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Discussion Starter #1
Looking to purchase a new universal aluminum rad and want to know what psi cap to go with. What are guys usually running at the track? The car will mostly be a track car with a little driving around the neighborhood.
 

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My machinist is above & beyond a master engine technician.
If he wants a higher one for better cooling, I listen.
It's been over 20 years, I don't remember the exact technical explanation.

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I run 25 lbs because that is what was recommended. I think it keeps the water in contact with the metal. May be wrong?
 

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I run 25 lbs because that is what was recommended. I think it keeps the water in contact with the metal. May be wrong?
I don't know and that's why I asked, I'm running an 18# cap and don't have a cooling issue but if higher is better for a valid reason.....
 

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Of course higher pressure raises the boiling point. The water in direct contact with the hot engine surfaces will start to boil and turn to steam at the metal/water interface. This is what you see at the bottom of a pot of water that is just starting to boil and is referred to as Nucleate boiling. This nucleac boiling can lower coefficient of heat transfer so it can't remove as much heat.

https://en.wikipedia.org/wiki/Nucleate_boiling

I think it's more important to have a higher pressure cap on an electric water pump application than a mechanical (centrifugal) water pump. That's because at higher rpm the mechanical pump's output pressure goes up which raises the system pressure. On my car with a 16 lb cap I see the cooling system pressure downstream of the pump go from 16 to 30 lbs at the higher rpms during a run.

An electric pump's pressure will stay the same during a run so it doesn't "automatically" compensate for the increased surface heat in the heads/block.
 

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Why so high as opposed to a 16 or 18?
I don’t want to take a chance on a lower pressure cap possibly opening and overfilling the 1 quart overflow and spraying water on the track near the back tire and then you eat shit into the wall.
 

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All I know is I run an aluminum radiator with an overflow tank and a 14 lb cap. Water expands into the tank and gets dwawn back into the radiator as needed. Mechanical water pump, BBC 11:1 compression.
This is in Florida, in the summer, in high humidity and it never goes over 180. If I put a 160 T-stat in it it would run 160. Water and antifreeze
 

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I don’t want to take a chance on a lower pressure cap possibly opening and overfilling the 1 quart overflow and spraying water on the track near the back tire and then you eat shit into the wall.
We may have been taking a chance, but I literally ran zero pressure on the cap, when I was running wildly ported iron heads that all leaked under pressure, and never watered the track down. But I went to the line as cool as possible to start, but I never overflowed the catch can.
 

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We may have been taking a chance, but I literally ran zero pressure on the cap, when I was running wildly ported iron heads that all leaked under pressure, and never watered the track down. But I went to the line as cool as possible to start, but I never overflowed the catch can.
Understood Randy. Not sure if my theory has any merit, just made sense to me.
 

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Understood Randy. Not sure if my theory has any merit, just made sense to me.
Well, you are right to be concerned. I’d never want someone to try it, and spill water on the track. I was very careful, and had no real problems. And, I might add, I wasn’t the only class racer to do this. Do what you are comfortable with, and good luck.
 

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A radiator system with a 16lb cap will NOT cause or allow anymore water to go to the reservoir then a 26lb cap. That is dependent on the system temperature.

Water is basically in-compressible at those pressures. It will expand exactly the same percentage based on temperature at both pressures.
 

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Discussion Starter #19
Of course higher pressure raises the boiling point. The water in direct contact with the hot engine surfaces will start to boil and turn to steam at the metal/water interface. This is what you see at the bottom of a pot of water that is just starting to boil and is referred to as Nucleate boiling. This nucleac boiling can lower coefficient of heat transfer so it can't remove as much heat.

https://en.wikipedia.org/wiki/Nucleate_boiling

I think it's more important to have a higher pressure cap on an electric water pump application than a mechanical (centrifugal) water pump. That's because at higher rpm the mechanical pump's output pressure goes up which raises the system pressure. On my car with a 16 lb cap I see the cooling system pressure downstream of the pump go from 16 to 30 lbs at the higher rpms during a run.

An electric pump's pressure will stay the same during a run so it doesn't "automatically" compensate for the increased surface heat in the heads/block.
I run an electric water pump,so what would you recommend for psi?
 

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Mine works fine with 10 PSI cap. BBC, Mezeri? 35 GPM electric water pump, dual pass 1" tube aluminum radiator, OEM G body plastic recovery tank. Pump and electric fan on. Pull into the water box at 140, leave the line at 160, 180 back at the trailer. Cools down to 140 in 15 minutes with water pump and fan on. Pump gas 582.
 
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