This is a huge problem. The Mazda serial cooling scheme
was done for low cost but it soon becomes a problem
when building high HP engines. It is also called axial flow.
The water goes in the front rotor housing  and works its way
to the back rotor housing. Consequently the rear rotor always
runs hotter.

Most car engines are the same serial cooling.

As far back as WW I parallel cooling was used on water cooled aircraft
engines. Each cylinder got the same water temperature.

Heat pipes may have a role to play helping to cool the
lower spark area.

Paul Lamar

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