[{HVY-E}Jinxx]

~S~
I have read that the only plane who's speed increased when radiator
was fully opne was the P-51. In the D model the speed actually increased
by 5-8 mph faster. I guess the radiator was design as not to influence the
drag, when the P-51 radiator is fully open the hot gas push out the rear of the radiator which caused the increase in speed. At what altitude did
this happen I am not sure, however it must be above 18-20,000ft.

The increase in speed is actually caused from a type of 'RAM AIR' effect as air enters through the intakes and is forced out the exhaust at the back of the aircraft.
There is a name for the effect, but it escapes me at the moment.
To my knowledge, it wasn't effected by alt, but by speed.