Al W;

 

I don’t disagree with your concept, but I think there are other considerations.

 

The air volume number of 2 cups only applies to your system.  The 7 psig you mention implies that you only get a bit over ½ cup expansion of coolant.  In my system I know I get at least 6 times that much.

 

In addition to the boiling point protection, there is the consideration of pump cavitation.  When I was designing my system, I recall some pretty awful photos (on the ACRE list) of what cavitation can do a pump impellor.  Since we generally run at high rpm, it’s likely an issue.  

 

And “ideal” pressure depends on where you measure it, and where the cap pressure is.  There is likely at least a few psi difference between the pump inlet and the outlet.  If the 7 psig (gauge pressure) you mention is on the outlet side, then it may be close to 0 psig at the inlet, and considerably lower behind the vane in the pump. At 180F water flashes to steam bubbles at about -7 psig (8 psia), so there could be a have high probability of cavitation.  And that’s at sea level. If the 7 psig is at the pump inlet, you’d be a bit better off.

 

That’s why I designed my system to have a ‘cap pressure’ of 23 psig on the pump inlet side, why I have no air under that cap, and why I have a pressure cap on the expansion bottle; so even when the coolant temp is decreasing, and the cap pressure is 0 psig, there is still some positive pressure at the pump inlet.  Of course designing for this consideration takes care of the boiling protection as well, and provides some margin for altitude, since we generally fly considerably above sea level. It has worked fine for me so far.  Concerning masking a compression leak, I think whether the pressure is 7 or 27 it is negligible compared to combustion pressures.  

 

I don’t know that this is necessary, right, wrong, or ideal; but it made sense to me.

 

Al G