Krusty, its a mmHG system.
Basically the "Red Zone" is the power settings for combat, and the "Black Zone" is lower settings for normal flight/cruise.
(besides, really, the Ki-84 has fuel efficiency great enough to not have to use cruise at all. More than 40 minutes at deck at full power - pull back on the throttle and RPM a little bit and the flight time easily soars over 60 minutes)