Radiator caps and pressure

[Kevin H]

Given that pressure has the effect of raising the boiling point of water, if an engine's normal operating temp is say 85-90 degrees Celsius, why does it need a radiator cap rated at 15 psi? - this means the boiling point would be over 120 deg C (at 13psi water boils at 120 deg C)

I can't see any point in pressurising the water system unduly, but there must be a point to it because every car manufacturer does it!

Does anyone actually know the reason for pressurising the water cooling system to 15 psi?

[Rubbond]

Kevin,
It's precisely to stop the coolant boiling! Otherwise you would have to keep
topping up.
Many engines are designed to run at about 105 degC, adding to the pressure
requirement.
Coolants contain many additives, but the most important are glycols (I
believe mostly ethylene di-glycol), to lower the freezing point, and
additives to supress corrosion. Not sure what their boiling point/pressure
diagrams look like, but no doubt the radiator cap resistance is designed to
be higher than the type of coolant recommended. In Canada people will use
higher concentrations of glycols, but leave the same coolant in the engine
in the summer.
Still, I can't see why these plugs should blow out!
Andrew.

[Kevin H]

Originally Posted by Rubbond

Kevin,
It's precisely to stop the coolant boiling! Otherwise you would have to keep topping up. Many engines are designed to run at about 105 degC, adding to the pressure requirement.

Maybe modern engines are designed to run at 105 degC, but these old Splodge and Chrysler V8s are not. Someone on here said that the 313/318 engines run at 85-90 degC and the 383/400 engines in the 411 run at about 95-98 degC.

I can see why some pressurisation is desirable - to keep air out of the system for one and to allow them to go a little bit over their normal running temp.

The 411 has a 13psi radiator cap, which means the water would reach 120 degC before it boils (at sea level). The 407/408 has a 15 psi cap - which means it can exceed it's normal running temp by more than 30% before the coolant boils. That seems rather excessive.

That's using water only - if you add glycol based anti-freeze the boiling point is even higher.

[RGSchmitt]

Hi all -

I'd be curious to know the direct source of data that the older Mopar
engines are "designed to run" at a particular (small) range of
temperatures. I note there can be about a 150 F degree variation
(more than 100 C) in US ambient temperatures. Certainly a design
challenge for the Chrysler engineers of the '50s-60s. And to judge
by roadside observations 30+ years ago, not always met.

Bob

[Kevin H]

Originally Posted by RGSchmitt

I note there can be about a 150 F degree variation
(more than 100 C) in US ambient temperatures.

I had to really think about that one Bob, but then I remembered Alaska is a US State.

What's important in the context of this thread is the maximum ambient temp, and even in the US that is always going to be way below the operating temp of the engine. So I guess they design the cooling system to cope with the max ambient and the thermostat regulates the flow of water through the radiator to maintain the intended operating temp of the engine.

The temps I quoted were not operating "ranges" they were approximate normal operating temp for the engines.

The 313/318 temps came from a Bristol 407 owner on this forum, I assumed from experience with his own car. The 383/400 temps came from me, based more upon Setright's notes about the Bristol 411 (the operating temp of my own 411 is not stable!).

If you have more information about the normal (or ideal) operating temps for these engines I would welcome it.