I got 4 12" 4ohms 300RMS Each And 2 400RMS Amps 2x 165RMS at 4 ohms and at 2x 200Rms 2ohms. When i at 4ohms with both amp there is no problem with power it jumps from 14.2V to 12.01V. But when at 2 ohms both amp 800RMS power jumps from 14.3V to 8.93V why is that 4 to 2ohms and its only for 180RMS more with 2ohms both amps and the 4 12"s.
when the load is decreased (ohms) the power (watts) increases. power = volts * amperes. in other words, as you use or get mroe power from the amplifier, you draw more current. if your car's alternator can't supply enough current, the voltage suffers as a result, and you see this drop or sag you mentioned. the problem with this, is it causes something called clipping which makes distortion and causes damage to the speakers and eventually the amplifier as well.
the solution is stay at 4 ohms, or get a larger alternator to handle the high current demands of the stereo system.
energy isn't free. it has to come from somewhere. in this case, it comes from the alternator, and there's no way around that. capacitors don't help and battery upgrades only help if the car is turned off.
as for why power doesn't double when you go from 4 ohms to 2 ohms, that has to do with the efficiency of the amplifier and it's power supply. Without getting overly complicated, an amplifier has a load at which it's most efficient. Usually 4 ohms. going to 2 ohms will make more power, but not twice as much because you end up losing a larger percentage to heat.