Also you better watch the freeze level in the summer time or you will have a frozen and leaking heater core. Needs to be protected down to about 10 degrees even in the summer. If you have had the experience of changing a heater core, you are very careful about this, I promise

 

Bill - Is it the antifreeze...or the pressurizes system that does the most to allow higher temps?
I don't really know...have always wondered.

HID - That's a great point....and one of the reasons for 25% antifreeze, which I believe is good down to about 0*.

RJ

 

Boiling temp vs cap pressure

0 psi - 212°F

5 - 227

10- 240

15- 250

20 - 259

25 - 267

Antifreeze also raises the boiling point, it usually says by how much right on the jug

 

antifreeze is also needed to prevent corrosion and keep the water pump lubricated....