On 12 Nov 2007 at 0:18, David Brister wrote:
> Is there a guru with a definitive answer to whether it's better
> to have an electric fan wired to run whenever the thermal switch
> says so?
(Do you mean manual vs.thermo-switch or do you mean post engine
shutoff vs. only when the ignition is on? Should I be offended that
you didn't believe my original post? Ah, okay, I won't be.)
I can't speak for Volvo or VW though I can make guesses as to why
they keep their fans on after engine shutoff. But for a TR the
answer is pretty simple. The fan blows air through the radiator.
With the engine not running there is no water flow, so cooling the
radiator does nothing for the engine. The radiator itself is not
affected by heat one way or another, and cooling it down from its
normal running temperature (180DegF) faster than it would cool down
by itself after you shut off the engine makes no difference at all.
Now about the Volvo and VW. As I said before, the only reason you'd
want a fan to stay on is if there is something in the engine
compartment other than the radiator which benefits from cooling. One
possibility is a turbo. Being attached directly to the exhaust it
might overheat from direct heat transfer from the hot manifold (or
whatever part of the exhaust system it is attached to). This is
especially a problem with the bearings as the turbo spins down
because oil pressure also goes away when you shut off the engine.
Another possible reason for after-shutoff cooling is all the engine
monitoring and fuel injection control stuff. Tight, heavily
insulated-for-sound engine compartments don't allow much unforced air
flow, and engine sensors might pick up heat from the exhaust
manifold. That might shorten their life or it might just make the
car hard to start or run poorly when restarted.
There may be other reasons. But certainly no TR or Spitfire has any
of those things unless it is heavily modified. A Spitfire or GT6 has
more space under the bonnet then a TR, but none of either is as tight
as most modern cars. None has anything that would suffer from heat
transfer from the hot manifold. (Do be careful not to re-route your
fuel line too close to the manifold.) And after-shutoff electric
fans sure weren't stock in any of our cars.
Now, automatic vs. manual operation is a different question. An
electric fan's value is that it needn't run if you are moving fast
enough, which is most of the time. An automatic switch never forgets
to turn on when required and off when not. With a manual switch you
must watch your temp gauge. The temp gauge probably reads the temp
at the head whereas the fan's switch might be on the radiator. But
that subtle difference shouldn't matter much.
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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