Spark plug will fire, but the fuel injector wont. That is the fuel saver mode.

Oddly, a lot of people report a gain in MPGs after turning off MDS with a tuner. Just a thought though.

 

It is done by de-activating the lifters, and sending the oil to the exhaust valve which opens to bleed off the air pressure, this picture shows the oil flow into the lifters and where it is shut off ,no fuel is sprayed either when this is happening and it also shuts off the spark on those coils.

 

So if it is shutting off spark and fuel to those cylinders would'nt they be cooler than the 4 remaining cylinders that are firing? My thought process is that the 4 hot pistons would be wearing their piston rings faster than the cooler ones, granted you wouldnt notice this for 100k plus miles.

 

I read/thought I saw in print that spark is NOT deactivated to those cylinders.

 

It is deactivated, here is a pretty cool clip of it working, but you have to watch it a couple of times and watch the outer shell of the lifter on the right it will not move with the rest of the lifter
http://www.youtube.com/watch?v=x4RjE...layer_embedded

 

Go to the official Dodge Ram site and watch the video and read about how it works. Its all there and pretty interesting actually. Also I read a long article in some car mag sometime back that although the same 4 cylinders are deactivated all the time, after hundreds of thousands of test miles, they found no wear difference between those and the cylinders that weren't deactivated. Looks to be computer controlled (of course) and works through the lifters on the cam if I remember right. I was convinced enough that I bought one!

 

I'm thinking of getting rid of the 275/70/17s on my TRX4 for some 265/70/17s because they are way lighter like 7 pounds per tire. I would bet although I might see 50 more rpm at 75 MPH, MDS would stay on longer. Less torque needed to turn them. I would hope to break even on highway mileage, but improve my in town mileage, and get better braking and cornering.

 

Lighter tires will make a big impact. THe tires I am looking at are 3lbs heavier than stock, so I expect to loose a couple tenths.