CR has nothing to do with how much air is drawn into the motor, and variable compression has nothing to do with valve lift. The SA engine has a static 13:1 CR. This is altered according to engine load by changing valve duration, holding the intake valves open into the initial part compression stroke.I honestly didn't even think about variable compression. My old car only had VVT on the intake cam but I never though in terms of compression changes due to the change in valve lift allowing more air to pass in to the cylinder altering the compression ratio. I'll have to read up on it to see how it's implemented and how far it's come! Seems pretty fascinating and really changes my mind on how these cars might react to boost.
I'm always sort of averse to retarding timing to stabilize since that sort of seems like a crutch but I'm also not an engineer or someone who tunes for a living, just someone who reads a lot of technical information all day.
Thanks for the excellent response! And if you could point me in the direction of some more in depth information on the design intents of the Skyactiv-G platform I'd love to dig in to it more to understand what this motor is doing so differently from everyone else. ?
Using variable valve timing to prevent knock is the best way to do it. I'm pretty sure that the building full of engineers at Mazda who designed the motor knew something about what they were doing....
As for more information about the SA engine, just google and you will find plenty of information. Go on Youtube and search for videos by Dave Coleman, the lead engineer for the Mazda 3, lots of information there.