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.

 

 .  

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.

 

 .

 

Ed

 

If there is any doubt about the hose I’ll remove it … I would never have guessed how that expansion tank is working with this setup.

Originally I wanted the tank to be the highest point in the system and be connected to the inlet side of the pump where pressures are lower and it would make an ideal filling point for entire system; however, there isn’t enough height under the cowl to mount it above the water pump outlet.

During first flight the pressure got pegged to >20 psi > cap pressure … I removed that cap and put back the old one at 16 psi … now if the system is pressurized through the snifter valve it is regulated at 16 psi by the cap, as I would expect. So I relieved the pressure to 10 psi and ground-operated the engine; it pressurized and regulated itself back to 16 psi from 130F water temp all the way up to 200F water temp … but what is startling is that the tank is actually cool to the touch … the water pump is hot, radiator and its hoses are hot … hot pressurized air is exiting the engine through that hose and 1 cup of coolant but having very little temperature effect on either … I love it … I believe the system is working perfectly and don’t see any reason to operate at pressure above 16 psi.

Jeff

(Just waiting for the weekend and flight #2)

 

 

Jeff, you might check with Tracy Crook.  Way back when, he attempted to use some clear plastic/nylon cored reinforced hose for his coolant system for the same reason (I think) – so you could see what was going on.  As best I recall, it was not up to the task due to heat and pressure – but these were the main line hoses and not just to the header tank. 

 

I can not recall the type – but think he got them at the local hardware store.

 

Ed

 

 

Thanks Kelly, I copied (Bill Eslick’s) installation (seen at Round-up) and the see-through aspect of that hose was a major selling point; it’s easy to see whether there’s air or fluid in the line, which is very helpful … ideally there should be fluid in the line and about 1 cup in the expansion tank; I suppose removing the cap would provide same information … but the tank has to be de-pressurized, cap removed and re-pressurized before flight.

Bill any comments on durability of that hose?

Jeff

 

 

-------------- Original message from Jeff Whaley <jwhaley@datacast.com>: --------------

Thanks to you all for the congratulatory comments … attached is a picture of expansion tank and preflight roll-out … the orange line is fuel return; the brass is pressure gauge pickup and snifter valve installation.

After reading Eds’ email I now know why the water temperature was so high.  I installed a thermostat with the onset of cold weather, but I couldn’t use the original Mazda thermostat because my temperature bulb protrudes across the bypass hole.  Originally the bypass hole was plugged and there was no thermostat; however, I removed the bypass plug and used a non-Mazda thermostat !! … so that’s why the underside of the Mazda thermostat is so long! Thanks Ed for setting me straight and based on your comment about 20% loss of efficiency, I could possibly see 230F x 0.8 or as low as 184F on next flight … that would be great! For now the thermostat is gone and bypass is re-plugged.

Closer examination of the belt and rubber deposits on underside of top cowling suggests the belt got jammed in the pulley and cut by friction.

I tried again last night to remove the alternator pulley nut … what’s the secret? Left-hand thread? Loctite or what? It’s so tight I’m afraid of damaging something trying to get it off.

Jeff

 

 

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