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I think, and I'm not completely sure, that GM's cylinder deactivation simply shuts off the pulse signal to the electronic fuel injection to certain cylinders to only fire 4 or 6 in a V8 depending on power requirements. the coils still fire a "waste spark" and the valves still open and close off the cam shaft lobes just like as if the cylinder was producing power with a fuel charge. I assume this is all computer controlled by the PCM (powertrain controll module) So if the computer can determine shift points, fuel injection pulse widths, fan on and off, etc. all based upon sensor outputs why is the power on demand cylinder deactivation not also able to be programmed ??
The pistons are still moving along with all other mechanical parts, there is no seperation so there is still a ton of wasted energy and inertia but for adding a few MPG on low load cruising it is worth it especially on trucks that are not always needed at full power.
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