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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.
Well, it seems like the hotter the engine is, the harder the radiator hose is, which seems like more pressure. I would think the 16lb cap would release and start filling the overflow bottle sooner than the 25 lb.
 

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I run an electric water pump,so what would you recommend for psi?
For a NA car, 16-24 is fine. A power adder car with more heat and an electric pump I would recommend the higher pressure.
 

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Well, it seems like the hotter the engine is, the harder the radiator hose is, which seems like more pressure. I would think the 16lb cap would release and start filling the overflow bottle sooner than the 25 lb.
As soon as the water starts to heat up it also starts to expand and increase the system pressure. This volume change initially affects the hoses, radiator and other non-rigid components that will expand as the pressure increases. At some point the temperature/pressure increases to the point where the expansion is greater than the amount the non-rigid parts can expand and the system pressure will overcome the radiator cap pressure.

So yes, in reality these non-rigid parts will expand slightly more at 25lbs than 16lbs. But the difference in volume absorbed by this expansion would be small compared to the overall expansion of the water itself. I've run both pressure caps and have never seen a significant difference. If someone has large water hoses that are soft and expand like balloons, maybe it will be significant.

What I've left out of this theoretical discussion are the gases that are in the cooling system. As soon as the water/coolant starts to heat up, gases will come out of solution and expand much more than the liquid. Even after we purge all of the excess air out of the cooling system these gases will still be present.
 

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One thing to keep in mind when considering what pressure cap to run is whether or not you have the stock heater core. Put too much pressure to one and it can blow.
 

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Basic rule for a track only car, 16lbs for standard head gaskets and 7lbs if your running copper gaskets. It would be a very rare case that you would need a higher them 16lbs cap on a drag race only car.

Water boils around 212 and each pound on the cap adds 3 degrees to that so a 7lbs cap should be good tell around 233 degrees before the coolant boils.
 

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Why the lower pressure for copper gaskets??
Copper gaskets are great for cylinder sealing but don't seal well at the water passages.
 

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I run a 16 lb cap and zero issues with copper gaskets. Have yet to have one leak...
I personally have no direct experience. I've just run across many comments from users of copper gaskets regarding water leaks.
 

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Is it normal to still have pressure in the system , even after sitting for 5 days or so ?

Started noticing this after putting a new cap on .

Its a 16lb lever style cap like this

Dave - When I had that happen to me on my car it was the result of water reacting with the system metals. Let's put it this way, I've never had that happen when my car had anti-freeze in the cooling system.
 

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Dave - When I had that happen to me on my car it was the result of water reacting with the system metals. Let's put it this way, I've never had that happen when my car had anti-freeze in the cooling system.
This is correct. you are likly having a chemical reaction with something in your coolanat system. Complety drain and flush and just use plain water(if you have good local water) and a little bit of coolant.

https://www.aa1car.com/library/cooling_system_electrolysis_corrosion.htm
 

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Depends on the head gasket too. If you’re using copper head gaskets, keep it at or under 8#. They can weep otherwise. The cylinders seal up fine, but the water can weep.

On everything else I run, I use a 16# cap with no problems.
 

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Why the lower pressure for copper gaskets??
What I’ve seen is, with copper and o rings, the cylinder gets good pressure to seal, but the water jackets can weep.
The problem is a little more common with aluminum blocks. The sleeves generally are .004” proud on the deck and reduces the clamping force around the water jacket. Also, many times the factory surface on the block before the sleeves go in is less than perfect, and then distorts a little from the press fit. They are always much nicer when we're able to correct the deck nice and then install the sleeves.

On my Mitchell X block, it will weep a little bit when it first goes in the car after freshen. Quits leaking after a few heat cycles and I also add a sealant tablet to the system. It’s not much, like a little drip on a corner or something, and then none. I use an 8# cap.

Iron blocks with a good finish are way less likely to leak, but being copper is dead soft, it will give over time. They can develop a leak with a high pressure cap occasionally.
 

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The fastest 4.6 modular engine builder in the world told me that you want higher pressure to push water to the metal surfaces of the engine under power, so there is no steam. i run a 30+ pound cap.
 

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The fastest 4.6 modular engine builder in the world told me that you want higher pressure to push water to the metal surfaces of the engine under power, so there is no steam. i run a 30+ pound cap.
That's a funny way to say "higher pressure raises the boiling point temperature". :)
 

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Do you think it’s possible it is creating a vacuum as it cools down?
A properly functioning radiator cap will not allow the system pressure to go much lower then atmospheric pressure as long as the water height in the overflow reservoir to close to the same height as the top of the radiator.
 

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The fastest 4.6 modular engine builder in the world told me that you want higher pressure to push water to the metal surfaces of the engine under power, so there is no steam. i run a 30+ pound cap.
This is why in some boosted applications a mechanical driven water pump works better then and electric water pump. Most electric water will only create about 5psi if you deadhead it. A mechanical pump can create 20psi or more. This is why I will almost alway recommend a mechanical water pump on land speed race cars.
 
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