This is a good question and we now have more information. I found some more photos to illustrate the differences

H75: Open combustion chamber
H89: Closed combustion chamber

Open (full-circle) combustion chamber is superior for lower NOx emisssions

Closed is superior for power, and superior for fuel economy. It is not due to higher compression ratio, it is due to quench and requires flat-top pistons to get the optimum power. In the JDM B310 H89 model they used dished pistons to lower the compression ratio (to reduce NOx emissions), however they are offset-dish which allowed some quench

H75 open chamber

Attach file:

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  H89 combustion face.jpg (61.73 KB)


  H89 closed combustion chambers.jpg (53.68 KB)


  H89 heart-shaped combustion chambers.jpg (58.83 KB)