Hi Ed, I understand what you’re saying, but as stated before  … “I would never have guessed how that expansion tank is working with this setup”.

I expected that if the cap pressure was exceeded, the system would purge some coolant; however, so far it is only purging air; the overflow bottle is bone dry.  The expansion tank is about 1 quart and it mostly empty; i.e., room for expansion.   I was only ground running the engine and never exceeded 4000 rpm. Do you expect it to behave differently at higher power settings or different at altitude?

Jeff

 

 

On thing to consider about the 16 psi pressure cap - back a few years ago a fellow or two were having problems losing coolant during a flight.  No clear evidence of why, but a cup – pint or more of coolant loss occurred just about ever flight of any duration.

 

 To understand what was happening its easiest to  look at your cooling system pressure and the radiator caps capacity in terms of Absolute pressure rather than the gauge or differential pressure we normally use and think in.

 

Atmospheric pressure is by definition Absolute pressure.  So at sea level without the engine running the absolute pressure inside and outside the coolant system is 14.7 psi.  So your coolant pressure gauge which is a differential pressure gauge is reading zero because coolant system 14.7 psi absolute – atmosphere 14.7 psi absolute = zero difference.

 

Now if you put a 16 psi radiator cap on your coolant system then whenever the differential pressure (absolute coolant pressure – absolute atmospheric) is less than 16 psi the cap will hold the coolant in – if the differential pressure is greater than 16 psi then it will vent.  So the absolute pressure capacity of your cap is 14.7 atmospheric + 16 psi coolant = 30.7 psi. – AT SEA LEVEL

 

So you run your engine at sea level and the pressure gets to 12.7 psi according to your pressure gauge (also a differential pressure gauge) then the total  absolute pressure is - 12.7 coolant + 14.7 atmospheric(already in the coolant system) = 27.4 psi absolute.  You’re OK because your pressure capacity is 30.7 psi absolute and your  actual coolant pressure is 27.4 psi absolute – so you have a margin of approx 3 psi at sea level..

 

Now fly to 8000 ft MSL or so where ambient pressure is around 11psi absolute.  Now you have the 16 psi   cap + 11 atmospheric for a total of capacity of   11 + 16  = 27  psi absolute before the cap vents. Note that this is 3.7  psi less than what its holding capacity was at sea level.

 

 IF (and it may not – probably will NOT for a lot of reasons) your coolant system is still producing the same absolute pressure (27.4 psi) as  it did at sea level (yes, the pressure gauge will show more than 12.7 because it’s a differential gauge and the outside pressure is less) but the absolute pressure of your coolant system would still be 27.4 psi).

 

 So  the absolute pressure  IN your system is still 27.4  psi absolute in this example, but your radiator cap (with the lesser ambient pressure) is only capable of holding 27  psi absolute at this altitude – in which case the cap will now release and coolant will flow through the radiator relief valve because your  Caps limit (spring + atmospheric 16 + 11  = 27 psi) is now less than the absolute coolant pressure  at altitude < 27.4 psi by 0.4 psi. 

 

Now in your case you  should be fine as it is unlikely you will still have the same heat and coolant pressure at altitude – but, I use a 21 psi cap because it provides a bit more margin.  However, you don’t want to go too much to the extreme, as a higher pressure cap may result in leaks around fittings and gaskets/seals of the cooling system (water pump for instance).  So I think 21 psi is a reasonable compromise – but, that is simply my viewpoint.

 

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In any case, the individual with the problem I mentioned earlier  tried using  a 21 psi cap and that immediately cured his problem. So I feel that 14.7 psi is simply to0 low for our applications, but  your mileage may vary.

 

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Ed