Frankenbiker,

I'd have to dispute the claim that the deactivation almost never occurs. Granted, I'm not terribly intimate with the GM system (I test drove one and saw the little 4cyl/8cyl display on the odometer area of the gauge cluster), but I'm pretty familiar with my Dodge after just about 5K now. The MDS is easy to detect, to a tuned ear...you can hear a difference in the engine sound and in the exhaust sound, especially with windows down. You can also feel the point in the throttle where the MDS turns off and all 8 cylinders go to town.

My truck spends a lot of its life with me and a toolbox as its load; I have to have one vehicle that will cover my commuting, hauling and towing needs. Living in Northern Illinois, I drive ~65 miles/day commuting. Of that 65 miles, 60 miles is on flat, country roads where I'm going 55-60mph. In those conditions, the MDS is on nearly 100% of the time...even when decelerating to 30mph to go through a town, and even when accelerating back up to 55-60. Sure, if I drive like a jerk, I can make the MDS stay off, but why?

I can say with great confidence that the MDS is active for 85% or more of my daily 65 mile commute. The remaining 15% constitutes the first little bit before the engine is warmed up (when MDS is inactive), and the last 5 miles in town. I can also say that deactivating the MDS causes a very significant drop in fuel mileage.

I'm getting mileage with this truck that is more than 50% better than I was getting with an Expedition 5.4L/4R100 combination...and that's assuming that the Expy got 15mpg (which it did not average over my 72,000 miles). Attribute part of that to 3:55 gearing, compared to 3.73 in the Expedition, but I attribute a lot of it to the MDS. Add to that an additional 85 very vocal horsepower over the Expedition, and you see where my original comment came from.

 

Now with that MDS, with it on, does it always use the same 4 cylinders, or does it periodically cycle through all 8 to keep engine wear consistant?

 

Not having really ever studied anything made by DCX except my Freightliner, and the Cummins-based Dodge trucks, I can't say I know a lot about DCX's system. I did take a little bit of a look at GM's system, though; not from a purchase standpoint, but from an engineering standpoint, to see how they were going to try to overcome the old Cadillac 6-8-4 variable displacement debacle. Most of my curiosities were satisfied, in that newer, much more powerful engine electronics, and drive-by-wire throttle control, were fixing the biggest complaints of the older system.

The system itself seemed to receive mediocre reviews from the automotive press. Not because it performed poorly, but because their testing showed that it hardly ever activated, based on spirited freeway driving. In this case, between Los Angeles and Las Vegas. Virtually none of I-15 is flat, and the average speed of vehicles with less than 10 wheels is 80-90 MPH.

Here's the difference. The system is designed with "relatively low speed" driving, which is rarely experienced by folks in Southern California or Michigan, where the various automotive press folks are located. 55-60 MPH on SoCal freeways will get you killed. In rural Illinois, it's normal.

Two different scenarios, two different results... both valid.

(no harm, no foul. )

-blaine

 

Blaine,

I agree...I'm not sure what the parameters are as far as max RPM or max speed that the MDS will operate (or if it has such a cut-off...it may be entirely based on engine load), but I've never had it operate above 75mph. I suspect the load would be too great above that speed (I had heard that it doesn't operate above 70, but have clearly heard it operating with the cruise set at 73.)

In answer to the previous question, it always uses the same 4 cylinders. The other 4 keep moving, but with the valves closed to reduce pumping losses as much as possible. The only difference in wear, then, is the fact that 4 of the cylinders don't have the force of compression and combustion. I can't see that making any significant difference over the reasonable life of the engine.

The DCX system uses oil to keep the valves shut, and cuts off spark and fuel to the 4 dead cylinders. It's a bit like the old slant 6 side oiling setup.

For my purposes, it works great--but I can see how it wouldn't help some others, under different circumstances.