when the car is moving it creates a pressure difference between the vessel and the outside, so any motion is enough to do this, but to completely clear the engine bay, I'm not 100% sure, but there wouldn't be any part of the engine bay that would allow a built up of static air when the car is moving, so there is always going to be a degree of warm air within the engine compartment since that is where it is being generated, but a static pocket is what would be required to cause serious heatsoak, and that can only happen at standstill, that's why rolling road tests are conducted with the bonnet open, I would hazzard a guess though, that anywhere above 25-30mph would make it very difficult to trap air in an engine bay