radiators act as surfaces that stick out the more you open them, which in turn disturb the airflow around them, affecting the aerodynamics of the AC thus reducing it's speed.
that's the case in RL, and portrayed with reasonable accuracy in CLoD. there's no reason to go fully open all the time, just monitor water and oil temps frequently and adjust accordingly.
as a rule of thumb you can go with oil rad at 50-60% but you'll have to work with the water rad much more frequently; worry if the oil rises above 85ºC, water must not exceed 102ºC for long, but you'll soon find that once above 98ºC you'll be struggling not to let it rise; above 105ºC it's an engine wrecker, if you can maintain it bellow 97-96ºC then you're in optimal regime. high RPMs tend to heat up the oil, but reducing RPMs and opening the oil rad up to 4/5 takes care of the issue; the water heats as the engine goes hotter, all you can do if it starts to overheat is go full open and throttle back for a while.
for more info you should take a reading at the many threads discussing this, it's not very difficult to figure
as always, practice makes perfect; happy flying
