alternator amperage

i have a 87 buick grandnatioal with a stock 120 amp alternator and i am having problems with voltage i am running a jbl 1200.1 amplifier and a power acoustik amplifier that is 100 watts rms x 4 and when i am on the expressway the system is fine but if i drive on the street the voltage that shows up on the digital capacitor is sometimes 11 volts this is when the engine fans are on headlights and heater how big of an alternator do i need to correct this problem i am running a optima yellow top battery,digital 2 farad capacitor with 4 guage wire

1600 watts, at 100 percent efficiency is 111 amps alone. The digital amp is about 75 percent efficient, so figure about 150 amps for alone. The PA amp will be around 50 percent efficient, so figure 55 amps for that one... 205 amps for the system alone, and then you need to add accessory draw, and whatever else. Look for a 250 amp alternator.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."