Thanks, Dave. I think I get it now.
John H.
>Very well put. The air flowing through the radiator has to come from
>somewhere but it also has to go TO somewhere. When you place an object in
>an
>airstream it will create an air pressure depression (see carburetor theory)
>and
>reduce air flow. But the object will also create a pressure pocket ahead
>of it
>also decreasing air flow. Most low profile fans have the motor in the hub
>which
>has a diameter of 3 to 4 inches (8 to 10 cm). This is a significant
>restriction of air flow. If this fan is bolted up intimate with the
>radiator there
>will be no air flow through the radiator in the area abuting the fan hub.
>Likewise the fln blades will restrict air flow in a reduced but similar
>manner.
>
>But the inertia of the air will cause the height of the air pressure bubble
>leading the obstruction to be lower than the height of the depression
>pocket
>trailing. So the distance required to reduce the effect of this
>restriction is
>much less for a fan following a radiator than leading.
>
>Look at any modern car and you will see a fan shroud. The shroud does two
>things: 1: it spaces the fan away from the radiator reducing the effect of
>the
>obstruction and 2: ducts air from the full face of the radiator to the fan.
>
>This second effect means that when the fan is running it will pull air
>through the whole of the radiator and not just the part in front of the fan
>blades.
>This means that the air is passing through the radiator at a slower speed
>creating less pressure drop, it means it has less inertia and can change
>directions more easily, and for a given amount of air flow there will be
>more
>temperature rise which means the air is extracting more heat from the
>coolant.
>
>Bolting a hayden fan to a radiator with the through-the-fins fasteners is
>truely convenient but not an optimum solution.
>
>The same can be done with a pusher fan but the distance between the fan and
>the coil must be much larger and there isn't room in any practical car
>configuration. But one advantage to placing the fan in front is that the
>fan will
>move more air because the air is cooler and more dense.
>
>One advantage to mounting a pusher fan is that you can leave the stock fan
>in
>place and get the advantage of both fans (and save the work of removing the
>stock one). But if you have problems with overheating at speed (where ram
>effect predominates) then perhaps the radiator needs servicing. Or
>enhancing*.
>
>
>Dave Massey
>57 TR3
>71 TR6
>80 TR8
>
>P.S. * removal of the crank hole.
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