NC hybrid setup/coolant blows heater core/how to avoid?

[Greg Hermann]

>It would seem to me that no matter what you do as far as restriction,
>as soon as the key is shut off, the increase in cooling system
>pressure will be equal throughout the system.  I find it hard to
>imagine that the heater core would be subjected to enough increased
>pressure from the water pump to be an issue.  The water pump mainly
>creates a pressure differential throughout the system so that coolant
>can circulate..... I suspect that there is very little if any increase
>relative to atmospheric pressure over what the static system pressure
>is.  A good heater core should be capable of withstanding any internal
>cooling system pressure likely to be seen in an automobile with a
>properly functioning pressure cap.         H.W.

Your suspicions and imaginings are incorrect on this one, Howard.

While the heater core may or may not take the pressure OK, the water pump
makes a substantial amount of pressure head inside the water jackets. And
this additional head is VERY important to preventing localized boiling
(beyond nucleate boiling) in the hottest parts of the heads. The head is
created by the restriction to coolant flow which is provided by the
thermostat, and is why a thermostat should not be removed (or, if it is,
should be replaced by something which restricts coolant flow to the same
extent as the stat did).

The pressure head which the water pump generates is proportional to the
square of the pump rpm, and at , say, 5000 pump rpm, for a pump with an
impeller the size of most water pumps, it would not be unreasonable to
expect to see 25 or 30 psi of pressure in the water jackets in ADDITION the
pressure from the radiator cap!! And if the thermostat restriction is
removed, the water pump will cavitate due to not having enough NPSH at the
resulting (higher flow at less developed head) point on its curve for that
particular impeller speed!

Regads, Greg