20SE05
This followup will be short, I promise.
Reduction in "pumping losses" is why Deac improves economy. By pumping losses I mean the work required to pull down on intake against the restriction of the throttle blade. When in 4 cyl mode, a deac engine has it's throttle blade further open than it would in 8 cyl mode at the same speed-load point. Think: working harder on the remaining four cylinders. Further open means air flows in easier, i.e. less losses.
If you mean the frictional losses of the piston going up and down with both valves closed all the time, yes of course that is an unavoidable loss of efficiency, but it's not that bad, cuz the work required to compress the air is recovered when it expands. You only lose the friction, which isn't terrible with a warm engine and the low tension rings they use now-a-days.
Saab's Variable Compression engine is no doubt still around, and still in development. (Did you hear about the one that sparked from the plug to the top of the piston? Those Swedes!) These things take years. I've also seen one from FEV, a Euro engine design company in their Detroit office. It's a steep uphill challenge: added complexity means lower reliability, and more variables means tougher to meet emissions reg's, which is the hardest aspect of designing an engine. And when a new engine line costs many hundreds of millions, they tend to be a conservative bunch. Believe me. Very conservative. On the other hand, engines are now extremely reliable when you think about it.
OK, not short, but let's call this one medium. See you around. Good questions.
Jim N
'73 2.0