Hi Neil,
I disagree with the idea of using racing rules in application to passenger cars.
- Racers generally have a much larger ratio of HP available to HP in continuous use (due to desire for highest possible acceleration), so if the racing rule references HP available than it will bias you strongly towards smaller openings. Please note again that my numbers above are in reference to HP in use rather than HP available - sorry, that does make them more difficult to apply. You can probably cut your HP spec by 2-4X, but I argue it makes more sense to look at the application rather than the engine. I included in^2/hp to make it easier to separate effects of power demand vs ambient conditions.
- Race tracks generally don't include sustained climbing. It seems quite clear to me that extended climbs present the most strenuous need for grill opening in passenger cars. [Edit - this next bit was irrelevant] In addition to my calcs, I remember that AndrewJ ran into his first serious issue with a fully closed grill while climbing a pass. (See http://ecomodder.com/forum/showthrea...cs-312-10.html) (This is also an argument in favor of some kind of remote or automated variable grill opening, unless you want to get out of your car in the baking sun and adjust your grill before and after you climb, or if you can avoid the mountains.)
- I'm pretty sure NASCAR doesn't run their Phoenix circuit in July or August(?). Even if you yourself never climb passes in life-threatening heat, it's worth knowing how strong the effect of ambient temperature can be, and when to expect your grill block design to be limiting.
I'm also not convinced that the drag through the cooling system affects the cooling system efficiency... as opposed to Cd.