Perhaps someone with air race experience will post here. While I've never attended an air race, I have been to several air shows over the years. My best educated guess is that there will be some sort of "air boss" calling the shots from the ground. Frequencies from 122.050 to 124.000 seem to be the most prevalent choice for the boss. Now, as to what extent the aircraft will communicate while competing, someone with first hand experience would have to post. Will the pilots talk to each other as they race? Will each plane use a seperate frequency to communicate with ground crew? Who coordinates frequency usage?
Since most air races occur out west, maybe we can search for and/or pose the question to groups out that way.