MDS Operation - 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.

Great Info here

Awesome info.

Comparing the two effects of running MDS or disabling MDS with a tuner, I have noticed a change in performance.

After running with MDS off for appx 3 months I changed it a few nights ago, I have noticed better shifting with MDS on. Truck actually seems to ride alot smoother on bumpy roads.

Is MDS something that shouldn't be disabled? With MDS off I no longer have exhaust drone, but I loose gas mileage and have different shifts.

It's a personal preference. Most people get rid of it due to the exhaust drone with aftermarket cat back systems.

Ok... Maybe I'm missing something but isn't your engine is working to repeated compress the gasses trapped inside the cylinder.

Or is it timed on the crank that when one cylinder is compressing another is decompressing therefor yielding no extra work? I still don't see how this will work because if I recall correctly volumetric compression in a cylinder isn't linear but exponential.

I believe the thinking behind this is that while the engine has to work to compress the air trapped in the cylinder, it also pushes down on the piston once it goes past top dead center and helps create power.

The theory I have Is the air / fuel mixture is compressing, but has little if any detrimental effect. Because the compression stroke is no way near the threshold of a diesel engine. Compression is not high enough too create heat for detonation and therefore allowing mds too work effectively. Course I'm usually talking up a good bs session at this point. Seeing as I have no idea what I just posted, lol.

BigHorn 4 Life said:

Is it ok to use this with a lifted truck with 37's? Or should I put it in manual mode?

Click to expand...

My guess would be it is totally fine. Since the system switching is based on load, throttle position, RPM, and gear it wouldn't matter if you had 47's. It just won't be in MDS mode as often as if you had 17's.

Sweet 17's. :********:

or stock 20's for that matter