Magnesium engine block fire + water
Here's a short video of what happens when you try to put out burning magnesiun with water. Makes you think those vehicles should come with a large warning for rescue personal. Sidenote: A CO2 extinguisher with also cause a really neat, uh, I mean bad reaction.

My theory about why magnesium engine catch on fire easier than mag wheels is the heat generated in an engine fire is much more contained and can't escape the engine compartment. The rims are exposed to the air and so the heat can radiate away.

-Blast