Loose cap allows boiling....


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Posted by ChrisCase [76.212.213.13] on Monday, November 28, 2011 at 12:23:34 :

In Reply to: Question on boiling point posted by Leon in PA [72.25.32.59] on Sunday, November 27, 2011 at 16:44:49 :

Vaporizing absorbs huge amounts of heat, it is how air conditioners work.

The corollary is that pressure raises the boiling point at 3° per psi. So a 15# cap system has a boiling point of 257. Higher with antifreeze.

So, in an engine with an overall temperature of only 180, not much to be gained as far as cooling goes. But the whole cooling system is not consistent. Heads get hotter, and the hottest part is the cylinder walls at the bottom of the stroke where the wrist pin pauses at the bottom. At that point there is usually cavitation damage inside the water jacket that is visible by removing the side freeze plugs. Bad thing that. Pressure helps prevent that local boiling and steam damage.

The pump will actually make about 10 psi max inside the block ( I checked) with a thermostat acting as a restriction. That is why you do want to always run a thermostat or at least a washer type restrictor plate. Add even a low pressure cap of 2-4 psi, and you can extend block demise.

So, Leon loosening his cap lets some water boil off, absorbing heat. But it can only help in a narrow range of deficiency in the cooling system, at possible permanent damage to the block.

My own truck had a deep neck radiator. I found a shallow neck pressure cap locally and made an aluminum spacer to shim the cap. Rubberized cork gasket under the aluminum, and the spacer cut to an octagon shape to allow the overflow to work. The spacer used to be a common item, made of hollow brass- think doughnut shaped carb or tank float.

And who says they don't overheat? What with the ubiquitous bad head gasket, Kaegi must be living a charmed life. Problem with the bad gasket in the cap is that if it does boil over, the spray on the windshield will blind the driver. Wipers don't like to clear antifreeze.



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