Greetings,
If the Supercharger fails, you roll on the Critical Hit table to determine how many crits are assigned. Then apply them.
Now, look at the Vehicle Record Sheet. There are three boxes for motive system crits and one box for Engine. If the Supercharger fails, mark off on of the Motive System boxes. Only when these three boxes are filled would the Engine be damaged.
I hope that clears up the confusion.
Best,
Joel BC