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RamTech 01-31-2010 10:30 AM

MDS - A Brief Overview
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.

BlueJet 01-31-2010 10:58 AM

Nice write up....thank you for this!

will 01-31-2010 11:33 AM

Very nice Randy. It still amazes me how this all works.

Toms Blue Ram 1500 01-31-2010 02:20 PM

I knew it was done threw the lifters but didn't know how . Thanks for the insight .

MADDOG 01-31-2010 02:36 PM

Great write up, Randy.


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.

RamTech 01-31-2010 02:49 PM

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.

MightyBlueRam 01-31-2010 03:47 PM

Thanks for the info! Very interesting stuff and it amazes me how complicated engines can be.

RamTech 01-31-2010 03:51 PM

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.

MightyBlueRam 01-31-2010 04:13 PM

Oh wow, I just noticed the new "ASE/CHRYSLER certified technitian" badge!!! Looks great.

RamTech 01-31-2010 04:15 PM

Just got that a couple of hours ago. Stacy made that one possible.

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