Hmm. Sounds like the well-known issue of the flow difference caused
by the 1-1/2 pass radiator, although it’s certainly a unique way to
measure it. It’d be fun to install a single-pass rad and see what
happens!
– Kirbert-----------------------------------------------------------------------------------
Or maybe not . . . . . . .
The HE engine in the coupe is basically stock internals, but has an
electric water pump feeding through a blanking plate into the same
place as the mechanical one did.
Still has the OEM radiator, recored once.
Has accurate temperature sensors on A and B bank close to the thermostats.
Both always seems to read within 2 to 3 deg C of each other.
Must admit that does not tell me what goes on at the back of the head.
As reported earlier, the cooling system maxes out at around 47deg C ambient.
Not sure if anybody has definitive proof, but I suspect the most
critical factor is how much air can be pushed through the radiator
rather than how much coolant can flow through the tubes.
With an electric water pump you could possibly test this.
First jam the thermostats fully open, then temporarily fit a flow
meter for the pump outlet.
Keeping all other variables constant, adjust the pump voltage and see
what happens. If my suspicion were correct, the coolant flow could
drop a fair bit before the coolant temperature starts to climb significantly.
I wonder how scientifically engineers design their cooling systems ?
My bet is they start simply by rule of thumb based on years of experience.
So they start with a proven water pump design driven by a belt and
pulley to give a reasonable RPM.
Worked before for similar size engines.
The radiator has to fit in with whatever the stylist thinks the front
end should look like.
In Lyon’s opinion it better be low and sleek.
So the radiator cannot be too high.
Try the one and a half idea because the slide rule ( yes, a slide rule
in those days ) reckons that the coolant and airflow will be at least
adequate and maybe better than adequate.
And the front is low and sleek.
Use the biggest and fastest fan you can fit and afford just this
side of objectionable noise.
Build it and try it.
Drive in conceivably worst case conditions.
If it is not good enough tinker a bit and make it work.
Project finished.
The end result is many owners running stock standard systems for 35
years in some cases, probably with good maintenance, and never any
serious trouble.
These owners may not race their cars, but they will endure traffic
crawls on hot days which is probably just about as bad.
Others suffer problems one way or another.
The best insurance is plan A.
Maintain it well and stop if in doubt.
Plan B is pretty cheap. I bough a box of 105deg C thermal switches
made for appliances.
Bankrupt stock, priced to clear.
I have one screwed to each head, just below the inlet manifold.
They are hooked up to a relay which drives a small buzzer in the console.
When the car had a dyno run the coolant got quite hot after 20 minutes work.
One of the switches activated and the buzzer was singing for 5 minutes
until cool down.
Normally I keep a good eye on the temperature gauge, but even so a
burst hose could catch you out and melt an engine within minutes.
In fact I bought the coupe in 1984 at 5 years old and within 2 months
had a burst hose, luckily it burst about 60 seconds before I arrived
to park at the factory which saved the engine.
That can happen with any car.
Richard Dowling, Melbourne, Australia.
1979 XJ-S coupe + HE V12 + 5 speed, 1988 XJ-S V12 convertible, 2003 XJ350
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