[TR] radiators, what vs. why

[Jim Muller]

This discussion of radiator fans has been interesting.  One thing to 
keep in mind is the difference between what it does and why it does 
it, or more to the point, why the engineers did it that way.

Sure, many cars today spin their fans after being shut off, even 
cycling on/off a few times as the radiator sensor cools, then warms, 
then cools.  That cycling may be from the water flowing inside the 
radiator due to temperature differences (i.e. thermal convection).  
It isn't obvious whether much water is flowing between the radiator 
and the block.  At some temperature the thermostat should shut off 
the flow between them.  Maybe that's when the fan stops for good.  A 
temp gauges should show a rise after the engine is shut off (if the 
gauge is still powered) because heat still propagates from inside the 
engine to the fringe of the head where the sensor is attached.

The real question isn't what a particular car does but whether it was 
intentionally engineered to do it for some specific reason.  It might 
be that they implemented some specific heat-management technique for 
post-shutdown, and that may have included thermal convection between 
radiator and block.  Or it may be that they were concerned about 
electronics or turbo bearings, or some other thing.  Or as Dave 
Massey pointed out, it might just be that the designers saved a few 
bucks on a relay and lower-current ignition switch, and judged the 
result acceptable.  Just because the fan stays on post-shutdown 
doesn't mean it was the engineers' primary intent.

It is a safe bet that few of those specific-engineering reasons 
applied to TR's except possibly keeping fuel lines and carbs cool and 
saving bucks.  If you want to wire your new electric fans that way, 
go ahead.  The pros and cons are obvious.  But it isn't required.  (I 
never saw a mechanical fan that would spin for long after the engine 
was shut off. :-)

Louvers are the ultimate answer, you know.  They look cool, require 
minimal tooling, don't run down the battery, don't need no Lucas 
connectors...


Jim Muller
jimmuller at rcn.com



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