If you are interested in capturing motor sports images, I would suggest that you look at some magazines to see how the pros do it. I agree with the suggestion that you should pan with the subject to give a more dynamic look. In these shots, you have captured very well the motion of the dirt clouds, but the cars look stationary. It might appear that they have run over some explosive device! Try panning shots of vehicles on the highway for practice. You can get some interesting images, even with ordinary vehicles.
Alex