Q, how high the coolant in the reservoir ? (Cold engine)

Here is a drawing to explain my point

1629388841820.png
 
Why am I asking this question?

Here is the reason:

1629389063842.png


On hot engine, if I set coolant level at 5cm, this is exactly half of the reservoir, ...the pressure reads 0,75Bar

But if the lever is at 7,5cm, pressure reaches 0,95Bar

Remember that 1Bar is the pressure that opens the safety valve of the cap

Hence my question

Is there anything in the books ? I saw nothing,...but
 
Pressure depends on mixture. Make it a little richer in coolant to bring the pressure down.
 
DWMBMW said:
The BMW Haynes Manual says no more than 2 cm below the base of the cap.View attachment 125016View attachment 125017
Click to expand...


i wonder if the expression of “fill to a point no more than 2cm below” is super clear ?

does this mean that 1cm below is acceptable ? it sounds ridiculous, i have witnessed that hot engine results in more than 1cm increase in coolant level with respect to cold engine level

or does it mean that 2cm below the base is the minimum for air to be there, so 3cm for example is acceptable ?
then 5 or 7cm will be ok too

i am afraid i do not understand that instruction
 
deQuincey said:
i wonder if the expression of “fill to a point no more than 2cm below” is super clear ?

does this mean that 1cm below is acceptable ? it sounds ridiculous, i have witnessed that hot engine results in more than 1cm increase in coolant level with respect to cold engine level

or does it mean that 2cm below the base is the minimum for air to be there, so 3cm for example is acceptable ?
then 5 or 7cm will be ok too

i am afraid i do not understand that instruction
Click to expand...
I read that as “to a maximum of 2 cm from the top” which would make sense if it expands.
 
As a starting point, could it be a simple option to just copy the height of the transparant reservervoir? Either distance to top, or distance to bottom?

I asumme the volume and dimensions of the plastic vs the brass version are quite alike. If so, then the level should be the same. ( note that I'm not anywhere experienced in engine things; it's just my practical reasoning)
 
Your pressure difference makes sense. As the air volume is smaller when filled to 7,5 cm the pressure will raise more compared to the pressure reached with a coolant level of 5 cm.
The air needs only needs to compensate for the growing volume of the coolant when heated. If the reservoir was a straight edged box, I would think the pressure increase is almost linear to the height. As we have hemispherical top shape,the linear pressure build up is pulled slightly up by the facts that the air volume reduces more between 5 to 6 cm, compared to 7 to 8 cm fill levels. Also the fluid above the 5cm level will add to the expanded volume, thus increasing the pressure a tad more vs linear.
 
eriknetherlands said:
As a starting point, could it be a simple option to just copy the height of the transparant reservervoir? Either distance to top, or distance to bottom?

I asumme the volume and dimensions of the plastic vs the brass version are quite alike. If so, then the level should be the same. ( note that I'm not anywhere experienced in engine things; it's just my practical reasoning)
Click to expand...


this is good too,
i can check the level at the transparent and judge,


but, was there a level mark?
 
eriknetherlands said:
Your pressure difference makes sense. As the air volume is smaller when filled to 7,5 cm the pressure will raise more compared to the pressure reached with a coolant level of 5 cm.
The air needs only needs to compensate for the growing volume of the coolant when heated. If the reservoir was a straight edged box, I would think the pressure increase is almost linear to the height. As we have hemispherical top shape,the linear pressure build up is pulled slightly up by the facts that the air volume reduces more between 5 to 6 cm, compared to 7 to 8 cm fill levels. Also the fluid above the 5cm level will add to the expanded volume, thus increasing the pressure a tad more vs linear.
Click to expand...

yes, in that system you can not compress coolant but you can compress air

my guess is that pressure in that system is due to coolant warm up and expansion, so the fewer the air in the reservoir the higher the pressure
 
deQuincey said:
....
but, was there a level mark?
Click to expand...

Yes there is; it is 6 cm from the bottom of the reservoir, which leaves 2cm to the "roof" of reservoir = 6 cm to top of cap attachment.
 

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The coolant will seek its own correct level (roughly speaking; not to the mm) by ejecting any excess once it reached the prescribed pressure release point of the cap.
 
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