This is a very good question. After thinking about it, I believe that the alternator will maintain power and the engine will continue running with either type of fuel pump until:
A. You switch off the key inside the car.
B. The High revving engine throws a rod or suffers other internal damage.
Scenerio C. If your car is like mine and it has electric fuel pumps and a carburetor. The ground connection of my fuel pump is connected to an inertia switch. If the impact was hard enough to trigger the switch, the engine will stop running when the fuel in the carburetor drops below the jets or it blows up. If the switch does not trigger (very likely in a soft rollover), it will keep running.
Scenerio D. If your car has elecric fuel injection with electric fuel pump, it probably has the Mustang's inertia switch, too. If the impact was hard enough to trigger the switch, the engine will stop running quickly when the fuel pressure drops. If not, it will keep running.
After considering this, I want to move the ground of my ignition system to my inertia switch along with the fuel pumps. Then, I will move the ground wire from the inertia switch to a separate connection on the external cutoff switch. If it doesn't have a second connection, I will use a relay to make sure that both circuits are turned off with the switch.
It is simple to test this. With the car idling in the driveway, turn off the external switch. Surprise!
Again, very good, thought provoking, question.
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