a cap can be charged to any voltage as long as the manufacturer's rating say it can handle that voltage. how long a cap can pump juice depends on the resistor and the cap itself. the time constant is calculated by R * C and since most caps that fit on the car have ratings in the micro farads, the time constant is not big enough for it to power the motor long enough for u to notice a speed change.
gosh, all those times i fell asleep in circuits class listening to the boring lecture, i mgiht have accidentally picked up something between my naps
