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
There's one
coolant design that's substantially better than the others. I discovered it
when doing severe ground testing...deliberately overheating my engine.
Three
basic requirements:
1)
Place your radiator cap and reservoir above engine. The higher the better.
This allows trapped air in system to rise out of the flow and stay there.
2)
Put a 24 psi cap on the system. You can throw away your overflow stuff. Not
needed.
3)
Always keep around 2 cups of air under the cap. This is the key item. It
brings a big safety advantage. It allows you to use coolant pressure to
predict well in advance how good your system is doing. It minimizes pressure.
Mine never exceeds 7 psi. But if something goes wrong, then my pressure rises
and I gain boilover protection due to the increase in system
pressure.
-al
wick
Cozy IV