[dsraven]

domestic water supplies can have varied degrees of minerals and different PH levels. that's why it is recommended to use distilled water when diluting coolant, filling a battery, etc. otherwise just buy the ready to go prediluted coolant. it is never advised to use straight water unless it is for a very short trip, like down the race track. plain water can start rusing stuff right away and rust is like insulation to a cooling system. heat can't get to the coolant if there is rust between the heat source, like the cylinder walls or head cooling jacket. coolants have corrosion inhibitors and water pump lubricants built in. a good rad cap is also a priority as a higher pressure in the system raises the boiling point of the coolant.