Ok, so now GM and Dodge are bragging about Mulitple Displacement or Displacement on Demand systems. Which, if you dont know, means that the engine in your truck will switch from 8 to 4 cylinders and back, when the need is there. It is supposed to increase fuel mileage but I dont know by how much at this point. When Ford was developing the Modular engine, back in the early 90's the two things they bragged about were parts interchangeability and multiple displacement systems. 15 years or so later, I have heard nothing new from Ford on when this might happen. What would be even better is if they could Flash the computer on older Mod motor vehicles like 02 Expedition to an upgraded program that would do this. They already have it set up that if your engine severly overheats it shuts off fuel and spark to half the cylinders to use them as air pumps to cool the engine off, so it would seem a no brainer to be able to re-flash and have Multiple Displacement. Of course the price would have to make worth while, if you get 1mpg better mileage and they charged you $500 to do (what a joke, you know they would charge more), it would not be worthwhile. It would take to long to realize your return on investment based on what it originally cost vs. savings in gas. Just my $.02, whats yours?

 

You could find out the firing order, buy some switches from radio shack, along with some good wire and splice the switches into the injector power line of selected injectors, route the switch to the cab and have your own multi displacement system for about $15.00.

 

There's a lot more to multiple displacement than killing 4 cylinders. You have to keep the valves open to prevent the cylinders from building up compression and working as a pump. If you've ever driven a vehicle with a bad plug, imagine driving one with 4 bad plugs. I think a lot of the variable displacement is to meet the government fuel averages and it doesn't work in real life. I just read a full test on the 2007 Silverado with the 6.0 engine. It was rated 15-19 but they got 14.1 on a long interstate trip. My cousin has a 2004 3/4 ton with the 6.0 and he can get 15 or a little better with the same type of driving.

 

I was kidding, I hope nobody takes that seriously.

Just a guess, but did Ford announce the multi-displacement 'after' Cadillac announced theirs? I'm guessing they announced it so the "Well, they have it so why don't you have it" crowd would be happy then they waited long enough to realize that it was a turd so they scrapped it. Maybe they've made some advancements in the last 15 years to make it feesible, maybe not, real world use will tell.

 

Yeah this has been tried several times over the past 20 30 years at least. The mileage improvements are not what you would think/hope. I. E. 4 cylinder mileage is 2x better than 8 cylinder, NOT. If I recall the latest Chryslers get maybe 20% improvement and the key to getting that improvement is driving the engine gently enough that it doesn't kick back into 8 cylinder mode.

The reason the improvement is not linear is that you still have all that metal mass and friction from the cylinders that are deactivated. With the valves open the engine still acts as an air pump so you have losses from that. With the valves closed the dead cylinders act as resistive springs.

I always worry this just makes for more complication and more things to fail and more expensive repairs. Not sure about the new designs but the early 80s GMs had terrible reliability records with the 4/6/8 engines.

Even if it works well I wonder about uneven wear and tear. Probably nothing, but makes me wonder.

Probably better to just drive gently with whatever you got.

I think they are a marketing gadget with minimal benefit to the consumer.

Just my opinion.

Jim Henderson

 

I believe there's even more to it than that. If you take a V8 and cut two cylinders off one end so you have basically the same engine but a V6, the only thing that has changed external to the motor is it weighs 25% less. You are still moving essentially the same mass/weight (the vehicle and load) which can be translated into an amount of work. That work is the same regardless of there being a 4, 6 or 8 cylinder engine in the vehicle (minus the difference in the engine weight). Work is performed in an engine by burning fuel, so it goes to reason that fuel consumption will not be drastically different.

Using the correct number of cylinders to make the horespower required for the task at hand will save a little in fuel just because too small an engine will be overworked and too big an engine is overkill, so you can even out some of those differences but you'll never get a proportional fuel savings for each cylinder taken away.

 

The old Cad 8/6/4 was a mechanical set up, and Ford and Eaton were working on it originally. Ford rejected it. Cad picked it up, and it failed as it was not a good system.

The new stuff closes the valves. The air in the cylinder acts as a spring. It has to be compressed, but it also pushes back on the downstroke.

According to magazines, these systems are "seamless" in their operation.

I haven't seen any numbers to show if they actually work well enough to justify the cost.

If you can get 20% by driving easy with it, I think that's about 10% more than just driving easy..... Your mileage may vary.

 

yeah, if you take away 50% of the cylinders, 20% of the time, that would be a max savings of 10% from the multi-displacement system. I wouldn't expect too much more than that.