Thanks, Ed, Al & Mark for you're input.
I
think I'll go with the simplest solution first, then add an expansion chamber
if that doesn't work.
I'm
thinking of getting one of those plastic overflow bottles with a screen
washer bottle fitted up next to it from a breakers yard. I'd mount this
high on the firewall. It would be simple to pipe the screen washer to the
plenum for use in hot day climb-out. Any thoughts?
21-24 PSI seems
a bit high for the cap, though. I have a 16lb cap. Do you think I should
change it?
John
John, Regarding
the radiator cap capacity. The radiator cap is a "gauge"
pressure cap in that it relies on atmospheric pressure along with its spring
to help it hold the coolant in. At altitude, with less atmospheric
pressure, the overall capability of the cap is lower. Also, reportedly
they tend to loose a bit of their capacity after a few heating/cooling
cycles.
I tried 16 PSI cap
and found I lost coolant on each flight. Once the cap releases fluid that
lowers pressure inside the radiators which if hot enough can then send a big
surge through the cap and overflow your overflow bottle.
I have recommend
to a couple of folks who reported loosing cooling using 16 psi caps to go to a
higher capacity cap and that solved their problem. In addition, most of
the literature on coolants indicate that you would like to keep the coolant
pressure high enough so that the coolant (water) would not have a tendency to
boil even at temps above 212F.
So you can try
your 16 psi cap, but if you find yourself loosing fluids (even with an
overflow bottle) on flights, then try a higher capacity radiator
cap.
Here is what on
radiator maker says about radiator caps
Radiator Caps
In a cooling system, a higher
pressure equates to a higher boiling point for the coolant. Higher coolant
pressures also transfer heat from the cylinder heads more efficiently. We
recommend using a radiator cap with the highest pressure rating that the
radiator is designed to accept. In general, performance radiators will accept 22-24
PSI, and professional racing radiators will accept a
29-31 PSI.
The coolant will typically only build to 16-18
PSI, due to expansion up to 200°F. However, if the engine does overheat due to
external factors, the pressure inside the cooling system could reach as high
as 28 PSI. Once the radiator cap has opened and vented coolant, the engine
will not cool down until it has been turned off. The radiator cap is basically
a "safety valve", so always use the highest
pressure radiator cap that the radiator will tolerate. If
you are unsure of the pressure rating for your radiator, check with the
manufacturer for the maximum recommended operating
pressure.
I don't quite understand their statement about the engine not
cooling down after venting until you turn it off. I would think
that reducing power and therefore heat into the coolant would cause the
problem to subside - assuming you have any coolant remaining {:>), perhaps
their statement "..and Vented Coolant" means ALL the
coolant.
Evaporator cores have a burst pressure around 250 psi, so that
is not your limitation. I would hesitate using more than 21-24 psi as I
would be concerned about pushing water out around the water pump
shaft.
In any case, your decision as always
{:>)
Ed