I am far from an expert, but there are two ways I know of to do something like variable compression, one is that Saab technique already mentioned, where the cyl

I am far from an expert, but there are two ways I know of to do something like variable compression, one is that Saab technique already mentioned, where the cylinder head is displaced to change the size of the combustion chamber. The other related scheme is the Atkinson cycle: https://en.wikipedia.org/wiki/Atkinson_cycle https://en.wikipedia.org/wiki/Atkinson_cycle The wiki page shows a complicated mechanical arrangement for having a smaller intake volume vs. expansion volume, but the "poor man's" way to do this is to just keep the intake valve open for the first part of the compression stroke. The upside is that you can then adjust the effective compression ratio with valve timing. The downside is that it reduces the intake manifold vacuum by blowing back some of the intake air, not sure how that's handled. Don't know what the Nissan guys are doing.