Coolant plumbing...
Turbo Mini Forums

I'll throw a spanner in the works Rod

The cooling system static pressure only rises when the engine is shut off, therefore, the rad cap maintains the maximum static pressure during heat soak.

With the engine running, the max available pressure at the suction of the pump will be zero gauge pressure since the cooling system on Minis excepting the MPi should never see 100 degrees for the water to boil sufficiently to raise the pressure above zero.

The rad cap is only ever there to prevent the cooing system boiling over when the engine is shut down and in heat soak, causing the coolant to boil rapidly.

An overheating engine will boil the coolant while it is running and as a result will lift the rad cap. But the rad caps purpose is not to maintain pressure in the cooling system of a running engine.

To pressureise the cooling system to 1 bar, the coolant temperature needs to be 120 degrees in the rad. and if I am not mistaken, the rad is usually the coolest place in the cooling system

So am I still OK with my water coming from the drain plug at the back of the block ?

All this science is making my brain hurt.

Sorry Colin, it must be the wrong sized spanner you handed me :)

Squeeze your top radiator hose with your engine cold (no pressure) then again with the engine at full temperature (still running) and you will feel the pressure as you try to squeeze the hose (unless you have a cooling system that leaks like a sieve so cannot maintain the pressure).

The temperature in the radiator does not have to be over 100 degrees to pressurise the system.

But you're making me digress, the real point I was making was that pumps make fluids move around a cooling system by creating a pressure difference. Just because we don't measure the pressure before and after the pump in a car doesn't mean the pressure difference isn't there. The fact that there is a static overpressure applied is irrelevant to the pressure rise the pump generates to move the fluid. The static overpressure simply adds a suitable margin between the normal operating temperature and the fluid's boiling point and helps supress cavitation at the pump suction.

And David, yes.

The back of the block is relatively cool water, at a slightly elevated pressure because it is after the pump so will give you the flow when running and if the turbocharger is physically above it you have the added advantage of a natural thermosyphon when the pump stops.

Schrödinger's cat - so which one am I ???

On 4th Feb, 2011 Sprocket said:
I'll throw a spanner in the works Rod

The cooling system static pressure only rises when the engine is shut off, therefore, the rad cap maintains the maximum static pressure during heat soak.

With the engine running, the max available pressure at the suction of the pump will be zero gauge pressure since the cooling system on Minis excepting the MPi should never see 100 degrees for the water to boil sufficiently to raise the pressure above zero.

The rad cap is only ever there to prevent the cooing system boiling over when the engine is shut down and in heat soak, causing the coolant to boil rapidly.

An overheating engine will boil the coolant while it is running and as a result will lift the rad cap. But the rad caps purpose is not to maintain pressure in the cooling system of a running engine.

To pressureise the cooling system to 1 bar, the coolant temperature needs to be 120 degrees in the rad. and if I am not mistaken, the rad is usually the coolest place in the cooling system

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Are we talking about a full to the brim cooling system, or a cooling system that most of us use with a vapour space at the top?

One with a "vapour space".

As the engine warms up, the water expands into the "vapour space" as you call it. As the air and water are in equilibrium, the compressed air in the space then exterts a pressure on the water. As long as the water/air ratio is unchanged, the pressure in the radiator is maintained by the compressed air.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

To my knoledge sprox is correct...

water expands very little with heat, iirc about 5% from almost freezing to almost boiling, its the air in the tank that expands.

are we off topic enough yet! lol

still dont see how you get a pressure greater than one atmosphere at a temperature lower than 100c. If anything with a cold engine, the cooling system is subject to a vacuum

Any school boy knows that water boils at 100c at 1 atmosphere, boils above 100c above 1 atmosphere and below 100c below 1 atmosphere.

I can kind of see where jamie is coming from (even if he is a bit cock sure). To pump fluid you need to create a high pressure to flow into a lower pressure. The water at the point it leaves the pump must be at a higher pressure than the water in the block. The water in the block must then remain slightly higher than the rad in order to flow back, the difference is probably not much, but big enough.

Basically everybody is right you just describe it differently and from different angles.

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Apart from the First Law of Thermodynamics (which applies the initial pressure), try schoolboy science lesson No 2.

1/2 fill a kettle and turn it on without the lid on. Watch the element - long before the water boils you will see thousands of tiny vapour bubbles rising to the surface.

These vapour bubbles are mainly from localised hotspots (there is a lot more complex explanation available called nucleate boiling ...) but as water vapour occupies considerably more space than the liquid it came from, put all those bubbles in the enclosed space and apply the universal gas law (pV=mRT) and you will see how the pressure is maintained.

Although the vapour bubbles will apply the pressure wherever they are in the system, in a well designed system, the enclosed space is the expansion tank - it is the highest part of the system and the vapour migrates there as it is less dense than water. Then a nice equilibrium is established as the vapour condenses in the expansion tank and is returned to the system (The bottom of the expansion tank is usually connected to the pump suction) and more is produced by nucleate boiling at the hotspots and fed to the top of the tank.

In a Mini sidemount rad, the expansion space is the top of the radiator.

If the system design is poor, the pressure is crudely regulated by the setting of the cap/relief valve. Otherwise the equilibrium established in the expansion tank maintains the pressure slightly below the relief valve setting.

And if any vapour is lost, when the system cools down, there is a small non return valve in the expansion tank (or radiator) cap to draw air in to reset the starting pressures when cold.

Don't get carried away by assuming the temperature is uniform in a system just because your gauge reads 90 degrees and hence assume there can be no vapour in the system (which is what eventually applies the pressure).


But if you don't like the science, just go and squeeze a hose on a hot engine (less than 100 degrees) and you will soon realise it's pressurised :)

Edited by Rod S on 4th Feb, 2011.

Schrödinger's cat - so which one am I ???

Ha, reading this is there no thread on tm that doesn't end without decending into smut or some scientific debate?:)

Schrödinger's cat - so which one am I ???

Schrödinger's cat - so which one am I ???

Despite all this erudite discussion (?) I,m sticking with what I,ve just made _ I cant be arsed to change it again- and I wont blame anyone on this thread if it doesn,t work.

Can we lighten up chaps and get back to some more interesting stuff ?.

Any bubles of vapour created from a localised hot spot will instantly colapse when free in the surrounding sub cooled liquid. They will never be able to reach the expansion tank!

Consider a saturated liquid. while there is evaporation or even boiling of the liquid, when in a closed container, there becomes an equalibrium as a result of the saturated pressure. Vapour released from the liquid returns to the liquid at the same rate. Raising the temperature will allow more vapour to be released, and as a result raising the saturated pressure.

where there is only water present, the saturated pressure at 100c is 1 atmosphere or zero gauge pressure.

If you add air to the equasion, the vapour pressure is an average of the vapour pressures of all the fluids involved. However, taking for example a cooling system with water, and a vapour space filled completely with 'air' at atmospheric pressure, when the water is heated, it is the partial pressure that determines the actual boiling point. but this is still way off the 2 atmosphere mark.

Besides this, most manufacturers ask you to warm the engine up with the ad cap off to purge the system of air so that when the cap is fitted, the majority of the vapour space is water vapour. As the engine cools, a vacuum is pressent, so the point that water boils at 100c at 1 atmosphere still stands, and I see no way of how anyone can achieve a posative pressure below 100c in the radiator, where the cap is normally located, and, as said before, is the coolest point of the whole system.

if the whole point of the rad cap is to pressurise a running cooling system to prevent it from boiling, why when you forget to put the rad cap back on and drive 30 miles up the road, you do not loose any coolant from the system? other than what has 'evaporated'

As for being non scientiffic about the feel of a hose, which hose are we talking about?

I enjoy reading the scientific debate, but I much prefer the smut

To finally solve this arguement:

I think a fight to the death with rubber chickens is in order.

Edited by Carlzilla on 4th Feb, 2011.

Schrödinger's cat - so which one am I ???

God love you Rod, but give it a rest for now Eh ?

I was under the impression the liquid itself isnt under all that much pressure, but the air in the system is, which is why the top hose is pressurised when you squeeze it, as there isnt water in that part of the hose in anything ive ever taken off when the engine isnt running, but pressurised air.

Of course there is some pressure applied to coolant when the engine is running, otherwise it wouldnt pump, when the pump stops, the coolant stays relatively still, as there is no pressure.

If you take the rad cap off an engine at normal operating temperature, you get steam, as the hot gas escapes. its only when the coolant is properly boiling water, that any of it comes out of the filler cap. If it does, theres something not right.

Edited by Carlzilla on 4th Feb, 2011.

I concede that if, and only if there is a total void of air above the coolant in the radiator, that, at 90c there might be a pressure of over 1 bar. As the coolant expands as it heats up, so does the air. On 3.5 litres of water there is roughly a 0.125L expansion between 20c and 90c. Assuming 300ml of air at 20c, at 90c this will have been compressed to a volume of 0.175L as a direct result of the expansion of the water. The air itself has also expanded due to the higher temperature, so using the combined gas law, the pressure would infact be in excess of 1 bar gauge at 90c, thus lift the rad cap and bleed the air out anyway. This has nothing to do with the evaporation or boiling of the coolant and I stand by my statement that coolant in the radiator at 90c will be at or below zero gauge pressure when you open the rad cap and perhaps relieve any air that might happen to be there under pressure.

By leaving the rad cap off while the engine warms up, the surface evaporation of the water would likely purge the air out of the rad anyway.

Any time I have removed the rad cap on a cold engine, there has always been a vacuum, which means that there is little air in there to begin with. so why are we talking about air in the cooling system.

Oh, and Jamie/ Dave, sorry for getting this way off topic

Dave, the way you intend to run the turbo cooling hoses will be more than adequate, since the coolant managed to find a hole in one of my heater hoses, with an idling warming up engine. It sprayed coolant everywhere. Fab might remember that one so there is plenty of flow there.

Edited by Sprocket on 5th Feb, 2011.

So to round it off... it would be fine for both water cooling and good flow to take the water feed from the coolant plug on the block and the return into rad top hose? :)

Geoff