conditions, wind, etc. think of it like this if you have ever had to push a car. When it's stopped it takes a hell of a lot of effort to get it going, but once you get it moving its pretty simple and you just have to keep your legs moving. Think of the strong man competitions when they pull the big rigs. Same thing.
 

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The wind is a formidable force to overcome at speed. Keeping 4000 pounds rolling takes much energy especially if the a/c is running.
 

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Yes. I understand that about the wind. I'm just trying to make it extremely simple for the OP. Once that bad boy is moving it does NOT take that much engery to keep it moving if you factor out every outside force except gravity. It's why I stated they can probably maintain 70 cruising with simply one cylinder firing in perfect conditions.
 

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My guess, and this is only a guess, is this:

It may make more sense to shut down 4 cylinders at cruise speeds because cutting the fuel on all 8 enough to equal the efficiency of 4 cylinders may cause a lean condition and possible overheating of the cylinders.
 

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Trying to think of a good way to explain this…… There is a minimum amount of fuel a motor needs to run properly regardless of power output. If you are feeding 8 cylinders the minimum amount of fuel to run properly it makes more power than needed for a cruising situation. Regardless if you do not need that power you still have to feed those 8 cylinders enough fuel to maintain reliable operation, wasting engergy. Obviously 4 cylinder MDS still makes enough power to maintain a cruise situation at the same minimal fuel consumption but it is not wasting fuel on those other cylinders. Shutting down cylinders is not detrimental to the engine, but leaning out the fuel mixture is, and causes driving issues. MDS would not be needed if they could lean all 8 cylinders out enough to reduce power output, but they can not do that with out causing a motor to overheat and cough and puke and not run well. It boils down to the V8 makes more than enough power at cruise to maintain speed at the absolute leanest fuel mixture to maintain reliability. Only option after leaning mixture is the kill cylinders until the power output is just enough to match cruising speed.
 

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Discussion Starter #12
Trying to think of a good way to explain this…… There is a minimum amount of fuel a motor needs to run properly regardless of power output. If you are feeding 8 cylinders the minimum amount of fuel to run properly it makes more power than needed for a cruising situation. Regardless if you do not need that power you still have to feed those 8 cylinders enough fuel to maintain reliable operation, wasting engergy. Obviously 4 cylinder MDS still makes enough power to maintain a cruise situation at the same minimal fuel consumption but it is not wasting fuel on those other cylinders. Shutting down cylinders is not detrimental to the engine, but leaning out the fuel mixture is, and causes driving issues. MDS would not be needed if they could lean all 8 cylinders out enough to reduce power output, but they can not do that with out causing a motor to overheat and cough and puke and not run well. It boils down to the V8 makes more than enough power at cruise to maintain speed at the absolute leanest fuel mixture to maintain reliability. Only option after leaning mixture is the kill cylinders until the power output is just enough to match cruising speed.
If an engine can run properly from idle to WOT, where does this "lean condition" concept come from? If that statement were valid, a car without MDS would run lean maintaining cruising speed.
 

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Cars are already as lean as they can be in the epa mandated ranges. Idle/cruise ranges.

The engine runs properly because it's NOT too lean, they can not take it any leaner in the cruise range. Car without MDS runs same air fuel ratio as one with or they are at least very similar. In MDS mode they might enrich to prevent detonation. Remember we are talking about cruise range, not WOT. That is a completely different air/fuel table. What I'm saying is if they could lean a motor out to say 19:1 AFR cruising and still make it run crisp with no stumbles or destination then mds would not be needed, but they can't. Even with that 19:1 lean condition if they could do it still might be making more power than needed. Cutting cylinder is the only logical way to reduce power output without sacrificing drivability.
 

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Here's how MDS works:

The Multiple Displacement System (MDS) provides cylinder deactivation during steady speed, low acceleration and shallow grade climbing conditions to increase fuel economy. Both four and eight cylinder configurations have even firing intervals providesmooth operation. Two cylinders on each bank are active when the engine is in four-cylinder mode – every other cylinder in the firing order. All of the cylinders that are deactivated have unique hydraulic valve lifters that collapse when deactivated to prevent the valves from opening. Engine oil pressure is used to activate and deactivate the valves. It is delivered through special oil passages drilled into the cylinder block. Solenoid valves control the flow. When activated, pressurized oil pushes a latching pin on each valve lifter, which then becomes a “lost motion” link. Its base follows the camshaft, but its top remains stationary, held in place against the pushrod by light spring pressure but unable to move because of the much higher force of the valve spring.

NOTE: It is critical to use the recommended oil viscosity in 5.7L engines that use MDS.

Deactivation occurs during the compression stroke of each cylinder, after air and fuel enter the cylinder. Ignition then occurs, but the combustion products remain trapped in the cylinder under high pressure, because the valves no longer open. No air enters or leaves. During subsequent piston strokes, this high-pressure gas is repeatedly compressed and expanded like an air spring, but fuel is not injected
 

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If an engine can run properly from idle to WOT, where does this "lean condition" concept come from? If that statement were valid, a car without MDS would run lean maintaining cruising speed.
Chris07's explanation was right on...but not following the followup question (maybe b/c its the day after St. Patrick's day). I think to Chris's point that you can't reliably lean the AFR in all 8 cylinders to reduce fuel consumption, so shutting down the cylinders via MDS is the reliable alternative.
 

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I think it's a gimmick to pamper CAFE standards is all..and doesn't belong nowhere near this car.. It's nothing but problems from what I see on this forum...2 cents
 
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