Bill;

 

Working just fine.  I haven’t noted a pressure over 30 psi, even at the highest temps.

 

Al

 

Al,

Now that you are flying, how is your series connection of coolant bottles with pressure caps working? I have constructed something similiar, and there was some discussion that claimed that it wouldn't work. I would be interested in what you found out.

 

Al, Rusty and others.

 

I've been a little confused with the set- up recommended. Do you have a pressure cap on the Radiator as well as the Expansion tank and do you have an overflow bottle attached to the expansion tank.

I've also seen where the rad pressure cap is not on the rad but on the return to the pump.

 

I would really like to sort this out so that I get a clearer picture of the correct set-up.

 

Advice appreciated.

 

George ( down under)

 

George;

 

My system has two pressure caps.  The filler neck on the left in the photo connects to the pump inlet side of the loop (lowest pressure point).  That neck has a 23 # cap; and when the system is full and purged, there is never any air at that cap.  Overflow from the main filler is plumbed to the bottom of the overflow bottle.  The overflow bottle normally is maybe half to two-thirds air as required to accommodate the expansion from cold to hot.  It has a 15# cap. 

 

The idea is to always have positive pressure on the inlet side of the pump to inhibit cavitation.  Without a pressure cap on the overflow bottle, expansion of the coolant in the engine pressurizes the system as it heats up. But with no air in the system, after the peak is reached, the pressure goes back to zero (or slightly negative to draw coolant back from the overflow bottle) any time the temperature drops a bit; as when you reduce power after climb-out.

 

There are no caps on the radiators (2) as they are mounted at lower levels.  There are air bleed lines from the highest point of the tank on each rad that go back to the filler neck; which is always at the lowest pressure in the loop so the air will go there.

 

With both caps fully latched, it is possible for the pressure to peak at about 38 psi (23 plus 15), at sea level, if the overflow bottle is full (coolant fully expanded).  This give more margin (higher boiling temp) and less likely cavitation at extreme conditions. Generally there is always residual air in the bottle that is compressing, so the pressure doesn’t reach peak.  Caps are ‘differential pressure’ so at higher altitude the absolute pressure in the system is lower.

 

A similar, and simpler approach is to have an expansion tank that has air in it connected directly to the system, say as a filler tank with a pressure cap, that is only half full when cold.  Pressure builds as things heat up, up to the max cap pressure.  The difference in my approach is that the pressure builds very quickly to the system cap pressure because the coolant is incompressible (not counting some expansion of hose connectors).  So even if the engine is not fully warmed up and I give it full throttle on takeoff, the system is at least 23 psi at the pump inlet.  Same is true any time later when power (and RPM) is increased.

 

Hope this all makes sense,

 

Al