Listers,
Most authoritys, including fan manufacturers, claim that a puller fan is more
efficient than a pusher, but the difference is not an order of magnitude. The
numbers usually given range from 10 to 30 %. If the fan you are adding,
coupled with your radiater/shroud arrangement, is capable of supplying 130 %
of your cooling needs, than it doesn't matter where you put the fan. If your
setup is marginal, then it will need to be installed as a puller.
As for the electrical demands, it's unfortunate that the fan and the
alternator work at odds with each other. The fan will be used the most when
you are sitting in traffic at idle, the time when the alternator is producing
the least amount of current. If you are stuck in a traffic jam on a very hot
day for a long time, you could draw the battery down quite a bit, as the fan
will be running almost constantly. When you are driving at high speed and the
alternator is producing its maximum, you probably won't have the fan on at
all.
A 28 amp alternator may only produce 14 amps (or less) at idle, and a heavy
duty fan can draw as much a 18 amps. Even some of the more modest fans can
draw 9 amps or more, leaving only 5 amps for the rest of the car. The coil
will pull another 3 amps or so, leaving only 2 amps for everything else. Any
items drawing current over that 2 amps will have to draw from the battery. An
18 amp fan and the coil together will draw 7 amps from the battery, with
nothing else on.
We get a little break here, in that traffic jams in very hot weather normally
occur in the daytime in the summer, so we can turn every thing else off and
leave only the fan and the engine to be supplied by the alternator. At night
time when you need your lights, or in the winter when you need your heater,
the fan will not be needed quite as much.
There are several choices, some of them OK, some of them not so OK. Just be
sure to consider all the parameters before you decide which way to go.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://members.aol.com/danmas/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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