[TR] Fan Hookup

[Jim Muller]

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 at rcn.com
'80 Spitfire, '70 GT6+



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