Quote:
Originally Posted by morris
I've been taking all ratings for caps as relative to atmospheric pressure. I've therefore also been assuming that a 0psi cap is simply just a cap and in no way inhibits the flow past the neck to the overflow.
I've pretty much come to the decision of fitting a 1.4bar cap on my header, a 0 to <1.4 on my rad and then a catch tank next to the header which will suck back any over spill of coolant as the system cools down.
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Hi Ian,
And there in lies my conundrum!!
Practically I agree with you and I am going down the same route. And I recognise that BMWs were built to run in all climates so in the Middle East for example, the higher the cooling system pressure, the higher the boiling point of the coolant and the less chance of the engine overheating.
But from a theoretical, and an understanding perspective, and taking atmospheric pressure as the reference point, if you use a 1.4 bar cap on your header, do you think the cap will release when the cooling system reaches 1.4 times atmospheric pressure or 2.4 times atmospheric pressure.
And if you look on the SVC website they have the same header tank as you have used and they offer caps to go with it ranging from 0psi to 20psi. If that figure is relative to atmospheric (and I assume it must be 'cus why would you want a pressure cap releasing below atmospheric pressure ie. a partial vacuum) then that will give a cooling system pressure of 15psi (atmospheric) + 20psi (the highest rated cap) which is equivalent to 35psi. Now as was said previously, 35psi is well into tyre pressure territory. Could a cooling system really run at that sort of pressure...........................??
So I remain confused. I know what I am going to do practically but what do the numbers actually mean?? Is it simply a terminology thing where, when using 'bar' the quoted figure is a multiple of atmospheric, but when using psi, atmospheric pressure is used as a reference and the psi rating is relative to atmospheric.
Nigel