Joe, once you leave the T-stat housing (including the hose to the radiator)
you're OK. I've got a similar setup in my car, but after replacing the
radiator I use the cap on the radiator as my primary cap. The other one in
the hose leading to the radiator doesn't suffer from the over-pressure
situation. After examining the thermostat housing closely I understand
why. The outlet from the TR thermostat housing has a fairly small inner
diameter and very thick walls. The restriction is almost as much as the
"dummy" thermostat body that many of us run within the thermostat
housing. The hose, on the other hand has a pretty big ID. The restriction
in the output of the thermostat housing is what keeps the pressure in the
block if you're not running a thermostat as near as I can tell. Next time
you have the thermostat housing off of your car, look at how small the ID
of the output pipe is compared to the input and you'll see what I mean.
When we had the problem on Jack's car, we could duplicate it by just
revving the engine without load. At a certain RPM, suddenly the water
began pouring through the overflow even with a 15 or 22 lb cap. (Cap on
the thermostat housing) We first suspected massive head gasket leak, but
were very pleased to fix the problem by plugging that particular
overflow. I was happy that Henry Frye had shared his experience (prodded a
bit by Kas's suggestions regarding flow and pressure through the block) in
plumbing a pressure gauge into the block. That's what gave me the clue as
to what was happening.
- Tony Drews
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