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Old 09-23-2019, 07:56 PM   #1
Drag Mouse
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Default What psi radiator cap

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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Old 09-23-2019, 08:04 PM   #2
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Default Re: What psi radiator cap

Pretty sure NHRA and IHRA have a minimum cap PSI. I cant remember what it is.
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Old 09-24-2019, 12:25 AM   #3
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Default Re: What psi radiator cap

I don’t recall seeing that spec in the rulebook, but I use a 25 psi Moroso cap.
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Old 09-24-2019, 07:33 AM   #4
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Default Re: What psi radiator cap

Zero, if you run a set of ported SBC Chevy iron heads. Ha!
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Old 09-24-2019, 09:54 AM   #5
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Default Re: What psi radiator cap

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Originally Posted by dakster1a View Post
I donít recall seeing that spec in the rulebook, but I use a 25 psi Moroso cap.

Same here, 25 or 29 on all mine.


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Old 09-24-2019, 10:23 AM   #6
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Default Re: What psi radiator cap

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Originally Posted by Randy Wilson View Post
Zero, if you run a set of ported SBC Chevy iron heads. Ha!

You are right on that. My head castings are 51 years old. I use the lightest one I can find
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Old 09-24-2019, 11:17 AM   #7
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Default Re: What psi radiator cap

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Same here, 25 or 29 on all mine.


.
Why so high as opposed to a 16 or 18?
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Old 09-24-2019, 12:05 PM   #8
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Default Re: What psi radiator cap

.

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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Old 09-24-2019, 12:52 PM   #9
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Default Re: What psi radiator cap

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Why so high as opposed to a 16 or 18?
Ya, iíd Like to know that one. I know the higher the psi, the higher the boiling point. Donít know any other advantage to running higher system psi.
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Old 09-24-2019, 01:39 PM   #10
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Default Re: What psi radiator cap

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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Old 09-24-2019, 01:41 PM   #11
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Default Re: What psi radiator cap

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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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Old 09-24-2019, 03:19 PM   #12
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Default Re: What psi radiator cap

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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Old 09-24-2019, 08:05 PM   #13
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Default Re: What psi radiator cap

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Why so high as opposed to a 16 or 18?
I donít 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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Old 09-24-2019, 09:04 PM   #14
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Default Re: What psi radiator cap

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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Old 09-24-2019, 09:45 PM   #15
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Default Re: What psi radiator cap

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Originally Posted by dakster1a View Post
I donít 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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