"So lets look at the example of an engine that is using higher octane fuel than it is designed for and that is therefore burning slower. All other things being equal, it will then make slightly LESS power, run hotter and use more fuel because it is effectively running retarded."
So that's the reason my Diahatsu ran hotter, cleaned the exhaust system and change the exhaust colour from black to
brown. It couldn't adjust the timing electronically! Led me to believe that higher octane ran hotter , but now I know it burns slower.