Subject: [Healeys] BJ8 radiator cap issue

From: [email-masked]
Date: Sun, 31 Mar 2013 15:48:36 -0400 (EDT)

Kees,
You are correct, however I am assuming an original Healey type cooling system
without an expansion vessel & an original style radiator cap.  I agree that if
the radiator is filled with water such that there is very little air in the
system, then initially the non-compressible water will expand & the pressure
will rise up to the pressure rating of the cap. At that point the "excess"
water will be expelled & the available space will be replaced with
compressible air.  From then on, I do not believe the heated water &
compressible air will ever reach the pressure rating of the cap, unless of
course the water temperature rises above 212F to the pressure/temp equivalent
of the specific cap.

David nock's explanation is based on the assumption that the thermostat is a
bellows type & the bellows is operating correctly at or below 7 PSI & the
bellows is collapsing at a pressure somewhere between 7 PSI & 10 PSI. This
does appear to be a logical assumption/explanation. However, I believe the
original post indicated that the temperature returned to normal after the 10
PSI cap was replaced with the 7 PSI cap. I would think that if the brass
bellows had collapsed it would not function correctly at the lower pressure.
Replacing the thermostat with the type David describes is the logical next
step. The only thing I do not like about those thermostats is that they are
160 F. Of course, most of our driving is such that the thermostat will be wide
open & the operating temp will reach equilibrium at some temperature
independent of the thermostat control temp.

Gary Hodson

From: Oudesluys <[email-masked]>
Gary,

      This is not necessarily correct. If there is little or no air in
the system the pressure will certainly rise to the limit of the      rad cap
as the water will expand and as it is non-compressable the      pressure will
rise considerably. This is often the case with an      airtight rad cap and if
a expansion vessel is fitted.

      If there is a lot of air in the radiator, the pressure will not
increase a lot as the air will be slightly compressed by the      expanding
water and the increase in temperature.

      Kees Oudesluijs
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From: Oudesluys <[email-masked]>
Date: Sun, 31 Mar 2013 23:05:56 +0200

Gary,

The bellows will not always collaps in the sense that they get damaged. 
Because of the higher external pressure the bellows will contract until 
the vapour pressure (sorry I do not know and cannot find the correct 
English term) of the medium in the bellows (probably aether, ethanol, 
methanol or similar or a mixture) is the same as in the cooling system, 
thus effectively (partly) closing the thermostat. This means that the 
coolant temperature has to rise to open the thermostat again, hence the 
higher readings. In effect by raising the pressure in the cooling system 
you also raise the opening temperature of the thermostat. By returning 
to the lower cooling system pressure the function of the bellows 
thermostat will return to normal again.
Raising the coolant temperature at the higher pressure is no big deal. 
In effect it will create a more efficient engine and a better heat 
transfer as the Delta T between radiator and air is increased.

I have always raised the operating temperatures ( usually between 
82-960C) on my cars, adding if necessary an expansion vessel, a bigger 
radiator and a thermostatically controlled electric fan and often also 
raised the pressure as to prevent boiling.
You can still use a bellows thermostat, but you cannot rely on its given 
opening temperature. Determine that when it is installed when you watch 
your temperature gauge.

Kees Oudesluijs
NL

Op 31-3-2013 21:45, [email-masked] schreef:
> Kees,
> You are correct, however I am assuming an original Healey type cooling 
> system without an expansion vessel & an original style radiator cap.  
> I agree that if the radiator is filled with water such that there is 
> very little air in the system, then initially the non-compressible 
> water will expand & the pressure will rise up to the pressure rating 
> of the cap. At that point the "excess" water will be expelled & the 
> available space will be replaced with compressible air.  From then on, 
> I do not believe the heated water & compressible air will ever reach 
> the pressure rating of the cap, unless of course the water 
> temperature rises above 212F to the pressure/temp equivalent of the 
> specific cap.
> David nock's explanation is based on the assumption that the 
> thermostat is a bellows type & the bellows is operating correctly at 
> or below 7 PSI & the bellows is collapsing at a pressure somewhere 
> between 7 PSI & 10 PSI. This does appear to be a 
> logical assumption/explanation. However, I believe the original post 
> indicated that the temperature returned to normal after the 10 PSI cap 
> was replaced with the 7 PSI cap. I would think that if the brass 
> Replacing the thermostat with the type David describes is the logical 
> next step. The only thing I do not like about those thermostats is 
> that they are 160 F. Of course, most of our driving is such that 
> the thermostat will be wide open & the operating temp will reach 
> equilibrium at some temperature independent of the thermostat control 
> temp.
> Gary Hodson
> This is not necessarily correct. If there is little or no air in the 
> system the pressure will certainly rise to the limit of the rad cap as 
> the water will expand and as it is non-compressable the pressure will 
> rise considerably. This is often the case with an airtight rad cap and 
> if a expansion vessel is fitted.
>
> Geen virus gevonden in dit bericht.
> Gecontroleerd door AVG - www.avg.com <http://www.avg.com>
> Versie: 2013.0.3267 / Virusdatabase: 3161/6216 - datum van uitgifte: 
> 03/31/13
>

-- 
Kees Oudesluijs
Dorpsstraat 183
2995XG Heerjansdam
T: 078-677 1233
E: [email-masked]

Technische commissie Jensen Healey en GT
Het Jensen Genootschap Holland
www.jensenholland.nl
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