Burnout Revenge

Play through the game and they unlock as you go or complete certain take-downs and race sheet requirements

There is really no definite answer to this question, because it depends on the car you are driving. The better your car, the better their cars. If you are in an extremely good car like the Limited R205 GT (which is unlocked late in the game) you will be more likely to encounter special cars, if you are in a special car, it is almost certain you will have the chance to Takedown a special car. And the higher rank races have more special cars. Try a high rank Crashbreaker Road Rage in a special car if you have one unlocked. Try for the Motor City Challenge Sheet first-it unlocks the Etnis Racer, which is a 209 mph Medium weight car that is hard for other cars Takedown. The heavier the better for doing takedowns!