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Discussion Starter #1 (Edited)
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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Great write up, Randy.

:rep:

So do you know what years/model RAMs were equipped with MDS?

Like, I'm wondering if my 2007 5.7L MegaCab has it. I think not.
 

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Discussion Starter #6 (Edited)
2006 was the first model year for MDS in the Ram. The Mega Cab wasn't offered with MDS even though it's rated as a half ton truck.
 

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Discussion Starter #8
When you tear one down, it's really not as complicated as it seems. Some of the components are different but the only real parts additions are the solenoids. Even the VCT isn't terribly complicated.
 

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Rep for the write up. I'm always amazed at how a simple solution(not necessarily easy) can accomplish so much. Thanks to Stacy for putting up the badge as well. All of Randy's posts show his knowledge, the badge makes it easier to know when you are getting a reputable source.
 

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Good one Randy. Rep for the info.

Like the new badge too. Stacy comes through once again.
 

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2008 RAM 1500 ST 3.7L Magnum V6 PS2
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Very nice write up Randy, thanks for the info and nice badge BTW!!! :smileup:

- Cajun :pepper:
 

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I'm surprised they don't hold the valves open a bit (not enough to hit the piston) so that it doesn't rob power on both compression/decompression strokes. Ideas?
 

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Discussion Starter #17
The idea behind this is that the same force that resists rotation of the crank on the up stroke will aid in rotation on the way down.
 

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Thanks Randy, that makes sense. I'm sure "they" did plenty of R&D by now to see what works best. Prob. a wash on loss of hp to rotate, since 4 of them are disabled.

Thanks!
 

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The 1500 MegaCab is really a 2500 with a "1" on the door instead of a "2". The HD trucks did not use MDS, thus the 1500 Mega did not either.

I read this somewhere recently about the MDS (multi displacement system) it sounds resonable
Heres how it works, when the valves for those particular cylinders are closed, the exhaust gas is still stuck in there from the previous ''burn'' (this is done on purpose).... as the ''non-working'' pistons move up they compress the exhaust gases, but since the valves are closed this creates something like an ''air spring''. When the exhaust gas is compressed during the piston’s upstroke, then push down on the piston during its downstroke. The compression and decompression of the trapped exhaust gasses have an equalizing effect – overall, there is virtually no extra load on the engine.
 

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.....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......
Does this mean there is a solenoid for each of the four cylinders so they can be deactivated 1 at a time....but in the same 2 rotation time line ?
 
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