A belly pan question

[wmhjr]

How do they design them from the factory?

With massive computer modeling to determine all the thermal and fluid dynamics. You have to worry about all kinds of variables, and it would take some serious track time and constant modifications to try and trial and error it. I would be very concerned about creating hot spots where heat would not extract, as well as unintended lift at speeds. You would also very possibly increase engine temp if done incorrectly, as you might create a high pressure area in the engine compartment, decreasing air flow through the cooling system