I use a 1.2 farad 24volt cap and it does help. If I haven't used my truck in a few days the lights will dimm a little at first. Once everything gets charged up I have no issues. My cap has a voltage meter on it and I don't go under 13.5-13.8 on big bass notes. My amps draw and make alot of power but
I think it has more to do with my altenator. I have a truck so I am sure it puts out more than say a Honda Civic or any car for that matter. I was on Rockford's website the other day and they suggest a 40 farad cap for each of my amps

I have had no issues with the way it is setup so I am not about to fix it....not yet anyway. Keep in mind my cap is only 1.2 farad but I paid over $200 for it:crazy: So maybe you get what you pay for.