So this winter/spring is phase 3 of my 3 year project. Paint and Body. Think I wanna swap out to a Trans Am hood. But had a really stupid question:

The louvers in it, where does the water fall thru exactly on the engine compartment. Would this create a negative effect or under hood rust on a car that does a lot of sitting? Especially in the winter?

 

Are you referring to the GTA style hood with the two front vents and two linear side vents?

On mine these are covered completely by the under hood structural supports and the insulation. They are not visible at all from the engine compartment. To answer part of your question, water goes down into the hood supports. I live in Arizona so rust isn't really an issue here.

 

In areas more prone to rain, is corrosion common in these hoods, due to water being trapped between the inner supports and outer skin? I've never noticed them rusting, but it seems like a logical concern.

 

There are drain holes. Cowl vents are drained to the sides, probably why the firewalls rust on some.
My air inlet Power bulge has a tray under the opening and a hose from there. The factory usually provides a drain path.

 

I have never had any problems with rusting around the louvers. Lots of snow and rain up here where I am. I would think if the hood was covered in snow long term, there might be a chance of problems then.