Back to Mazda. The major thing they did to allow that high compression ratio, up to 14:1 in Europe on premium pump gas, was to decrease the temperature of the incoming air charge.
They do this two ways: first, some fuel is injected on the intake stroke, evaporation cools the incoming air, then more fuel is injected during the compression stroke.
Second, the exhaust gas residual in the chamber was found to have a major heating effect on the intake charge, so they cut it in half by using a 4-2-1 header. There is also some business about the piston top, which features a nice hole right in the center. I did not understand the explanation they gave, something about not heating the top of the piston.

It's fascinating to see developments like this.

R.