Yeah, flash is a PITA, when there is any dust, smoke or fog in the air, and at a dirt track, theres, a pretty good chance of having that happen.
I just adjust ISO to get the shutter speed I need, depending on track conditions. If the track is slick and hard, I can use a slower shutter speed. When the track is soft and gets rough, I need to bump everything up, due to the fact the cars are bouncing around violently. Also some areas are lit better then others, and that will have a bearing on what settings I use. Also it depends on how well I am able to track cars, on any given evening. Hoping I bring my A game when the WoO are in town next week.
If I had a camera that handled higher ISO's better then the 40D, I'd be shooting diferently at times, then what I do now. At night I use my Canon 24-70 f/2.8L, and may mount up my Sigma 70-200 f/2.8 for WoO, instead of my 70-200 f/4.0 IS.
There is no "magic" settings to success.