Comparing 16 PSI Radiator Cap to Carbonated Soda Can Pressures

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dibbons

dibbons

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I always imagined our coolant systems always being on the verge of blowing sky high when using the standard 16 PSI radiator cap. After researching soda can pressures, I guess 16 PSI is no big deal. It's just that boiling coolant/water likes to shoot out like a geyser

100 kilopascals is about 14.5 PSI. According to the article, even a cold soda has more pressure than that.

Pressure in a Can of Soda - The Physics Factbook
 
I believe NASCAR cooling systems run at 100 psi.
 
Every pound of cooling system pressure adds 3 degrees to the boiling point. 15# gives you an extra 45°. It gets hot and vents but it really geysers when you loosen the cap. All of a sudden your boiling point goes from 257 to 212 in a second. The coolant was at 245° :BangHead::BangHead::BangHead:

Co2 builds pressure inside a pop can, boiling water inside a radiator. Other than pressure inside a closed container, neither are similar.
 
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many years ago one of those great old radiator shops that you can hardly find anymore suggested that I use a 7 lb cap on a recored radiator. I did it and never had a lick of problem with it. I've used 7 lb caps ever since. I guess from the info above the boiling point of the contents is lower because of it, but again, never had a lick of problem using them. I've never understood if there was a reason to put the extra strain on the cooling system with a 16 lb cap - maybe someone can educate me?
 
I too use the 7 pounder, and run a minimum coolant temp of 205*.
And my thermostatic clutch fan keeps it hovering just a tic higher.
Really I could probably run at atmospheric.with periodic top-ups to replace some coolant lost to steaming.
The bonus of low-pressure is ;
I think my top hose is circa 1973; the rest are 1999. The rad is a 1973 as well.
If your cooling system is doing it's job, I see no need for mega pressure.
For my system, running a 16psi cap on a 46 year old rad is just asking for trouble, creating the conditions for a catastrophic failure.
Some literature suggests that there are pockets in your factory ironheads that collect air and at atmospheric pressure, that air would never come out. Ok fair enough, I got rid of those factory iron air-collectors, and got me some Eddies. I think those aluminum heads suck so much heat out of the engine, it just might be impossible to overheat it. lol..... I even put several coats of paint on them to try and keep the heat from escaping. Heat is power. The hotter the engine is engineered to run, likely, the more power it will make. Right up until the oil boils off or cokes.
 
When you loosen a radiator cap on a cooling system that is at or above 212 F. The pressure in the system drops to atmospheric pressure and the liquid coolant flashes to steam. When water flashes to steam its volume increases by a ratio of 1600:1
So 1 cu ft of water makes about 1600 cu ft of steam. That is where your "geyser" comes from.
It is the rapid expansion of the volume of the coolant changing states that creates the risk. That would also why a soda can at 16 psi is not a comparable situation.
 
I wonder how that factors with aluminum heads...
 
AJ/FormS said:
For my system, running a 16psi cap on a 46 year old rad is just asking for trouble, creating the conditions for a catastrophic failure.
Click to expand...

I blew the top tank almost completely off an original radiator with a 16lb cap once.
Actually surprised me that a hose didn't go first, but it made me question the solder joints strength because it looked like it had been re soldered at some point in the past.
(Big hill in traffic on a real hot day.)
 
694spd said:
When you loosen a radiator cap on a cooling system that is at or above 212 F. The pressure in the system drops to atmospheric pressure and the liquid coolant flashes to steam. When water flashes to steam its volume increases by a ratio of 1600:1
So 1 cu ft of water makes about 1600 cu ft of steam. That is where your "geyser" comes from.
It is the rapid expansion of the volume of the coolant changing states that creates the risk. That would also why a soda can at 16 psi is not a comparable situation.
Click to expand...

Explosive Decompression .
 
rklein383 said:
Hot water heater exploding.....
Click to expand...


When I was a kid, we went to a funeral for an uncle. There was a woman there (family friend) that was scarred, and I mean scarred all over her face, hands, arms, and presumably under her clothes, too. My mom leaned over to me and said that she had been the most beautiful woman in the county, until the furnace exploded when she was next to it. I'll never forget that, and that's the same reason that I get nervous around hot water heaters and I try to work the over-pressure valve on my water heater on a regular basis. I steer clear of radiator caps, too.
 
jos51700 said:
I wonder how that factors with aluminum heads...
Click to expand...

With all the properties of aluminum compared to iron, it might be a wash. The water gets heat faster with aluminum heads but carries it away, and since the aluminum has lower density it retains less heat overall. IDK thinking out loud...

Raising the pressure raises the boiling point as already said, it also reduces boiling inside the hot parts of the head.
 
I use a 16 lb cap but I drilled some small holes in it to vent it. The cars were originally made with a vented cooling system and the reduced strain on the radiator is a good idea.
I run straight water and drain the radiator after driving it.
 
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