Coming from someone who doesn't know jack shit himself.

Caps are great for solving certain kinds of problems...

What you need to do is figure out what issues you have, and what will solve it. A good logging voltmeter is the perfect tool for troubleshooting your amp rack wiring, but any reasonably good analog meter will do fine.

First off, set up the pieces. Start the car, and have someone keep it at 2500 RPM or so. (I used 2800, because that's where it stayed when I proped the gas pedal with a stick. ) Turn on accessories like A/C (or heat, whichever is appropriate) and turn the fan on high, too.

Now pick something with a fair amount of bass. Probably in the heavier range of your listening preference. If you don't tend to listen to too much bass, then borrow something. Turn it up!

Measure the voltage at the amplifier terminals. (Not just the sub amplifier, check all system amps!)

Whenever a bass note hits, you will probably get a dip in voltage. Record the peaks at both the top and the bottom of the dip. While maintaning listening level and RPM check again in a couple of minutes. (5 or more)

Now, here's where the diagnosis comes into effect.

If your peak voltage is below 13.5V in your first test, then your wiring is probably weak and should be upgraded, or you left hte tunes cranked for too long before you started your test. Your amp is choking on what you can feed it through the wire that you've run. Of course this assumes that your charging system is working properly int he first place. In my case, 13.8V -- Not a problem.

If the peak voltage is lower in the second test than it is in the first test, your amps are drawing more current than your electrical system can provide. Some drop might be expected, but anything noticeable indicates that your electrical system is overloaded. (This is why keeping the engine at or slightly below highway cruising RPM is needed -- to gauge how effective the alternator is.) Again, in my case, 13.8V -- Not a problem.

In either of these cases, a capacitor will be of no use to you, although it will still resolve the following:

Now, it's time to look at the dips in voltage whent he bass notes hit. In both your tests, they should be about the same magnitude. In general, the rule of thumb that I've seen get reasonable results is 1F for every 1V range for every 1kW of RMS class B amplifier power. If your voltage range is larger, you need more, similarly, if your amount of amplifier power is greater, then you need more. Note: Class "D" subwoofer amplifiers take considerably less current than class "B" amplifiers. (If you have class "A" amplifiers, then you ought to KNOW what your amp needs to feed it! Class A amplifiers need completely different troubleshooting, because they drain power differently than class "B" and class "D" amps do.) For class "D" I'd say that you'd take it at a 1/4 value for that rule of thumb, so 1000W RMS of class "D" power can be treated as 250W RMS of class "B". -- That's not to say that it's less output power, but because it's more efficient, it IS less INPUT power.

The reason is simple. Long wires act as inductors, so the big long power wire to your amp rack resists changes in load. The capacitor, in parallel with the amplifier, and at the amplifier end, balances the changes out, by supplying the transient current needs (Note: Not voltage, but CURRENT) when the wire can't keep up. By doing this, it helps regulate the demand on the electrical system, which can mask signs of an inadequate electrical system. If the voltage dips aren't just happening at the amplifier, but, through the system's main wire, are happening throughout the car's electrical system, then a big capacitor will, in effect, do the same thing for the whole car. Since the whole car represents a peak load of less than 1kW (Unless something is SERIOUSLY wrong) it can stabilise the voltage, albeit at a lower than expected value. This is great news for the people selling 5F capacitors to kids with 300W amps, because it makes it SEEM like the cap is halping the alternator, when in fact, nothing could be further fromthe truth...