50% H2O / 50% coolent-antifreeze is normal.<BR>This mix lowers the freeze point to somewhere around -20C and raises the Boil point to about 110C<P>100% H2O is the best at transfer of heat yet will freeze at 0C and boil at 100C @ 1-Atm (normal sea level pressure)<P>100% coolent-antifreeze will change the freeze and boil points, yet will not transfer heat (Our goal) as well.<P>Also make sure that your pressure cap is working properly, I replace about every 5 years to be safe. 14 lbs-sq in (2-Atm?) will raise the boil point above 120C. This is the pressure cooker principal. You can raise the temprature past boiling IF you increase Pressure. Thus a defective cap will cause early boil over. <P>BOYLE'S LAW (usually for gases): PV/T =C; Pressure * Volume divided by Temp. is constant. Volume is constant in a cooling system... so If the temprature goes up(even past the boiling point), so will the Pressure. If we exceed the pressure of the cap, the pressure goes down but the temprature is the same and the volume goes up (and fluid spills into the expansion/overflow chamber) when the temprature cools, the pressure drops and sucks coolent back into the rad. When the boilover happens, the volume expands quickly. <P>Ever notice that a hot engine will almost always boil over if you remove the cap.