ok, 2 ways i can think of doing this. Its mainly in the visuals:
1. Change the animation of the back of the car exhaust. Currently its bluish. Make is more intense and look like fire. U can also add some "sparks" particles at the wheels to intensify the effect.
2. another way, is a costly way (in terms of computation): make a motion blur effect (similar to Need for Speed) when u power up things. U will need to apply a motion blur shader to the screen for this. (Its quite complicated).