Cylinder deactivation works to save fuel, but it may not be for the reasons you think. The main misconception is that shutting down half the engine's cylinders reduces fuel consumption because the half that are shut down aren't using any fuel. The reality of what's happening is a little different, because at whatever engine speed the switch from 8 to 4 cylinders occurs, the engine will put out the same amount of power (if designed correctly!) whether it is in 4- or 8-cylinder mode. When the change between 8- and 4-cylinder operation happens, things get busy in a short period of time, and understanding what's happening is not only very cool, but can give you the upper hand when deciding to modify your Hemi engine.
During MDS activation, cylinders 1, 4, 6, and 7 are shut down in order, with one cylinder being deactivated every 180 degrees of crankshaft rotation. Part of the cleverness of MDS is that a change to 4-cylinder mode
—called "Eco" mode as displayed on the instrument cluster
—happens without any apparent loss of power. In order for the driver to perceive no loss in power, the ECM has to ramp up both the throttle opening and injector pulse width of the cylinders that remain running. So as cylinder "1" goes off line, the next cylinder in the firing order
—number 8
—must receive an increase in fuel and air by the same amount taken away when cylinder number 1 was shut down. Two crankshaft revolutions later, all four of the deactivated cylinders have shut down, and the remaining four holes have been given the shortfall of fuel and air missing from the deactivated holes.
The Real Reason Cylinder Deactivation Improves MPG
Cutaway version of a Hemi MDS lifter. The key thing here is the transverse pin exposed to oil pressure from the orifice in the lifter body. As the fluid circuit energizes, the pin is pushed in, unlocking the plunger inner body from the lifter's outer body. In this manner, the roller tip can follow the cam lobe without the plunger moving the pushrod up and down.
If you're asking "where's the fuel savings in that?" you aren't alone. The 20 percent improvement in fuel economy from cylinder deactivation isn't from four cylinders of fuel going away, it's actually from a far more modest reduction in pumping losses. In its simplest explanation, pumping loss refers to the power an engine spends sucking in fresh air and fuel past the throttle, then pumping the spent exhaust gases out. It's a mechanical tax, and it works like this: Imagine a big V8 running down the road at a cruise speed of 70 mph. At low load and low rpm, the throttle on that big motor is barely cracked open.
That sounds great, until you pluck that imaginary big-block from between the fenders and stick an in-line four in its place. If you could swap engines that quickly—in a heartbeat—you'd notice that to keep the same speed, you'd need to open the throttle much farther. That little four banger is getting better fuel economy, and because the throttle is nearly wide open, the engine doesn't have to work so hard to suck air past the throttle. You'll never think of WOT the same way again because WOT is actually when your engine is most efficient—not the least efficient.
