It's bad for the engine, but good for the car (6)
I believe the answer is:
carbon
But I can't explain it at all! Could you help me understand?
(Other definitions for carbon that I've seen before include "Non-metallic element, C" , "Flimsy" , "Eg, diamond or graphite" , "Abundant chemical element, symbol C" , "C (with a footprint)" .)