It just occurred to me that my idea of screwing with the cam timing on a conventional engine probably wouldn't work. The compression ratio would probably end up way lower than you would want it, taking away some of the inherent efficiency of an atkinson cycle engine. I guess you would have to mess with the cam timing and stroke it or higher CR pistons.

I understand that a somewhat lower CR would prevent knock, but if you take it too far you start to lose thermodynamic efficiency again.

Atkinson cycle engines use a cylinder head or piston deck height that would produce a high (~12:1) compression ratio in an engine with conventional valve timing. This engine would knock severely on pump gas.

So, Atkinson's late valve closure reduces the compression ratio to something manageable (~9:1?) bet the expansion ratio stays at a nicely efficient 12:1.