50D's, either a 24-70mm L or a 70-200mm L...........and whatever ISO it takes to get my shutter speeds to what I want, as there is no set formula I use, as it depends on whether I am shooting pans from the side, head on shots that are backlit, or head on shots that are front lit, or if the track is rough or smooth will also have a bearing. Even the type of car on the track will have a bearing as to what S/S and ISO I will use, as they all corner differently. I even find a difference needed between shooting the 360's and the 410's, as the 410's are much more violent in their acceleration.
It's alot of trail and error, and then toss in that every week, the light is changing and the sprint cars don't always run at the exact same time, it makes you work, but the effort is worth it, since I don't end up with basically the identical shot week after week.