RE: How does a gas engine act when hot?
Louvered or hooded exits work as long as the prop is turning and makes air flow past them. Certainly higher air flow results in higher negative pressure production.
However the lower airflow at idle doesn't mean overheating...don't forget the engine is at or near idle which means less heat generation and lower cooling demand
If some don't understand what I mean by "hooding" you can see a few snaps at my Derivative thread in the Pattern Forum. On my latest piped DLE55 design, inlet area is around 7 squares directed onto the fins while exit area is only8 squares. There is NO issue with cooling no matter how extended the vertical in 90 degree environment; that's how effective this is. Try it, it's straight forward!!