I saw some thread they always 70% water / 30% coolant mix it .I just want to know why don't just put 100% coolant in radiator?

 

because the straight coolant will not cool your engine efficently. Water will tho and if your in a warm all year climate then you can use straight water. The coolant is there to keep the water from freezing in cold climates.

 

Make sure you use distilled water.

 

i use 100% coolant in my car, but i use evan's npg so it's okay.

 

70% water is NOT recommended by Mazda. Read your decal. It specifically says:
"Coolant concentration must be maintained at 45% or higher. Example: 45% or higher coolant, 55% or lower water."

 

Coolant has anti corrosion and lubrication properties that water does not provide. Water does have a higher heat transfer capacity than coolant so those in warmer climes may want to use a higher water percentage.

I think Evan's is propylene glycol which is essentially non-toxic (ethylene glycol is very toxic) but propylene glycol has much higher viscosity in cold weather.

 

Straight from the FSM