At 09:19 PM 6/6/2003, Dennis Haynes wrote:
>The spring in the rpesure cap will have the same force regardless of
>altitude, so the pressure inside the colloing system will not be

I think we tend to run into a difficulty with this sort of thing because
it's easy to measure pressure relative to whatever the local atmosphere is
-- so many psi if it's more, so many inches of  mercury if it's less.  We
get used to thinking of pressure and vacuum as two different things.  But
lots of things (like boiling point) depend on the absolute pressure which
includes the pressure of the atmosphere and anything else that happens to
be pushing on the situation.  So the spring cap will keep the pressure
limit constant relative to the local atmosphere (just as the fuel pressure
regulator keeps the fuel pressure constant relative to the manifold
pressure), but as atmospheric pressure falls (about an inch of mercury or
half a psi per thousand feet altitude for reasonable altitudes) the
limiting absolute pressure, and hence the boiling point, will fall accordingly.

If your cooling system is running at 5-6 psi(gauge) which seems to be
typical running down the road, you won't run out of room until you reach
around 16,000 feet -- and the engine will be making less power and the air
will likely be pretty cold, so there should still be a margin.  If engine
output falls off linearly with pressure drop I suspect things will tend to
even out.  Less heat capacity in the air, but cooler air; and less
heat-rejecting ability in the system, but also less heat input.

david

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"