It's the thermostat that sets the 'stable' temperature, and you certainly
won't get 187 degrees with a 165 degree stat *as long as* the radiator can
dissipate at least as much heat as the engine is generating. Under these
conditions *any* thermostat, within reason, will be partially open and
maintaining *its* given temperature. Neither will a lower (or higher)
ambient, within reason, give a different water temperature with a given
tat - the whole point of the stat is to *maintain* a temperature after all.
Three different thermostats will *always* result in three different temp
gauge readings under normal operating conditions. The only time any
thermostat is wide open is when the engine is generating more heat than the
radiator can dissipate, i.e. slogging up a steep hill on a hot day, and you
will see that as the temp gauge needle being above its 'normal' point,
whatever that may be.
Stats aren't just open or shut. There is a temperature at which they start
to open, and a higher temperature at which they are fully open. And the
only time they cycle open and shut is when the engine is in the process of
warming up, and you can see this on the temp gauge. Also they have
hysteresis which means that they will be open more at a given temperature
point when the temperature of the water is dropping than when it is rising
(just like the electric cooling fan stat turns off the fan at a lower
temperature than it turns it on). You can see the effect of this on the
temp gauge by slogging up a hill at a given speed, then cruising down at the
same speed on the overrun in the same gear. The radiator is getting the
same amount of cooling but the engine is producing more heat going up than
down, and the temp gauge will be higher than normal going up and lower than
normal going down.
Have you ever wondered why the temp gauge goes up after you have switched
off? And goes up more on a hot day than a cool day even though prior to
switch-off the temp gauge reads pretty much the same? This is the effect of
the higher temperature in the combustion chambers than the water jacket
while the water is circulating - the temperature gradient - and it is higher
on a hot day because the engine is getting less auxiliary cooling via its
external surfaces due to the higher ambient. As soon as the water pump
stops the coolant nearest the combustion chambers heats up, and
thermo-syphonic action distributes this around the top of the engine and
hence the temp gauge sensor.
The cooler stat is intended to be used it hotter ambient conditions (i.e.
the use of the heater would probably be inappropriate) so that as long as
the radiatior is still capable of dissipating the heat the engine is
generating, the coolant temperature will be lower than if a normal stat were
used in order to maintain the internal temperatures of the engine closer to
normal.
IMHO
PaulH.
----- Original Message -----
From: Max Heim <mvheim@studiolimage.com>
To: MG List <mgs@autox.team.net>
Sent: Friday, April 21, 2000 9:36 PM
Subject: Re: (Paul) Shroud Question
> I still don't see how fitting a cooler 'stat makes the engine run any
> cooler, if in warm weather the 'stat is wide open anyway. At this point,
> with the engine fully warmed up and operating at a stable temperature,
> say, 187 degrees, both the 165 and the 180 degree thermostats would be
> wide open, and there would be absolutely no difference between them.
> Meanwhile, a 190 degree thermostat would be cycling open and shut,
> presumably. Now if the ambient temperature were lower, so that the engine
> would reach equilibrium at, oh, 172 degrees, then you would notice an
> effective difference with the "cooler" thermostat. BUT (and this is a big
> "but"), from my experience, at this kind of ambient temperature you are
> more likely to want to run the heater at full efficiency, and are not
> concerned at all with overheating or low oil pressure. In other words,
> it's cold out, and you would prefer the high temperature thermostat at
> this time, anyway. This is why I don't believe in fitting a unreasonably
> low temperature thermostat, like 165 degrees. The engine will never
> stabilize at that low a temperature EXCEPT when it's already cold out
> (and you don't want it to).
>
> Obviously, if you have fitted a larger radiator, larger oil cooler, or
> extra cooling fans, this may not apply to you. But then you have to
> consider whether 165 degrees is the most efficient operating temperature
> for the engine. IMO it is not.
>
> Paul Hunt had this to say:
>
> >The other thing, and the real reason why hotter and cooler thermostats
are
> >recommended for extremes of climate (well, extremes when compared to the
UK)
> >is that there is always some cooling via the engine surfaces. Small
engines
> >in very cold weather may never get up to temperature when cruising, which
is
> >why people blank off the radiator in winter. They are not really
blanking
> >off the radiator of course because the thermostat is closed anyway, what
> >they are doing is reducing the icy blast through the engine bay and hence
> >the block cooling.
> >
> >When an engine is run in a relatively hot climate there is relatively
less
> >block cooling for a given load, so more heat has to travel through the
> >radiator to get cooled, and heat is retained in the engine for longer,
> >raising the internal temperature of the engine above what is ideal. A
> >cooler stat helps to compensate for this reduction in block cooling.
> >Indeed, at one time the cooler stat was fitted as standard in order to
> >increase (or rather restore) oil pressure. This works by reducing the
> >internal temperature of the engine, so running with a cooler stat really
> >does allow the engine to run cooler. Likewise a hotter stat in cooler
> >climes helps to compensate for the *increased* block cooling and
resulting
> >lower internal temperatures.
> >
> >PaulH.
> >
> >----- Original Message -----
> >From: Bruce Burrows <bburrows@webtv.net>
> >To: <mgs@autox.team.net>
> >Sent: Wednesday, April 19, 2000 4:39 PM
> >Subject: Re: (Paul) Shroud Question
> >
> >
> >>
> >> --WebTV-Mail-8772-525
> >>
> >> Your reply (which appears below) was a good one, Paul, and much
> >> appreciated. I have to say you forced my mind to work just a bit!
Here
> >> is my best take:
> >
> >...
> >
> >
>
>
> --
>
> Max Heim
> '66 MGB GHN3L76149
> If you're near Mountain View, CA,
> it's the red one with the silver bootlid.
>
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