Bob:
The pressure rating on the cap raises the boiling point of the
coolant. A 50/50 mix already has a slightly higher(?) boiling point than
plain water, and under pressure the boiling point goes higher still. The
cap has NO effect on operating temperature of the motor, only the
maximum temperature that the cooling system will tolerate.
The idea is simply to keep your coolant from blowing itself all
over the pavement as the engine gets hot. The pressure rating of the
radiator cap has no effect on engine temp. When the coolant boiling
point is reached the coolant turns to steam and QUICKLY leaves your
cooling system. With the coolant gone, the engine temperature reaches
unacceptable values and you must shut the motor off to avoid damage. The
higher the pressure rating on the cap, the higher temperature needed to
boil coolant. By the way, the upper limit to this little scheme is
determined by when a gasket or a hose blows out under pressure. The goal
here is to let the pressure climb to resist boiling, but not to let it
climb so high that a gasket or hose lets go. So the pressure rating on
the cap should be lower than the maximum pressure your radiator hoses
will tolerate. 4 PSI is the absolute minimum a decent cooling system
should handle.
On an old or poorly maintained system, the lower pressure cap
will avoid coolant spraying out around fittings and clamps under
pressure. But the correct way to deal with this is to replace those
tired hoses, clamps, etc so that maximum operating pressure can be
maintained. Higher pressure gives you more operating margin, but if your
DPO was too lazy to replace worn out hoses, then a lower pressure cap is
a ghetto solution to incessant leaks. Instead the coolant vents into the
overflow bottle.
The engine will run happily (but with reduced power) at say,
220F, but without a sealed high pressure system the coolant will not
stick around and your trip will be cut short. The center of your temp
gauge is approximately 180F, as I recall.
Vance
-----Original Message-----
From: owner-6pack@Autox.Team.Net [mailto:owner-6pack@Autox.Team.Net] On
Behalf Of Bob Danielson
Sent: August 13, 2007 5:38 PM
To: triumphs@autox.team.net; 6pack@autox.team.net
Subject: [6pack] Radiator Cap Pressure Rating
While poking through the 6-Pack forums, I saw a post on the impact of
radiator caps and thermostats on cooling the car. I understand the
thermostat impact but what does the radiator cap do cooling wise? It
somehow
raises or lowers the boiling point? I checked and my car has a 4 lb cap
while Moss & TRF state that a 13 lb cap is needed for my model year.
Does
the 4 lb cap make the car run hotter or cooler?
Thanks
Bob
Bob Danielson
1975 TR6 CF38503U
Running w/ Throttle Body Injection
http://tr6.danielsonfamily.org
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