[RICK LAKE]

Xavier You would be better to have a 100 amp alt over a 70 amp setup. Here are basic amps for things on your car
cooling fans 6-15 amps each for draw
msd system 8-36 amps depending on system. I don't believe in the story they tell on the 6al setup of 2-3 amps. This doesn't include the coil.
low beam head lights 8-10 amps
high beam lights 10-15 amps
ipc and gauges 2-5 amps with lights
tail lamps 3-7 amps
fuel pump depending on size 3-10 amps per unit
FI system 5-15 amps.
battery size and recharging ability 1-30 amps depending on loads and temp
Are you running under size pulleys on the motor?
What is the rpm idle of your motor?
At idle the motor is not making 100 amps, maybe 40-60 amps, you can add up the numbers and see the problem.
I ran a 70 amp system and had to switch to a 130 amp system because of the under size pulleys not spinning the alt fast enough and keep a full charge on the battery as well as 2 fuel pumps,msd setup, 3 cooling fans, lights. Idle with 750 rpm.
You also need more amps from the alt if the cables are under size for the load or you have weak grounds. 2 grounds min on the car, I have 3 grounds for my car because of the extra curcuits.
There is a 10% fudge factor for alt outputs. Just because it said 100 amps doesn't mean it can mantain this rate all the time without cooking the regulator. You also have to include possible belt slippageof 1-3%.
Remember that is the alt can't supply ALL the power to the car running, after a while your battery will be low on CCA and not able to start the car because of the drain. The battery will not be fully charged either and shorten the life of the battery. That pretty much covers alt and charging 101. In this case bigger is better at 100. Rick L.