Original question: 100% water “cools” the best. In other words it takes the most heat out of the engine.
Then the questions are:
A)Is there enough coolant flow to remove the heat the engine produces?
B) Is the coolant rejecting enough heat to the air?
This gets difficult to sort out because the change in temp of the coolant from radiator in to radiator out is usually under ten degrees F so we’re looking st pretty small changes in coolant temp.
I think you suspect your problem is question B and that makes sense to me. Then the issues are radiator size and adequate airflow. And I would try to improve airflow.
