I think it would be ideal if you open at 12 but start racing before 5 (say, 2pm or 3pm). The problem I see is not when you open, but when we start racing. There's a 5 hour gap there. Opening at 10am and racing at noon would not leave much time for people to run errands in the morning (if they have to) and make it home in time to get all of their equipment and drive to the track.