I have done the same thing. Nice glowing catch fence and a black background, no race car in sight. There used to be a guy who posted here named Chris Greenburg and he was shooting professionally on the ALMS schedule. He had really nice gear and shot mainly ISO64 slide film and used a powerful TTL flash to supply fill and better freeze the action. A flash with a decent guide number like 150 or better cost more than my entire camera kit!
I have found that the flash will only work if you are unobstructed viewing. Anything in the way and it makes a giant reflection and kills the shot.
I have a nice new book by John Shaw called "John Shaw's Nature Photography Field Guide". He shoots a lot of birds and other animals in action from long distance in broad daylight. In it he describes all kind of ways to get great shots without spending huge money. He uses a standard SB-24 Nikon flash with a fresnel concentrator. Its is not cheap at US$40 but far cheaper than a US$300 medium range flash. It can be found at
www.kirkphoto.com. I am thinking about getting one to use with my Sunpak manual flash. I have not used the flash very much becasue it is just too short range for what I am usually doing and my Canon can only run 1/90th a second max without TTL flash. Thats just too slow for most motorsport pics.
I normally shoot at 1/250th or 1/350th a second and the f-stop wide open (as I can get it anyway).