basically there's thigns that make power and things that take it. Caps certainly don't make it. Also, OF 1farad of capacity, you've only got a small fraction of that that can be used seeing as you're only dropping voltage from 14.4 to say 13v the full capacity of the cap would require draining it to 0 which would mean your car's dead. Now say you're driving, and playing a song with a rolling bassline. the cap drains, then starts pulling heavily from your alternater to recharge it, but as it's doingso, another bass note hits. so now you're essentially worse off than you were before because not only does your alt have to give power for the amp, the battery, and all of your assorted running gear, but it's got a very power hungry capaciter greedily trying to suck up whatever juice it can get as fast as it can as well.