Most engines are thermally more efficient at a coolant temperature of 195 to 200 degrees Fahrenheit. Pressure is also a critical function of coolant efficiency. A typical street-car cooling system operates at 15 psi. This pressure also increases the boiling temperature of water. As a rough rule of thumb, for every 1 psi of cooling-system pressure, the boiling point of straight water will rise between 2 and 3 degrees. Water boils at sea level at 212 degrees Fahrenheit, but at 15-psi gauge pressure, water boils at 250 degrees Fahrenheit.