I have never seen a car fire where a magnesium wheel or engine block have gotten hot enough to burn. I have had large bon fires in which we threw mag engine blocks in, and let me say, WOW , now thats a fire, super white super bright, and hot, and if you put water on it it aggitates it and makes alot of popping and flashing. IT isnt recommended to use water to extinguish a mag fire. Dry powders and chemicals are more efficient.