Greetings,
With the recent talk of bypass hoses and cooling problems in cars with
skirted thermostats, I thought I'd take the liberty of posting the
following technical description from an Australian Triumph club
newsletter. Thanks to John Pike for forwarding it to me in the first
place, and I hope you don't mind John, if you're still on the list, as
I'm posting this in the hopes of benefiting everyone who may still
have some doubt about these issues.
****************
"Last newsletter carried an article that recommended the blocking of
the radiator bypass system when a skirted thermostat is not available
and a non-skirted type has been fitted.
I most definitely do NOT recommend that this action be taken, for the
following reasons.
While the radiator bypass serves no useful function when the engine is
at normal operating temperature and the thermostat is open; it serves
a vital role when the thermostat is closed.
When a cold engine is started the thermostat is closed and coolant is
prevented from circulating through the radiator. However, under the
influence of the water pump coolant does circulate through the block
and the head via the bypass hose, ensuring an EVEN distribution of the
generated heat throughout the engine.
As the coolant must pass the thermostat on its way to the bypass
outlet the thermostat heats up at the same speed as the coolant so
when the temperature reaches the specified temperature for the
thermostat, the thermostat begins to open, admitting a small amount of
cold coolant from the radiator at first, which has the initial effect
of lowering the temperature of the coolant in the engine and slowing
the rate at which the thermostat opens; this is important to prevent a
sudden surge of cold coolant into a hot block and head.
When the coolant temperature in the whole system has stabilised the
thermostat will continue to vary as necessary to maintain the correct
operating temperature for the engine.
If the thermostat is closed and the bypass blocked, coolant cannot
circulate and coolant heating is localised to areas adjacent to the
top of the cylinder bores; coolant does not pass by the thermostat and
heat only reaches it by conduction.
In this situation several things can happen and none of them are good
for the engine.
Since heat cannot be quickly conducted away from the cylinders, hot
spots develop. These can be hot enough to cause local boiling of the
coolant. The steam generated forces coolant back through the water
pump to the bottom radiator tank and then up and out through the
radiator cap and overflow pipe.
The steam forces the water out of the head and then reaches the still
closed thermostat which opens immediately, allowing the steam to pass
into the cold coolant of the upper hose where it condenses
immediately.
The heated coolant which was forced into the lower radiator tank
returns to the block and head and passes straight through the now wide
open thermostat to the top radiator tank. The very hot coolant is
immediately followed by the remaining cold coolant from the radiator
with consequent rapid cooling of the head and block - an excellent
recipe for cracking both of these.
Another scenario is that the coolant does not boil, but coolant that
is hot enough eventually reaches the thermostat and it begins to open.
Coolant begins to flow under the influence of the water pump and,
because the rest of the coolant in the head is much hotter than that
which first reached the thermostat, the unevenly heated block and head
are quickly cooled by the cold water from the radiator, with a similar
recipe for disaster as before.
If you are one of the growing number of TR owners unable to find an
original type thermostat, I can recommend the following procedure.
Use an off-the-shelf unskirted thermostat with a heat rating as near
as possible to the recommended rating of 70 degrees C. Do not use a
thermostat with a rating over 80 C.
It is permissible to reduce the bypass access to a minimum of about
3/8 of an inch. The important thing is to maintain at least some flow
during warmip. It should be pointed out that the skirt on the
original type thermostat did not seal off the bypass, but rather just
directed coolant flow more towards the top hose outlet. "
and John added:
Hope that helps a bit. You mentioned that your old Volvo thermostat
fitted the TR. However, the inside diameter of the opening is as
critical as the outside. The original TR thermostat had an outside
diameter of 54mm (sorry about the change in units), and an internal
opening diameter of 33mm. Many after market thermostats have an ID of
only around 25 mm, and can therefore restrict flow through the system,
leading to the inevitable overheating. One of our local TR drivers did
some research on the matter, and finally came up with a thermostat
which has an OD of 64mm and an ID of 44mm. It's a very simple matter
to grind the OD down to 54 mm and away you go. I have fitted one of
these in my TR, and as I said last night I have no overheating
problems at all. The particular thermostat is made for our local
Holden models and for the Nissan Skyline 3 litre. It's made by Dayco
Pacific, part number DT19E-BP, and has an opening temperature of 77 C.
***************
This all makes sense to me. I guess the bottom lines are:
1. Don't completely block off bypass hose.
2. Check the thermostat opening's diameter.
3. You can make something up that will cool your car properly.
Regards,
Jim Wallace
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