The Bi Turbos are really bad for this and will not allow much devition at all. But, basically you should see an OverHeating icon in the upper left corner of the screen. The best way to deal with that right away is to adjust your driving style and lower the rpms some while keeping up the speed for the air flow around the engine.
In the garage, increase the Radiator Size for more coolant flow, adjust the transmission for lower rpms (ie a longer gearbox), reduce downforce to reduce drag, and increase the tire pressure.
Sooo, rots-o-ruck mon.