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