Radiator cap pressure question?

[Bruce B.]

On my 57 Corvette Gasser I am using a stock 57 radiator with a 7 pound cap and the engine does not overheat under normal conditions.
My question is , can I use a 13 pound cap to improve the cooling by raising the boiling point?
Thanks in advance.
Bruce B

[Michael W.]

Re: Radiator cap pressure question?

Raising the system pressure does not improve cooling.

[Duke W.]

Re: Radiator cap pressure question?

A higher pressure relief cap will give more boilover margin, but it you're not having boilover problems now, it won't have much effect. Beyond that a higher pressure cap likely will not overstress the cooling system components if the pressure gets over 7 psi.

Duke

[Bruce B.]

Re: Radiator cap pressure question?

A higher pressure relief cap will give more boilover margin, but it you're not having boilover problems now, it won't have much effect. Beyond that a higher pressure cap likely will not overstress the cooling system components if the pressure gets over 7 psi.

Duke

Thanks for the info.

[Joe L.]

Re: Radiator cap pressure question?

A higher pressure relief cap will give more boilover margin, but it you're not having boilover problems now, it won't have much effect. Beyond that a higher pressure cap likely will not overstress the cooling system components if the pressure gets over 7 psi.

Duke

Duke and Bruce-------


Yes, some time ago I did some pressure tests on a waterpump that I rebuilt myself. I found that the pump would barely hold pressure much above 15 PSI, with leakage always occurring only at the back plate. This was with a pump with the later style "curved edge" back plate with impressions between bolt holes. So, out of curiosity, I installed the earlier style back plate with "curved edges" but without the impressions between the bolt holes. That plate would not even hold 15 PSI. Finally, I decided to try the old-style flat back plate. That wouldn't even hold 10 PSI.

I really think that leakage at the waterpump backing plate is the main problem with increasing system pressure. I think the rest of the system, even for older Corvettes, would hold up to 15 PSI.

[Bruce B.]

Re: Radiator cap pressure question?

Joe,
Thanks for the info.
I am running a Edelbrock performance aluminum water pump and I believe it is capable of increased pressures so I will give the 13 pound cap a try. The cooling system is in good condition so leaks should not be a problem.
In addition to your and Dukes info I did some digging concerning coolant boiling points.

The following information tells the story.

A system under pressure can handle higher temperatures, and offers a higher static boiling point.
Most liquids have a specific "boiling point", which is the temperature at which the liquid begins to
change to a gas. If pressure is applied to the liquid, it must become hotter before it can boil. Pure

is less than at sea level.



before it can boil. Even at this temperature the water is able to circulate through the engine, cooling
parts that are at a much higher temperature without the water boiling. As long as the coolant
remains in liquid form it can do it's job and transfer heat to the radiator or heat exchanger so it can be dissipated.
Coolant that is boiling cannot transfer as much heat and overheating is likely to occur if the coolant turns to a gaseous state. Steam adjacent to a hot surface, such as a combustion wall, can actually act as an insulator- thus preventing any heat transfer to the coolant.

[Michael W.]

Re: Radiator cap pressure question?

Joe,
Thanks for the info.
I am running a Edelbrock performance aluminum water pump and I believe it is capable of increased pressures so I will give the 13 pound cap a try. The cooling system is in good condition so leaks should not be a problem.

What problem are you trying to solve? Again, raising the system pressure and in turn the boiling point of the coolant will not improve cooling.

[Bruce B.]

Re: Radiator cap pressure question?

Michael,
Why are higher pressure radiator caps used in some applications?
In my opinion they prevent boiling and therefore prevent overheating.

[Michael W.]

Re: Radiator cap pressure question?

Michael,
Why are higher pressure radiator caps used in some applications?
In my opinion they prevent boiling and therefore prevent overheating.

True, but you stated that your car does not overheat with a 7 lb cap. If it does not overheat, the boiling point of the fluid is irrelevant.

[Duke W.]

Re: Radiator cap pressure question?

Back in the fifties, alcohol-based antifreezes were used along with 160 degree thermostats. The reason was that ethyl alcohol boils at 170F (at sea level), so higher temperatures even with higher pressures would cause the alcohol to boil off more quickly.

The switch to ethylene glycol (late fifties/early sixties) was accompanied with higher rated pressure caps and 180 degree thermostats. A 50/50 glycol.water blend with a 15 psi cap boils at 265F, so there is lots of boilover margin, but once you get to 250 the engine may start detonating.

Back in the thirties when the Army Air Corps embarked on their "hyper" (high performance) engine projects, an original objective was to run straight glycol at 300F, in order to reduce heat transfer for greater thermal efficiency, but that proved to be unworkable due to detonation with the typical 15 psi boost pressure used at takeoff.

Duke

[Bruce B.]

Re: Radiator cap pressure question?

Michael,
I have been in stopped traffic where my ECU has indicated a coolant temp of up to 211 degrees F before I started moving.
My interest in radiator caps is more of a preventative measure to avoid potential boiling over.
That's all, no big deal, but since I drive my 57 a lot I want to prevent any problems.

[Michael W.]

Re: Radiator cap pressure question?

Michael,
I have been in stopped traffic where my ECU has indicated a coolant temp of up to 211 degrees F before I started moving.

Assuming that the car is using a 50/50 mix of coolant and water, the boiling point with a 7 psi cap is 240*F. It appears that you have plenty of margin already.

[Duke W.]

Re: Radiator cap pressure question?

Michael,
I have been in stopped traffic where my ECU has indicated a coolant temp of up to 211 degrees F before I started moving.

...ECU on a '57???

Since you appear to have modern components, I recommend you install a 15 psi cap, and if the heads are aluminum consider a 195 degree thermostat. My Cosworth Vega experience has convinced me that aluminum engines (or at least aluminum heads) run better the hotter they are, up to a practical limit, say 230F.

I believe some late model cars and NASCAR are using 21 psi caps.

Duke

[Bruce B.]

Re: Radiator cap pressure question?

Duke,
Thanks.
I have a 13 lb. cap and I will give it a try.
A little extra safety margin can't hurt.
Bruce B.

[Mark B.]

Re: Radiator cap pressure question?

Modern antifreeze is not "coolant", and is a crappy heat transfer material and only serves to prevent freezing and lubricate the water pump marginally.
The higher pressure cap is a band-aid solution to allow "antifreeze" mixtures (50/50 or otherwise) to try to transfer heat as good as water.
It's arguable that at high altitudes a higher pressure cap would allow any given cooling liquid to reach the same temp before boiling as an engine at a lower altitude with a lesser cap.

Pure (de-mineralized) water is still the absolute best heat transfer liquid. The lowest pressure cap you can run and not overheat is the best pressure cap.
The best "coolant mix" would be the de mineralized water, an adhesion promoter, a water pump lubricant and a corrosion resistor.
"Water wetter" (a glycerin product the promotes water adhesion, and contains a water pump lubricant, and corrosion resistor) is a product I use.
Many people who race cars will be familiar with "water wetter" or "30 below" or other non-antifreeze products and water combos as antifreeze is normally forbidden at the track..