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Old 05-12-2011, 06:47:50 PM   #1
siberbunak
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How does radiator cap psi affect the operation of the engine

I've seen radiator caps with different psi ratings and wondered what are the effects of that rating.

Anyone should help ?

Also what is the adequate psi for a near to standard street engine or what is the factory spec.
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Old 05-12-2011, 08:41:59 PM   #2
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Each psi higher raises the boiling point by almost 2.5f degrees iirc
Most cars I have owned are 15# or 16#
I have ran a 24# cap before and the hoses are looking like they are ready to pop.
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Old 05-13-2011, 12:01:06 AM   #3
Todd80Z28
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Yes, it's all about boiling point. 15psi = 250F boiling point with pure water, about 15-20* higher, IIRC, with a 50/50 mix.
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Old 05-13-2011, 12:23:56 AM   #4
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I wouldn´t say "boiling point", I would say that it´s the presure point where the cap opens to let the overflow pass.

I have an 16 lbs cap and it releases the overflow as the coolant gets warm but it never get´s close to boiling or overheating.
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Old 05-13-2011, 01:05:23 AM   #5
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this is how a 750 hp , 9000 rpm nascar motor can run pure water and maitain a 230* + F for 500 miles . water boils at a higher temp at pressure .
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Old 05-13-2011, 01:39:20 AM   #6
ZS10
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Quote:
Originally Posted by Coolvette
I wouldn´t say "boiling point", I would say that it´s the presure point where the cap opens to let the overflow pass.

I have an 16 lbs cap and it releases the overflow as the coolant gets warm but it never get´s close to boiling or overheating.
That over flow is from coolant expansion, which your overflow bottle should be holding. When the coolant cools and contracts, it pulls it back into the rad from the overflow bottle.

The job of the rad cap is to maintain a pressure level and raise the boiling point so that part of the engine can operate at a temperature higher than the boiling point at atmospheric pressure. If left at atmospheric pressure the coolant would boil as it passed by the hotter passages, ie., around the exhaust ports in the heads.
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Old 05-13-2011, 03:37:12 AM   #7
siberbunak
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Thank you all for resourceful posts.
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Old 05-13-2011, 07:43:46 AM   #8
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Quote:
Originally Posted by ZS10
That over flow is from coolant expansion, which your overflow bottle should be holding. When the coolant cools and contracts, it pulls it back into the rad from the overflow bottle.

The job of the rad cap is to maintain a pressure level and raise the boiling point so that part of the engine can operate at a temperature higher than the boiling point at atmospheric pressure. If left at atmospheric pressure the coolant would boil as it passed by the hotter passages, ie., around the exhaust ports in the heads.


OK, point taken and lesson learned...

The use of an expansion bottle requires a different radiator cap I think as the one I have stops the return flow.
It seals off both in the botton and on the top of the radiator neck and I think that the bottom seal works as a one way valve, the high pressure from the radiator can pass but the spring pressure seals it off the other way.
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