How much air gap for a header tank?

SuperV8 · Post by **SuperV8** » Fri Mar 25, 2011 2:55 pm

I have a 4.6 rover V8 with a 'custom'

cooling system and header tank.

How full should I fill the header tank?

Obviously the level needs to be the higher than the highest part of my cooling system, but how much air gap would there need to be for coolant expansion?

Anyone know how much air gap in the header tank there would be in a rangerover header tank?

Tom.

DaveEFI · Post by **DaveEFI** » Fri Mar 25, 2011 4:20 pm

A header tank usually has a pressure relief cap - so just fill it right up and let the pressure eject the excess and find its own level.

ChrisJC · Post by **ChrisJC** » Fri Mar 25, 2011 5:40 pm

Guessing, my P38 4.6 has about 2 inches air-gap.

Chris.

Ian Anderson · Post by **Ian Anderson** » Fri Mar 25, 2011 7:03 pm

GT40 looses about 4" with expansion

Ian

SuperV8 · Post by **SuperV8** » Mon Mar 28, 2011 11:37 am

Thanks chaps, I think I can get away with 2" gap. If not i'll need to make a new larger header tank!

Doing some research I actually found a graph which shows the expansion of water over temp, I've worked out 6L of water would expand approx 0.5L at 95Degs. A 30mm gap in my header tank works out at 0.75ml so i'll see how that goes? Obviously pressure is involved also and maybe i've over simplified things (As is often the case

)

Tom.

DEVONMAN · Post by **DEVONMAN** » Mon Mar 28, 2011 7:22 pm

SuperV8 wrote:Thanks chaps, I think I can get away with 2" gap. If not i'll need to make a new larger header tank!

Doing some research I actually found a graph which shows the expansion of water over temp, I've worked out 6L of water would expand approx 0.5L at 95Degs. A 30mm gap in my header tank works out at 0.75ml so i'll see how that goes? Obviously pressure is involved also and maybe i've over simplified things (As is often the case )

Tom.

Liquids don't compress measurably under pressure so pressure has no significant effect.

Regards Denis

Bryan · Post by **Bryan** » Tue Mar 29, 2011 8:14 am

Liquids don't compress measurably under pressure

Maybe not, but the air in the gap will compress as the water expands.

Without considering the thermal issues, compressing the air from .75L to .25 L (ie. 0.5L expansion of water) will give a system pressure of approx 30psi, which will probably cause the cap to vent. Obviously, this is for the 30mm gap - the pressure will be much lower for the 2" gap

DEVONMAN · Post by **DEVONMAN** » Tue Mar 29, 2011 10:54 am

Bryan wrote:
Liquids don't compress measurably under pressure
Maybe not, but the air in the gap will compress as the water expands.

Without considering the thermal issues, compressing the air from .75L to .25 L (ie. 0.5L expansion of water) will give a system pressure of approx 30psi, which will probably cause the cap to vent. Obviously, this is for the 30mm gap - the pressure will be much lower for the 2" gap

I don't disagree with what you are saying, (Boyles Law etc) but, in this case it is surely irrelevant. The water will not compress and will expand and fill the space. If the space is not big enough it will flow out of the cap which is what SuperV8 is trying to avoided.
Regards Denis

unstable load · Post by **unstable load** » Tue Mar 29, 2011 12:30 pm

If the tank is filled prior to the first run, there will inevitably be some run out occurring from the expansion of the system under normal operating conditions. This will stabilise after a while and that level it settles at will become the engine's normal operating level for the header tank.

When the car has finished pushing the water out and is switched off, the natural cooling of the block and the rest of the components will reduce the pressure in the cooling system and air will flow back into the header tank through the radiator cap and it will stabilise around a particular level which will remain pretty constant for that car over time as long as the cooling system retains it's integrity.

The mark on your Ford/Vauxhall etc that shows how far to fill the header tank is a representation of the level for that particular system and is the end result of the path that SuperV8 is starting down now.

unstable load · Post by **unstable load** » Tue Mar 29, 2011 12:31 pm

If the tank is filled prior to the first run, there will inevitably be some run out occurring from the expansion of the system under normal operating conditions. This will stabilise after a while and that level it settles at will become the engine's normal operating level for the header tank.

When the car has finished pushing the water out and is switched off, the natural cooling of the block and the rest of the components will reduce the pressure in the cooling system and air will flow back into the header tank through the radiator cap and it will stabilise around a particular level which will remain pretty constant for that car over time as long as the cooling system retains it's integrity.

The mark on your Ford/Vauxhall etc that shows how far to fill the header tank is a representation of the level for that particular system and is the end result of the path that SuperV8 is starting down now.

unstable load · Post by **unstable load** » Tue Mar 29, 2011 12:31 pm

If the tank is filled prior to the first run, there will inevitably be some run out occurring from the expansion of the system under normal operating conditions. This will stabilise after a while and that level it settles at will become the engine's normal operating level for the header tank.

When the car has finished pushing the water out and is switched off, the natural cooling of the block and the rest of the components will reduce the pressure in the cooling system and air will flow back into the header tank through the radiator cap and it will stabilise around a particular level which will remain pretty constant for that car over time as long as the cooling system retains it's integrity.

The mark on your Ford/Vauxhall etc that shows how far to fill the header tank is a representation of the level for that particular system and is the end result of the path that SuperV8 is starting down now.

unstable load · Post by **unstable load** » Tue Mar 29, 2011 12:32 pm

Sorry about the multiple posts, I was getting a weird error message and kept retrying it...

SuperV8 · Post by **SuperV8** » Tue Mar 29, 2011 1:58 pm

Thanks for the comments.

My cap is a plastic 'performance!' screw on type (yellow) I think with a higher relief value than the regular bayonet type pressed steel version. I wouldn't want to see 30psi so think i'll try and get more than 30mm gap in the tank. It will be dependant on the highest point in the system which is the top of my rad. I think I will fill it just as high as necessary just a little higher than the rad.

One more point i'm just musing about, if my car's on a level surface and the cooling system has been bled what happens if I drive up a steep hill? That would then make the rad the highest point, would that mean air could collect in the top of the rad or once it's bled would the air stay in the header tank?

Tom.

SuperV8 · Post by **SuperV8** » Tue Mar 29, 2011 2:00 pm

I also got an error message! but turns out the message worked! Wierd

Ran into problems sending Mail. Response: 502 unimplemented (#5.5.1)

DEBUG MODE

Line : 126
File : smtp.php[/quote]

DEVONMAN · Post by **DEVONMAN** » Tue Mar 29, 2011 3:42 pm

One more point i'm just musing about, if my car's on a level surface and the cooling system has been bled what happens if I drive up a steep hill? That would then make the rad the highest point, would that mean air could collect in the top of the rad or once it's bled would the air stay in the header tank?

Tom.[/quote]

Tom,
With the system bled, once the engine is running the air in the header tank will stay at the top, provided there is some coolant in the tank, and the relative level of the rad is therefore not important, unless the system boils and bubbles are then trapped in the top of the rad. It is possible that some minor bubbles will be formed in the engine block as a result of local boiling at hot spots but these if trapped in the rad will get purged to the header tank at the next warm up session.

Regards Denis