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X-fan location


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I received my X-fan this morning (excellent service and fast delivery as always. Thanks Si).

Reading the instructions it states that "The adapter should be fitted in the bottom radiator hose".

I would have thought the top hose would be more appropriate.

If, for example, you are travelling at high speed, engine working quite hard (=hot) but the radiator is working efficiently, then the temperature in the bottom hose may not rise high enough to trigger the switch. This could allow the engine to rise to a high temperature - even though the output from the rad stays low.

If however, the switch was fitted in the top hose, it would sense the rising engine temperature (despite the output from the rad being low) and turn on the fan. This would have the effect of lowering the temperature at the output from the rad and cooling the engine more, bringing it back down to temperature.

Certainly this is the system used on ship's engines - the temperature sensor is fitted at the output from the engine and keeps the engine at a constant temperature - the water going in to the engine will vary in temperature keeping the engine temperature constant - ships use a PID control system for this rather than an on/off switch.

So which is it with a car engine? Top or bottom? Have I opened a can of worms? :blink:

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I think it makes sense to have it in the bottom hose because if you are stationary then the rad wont work as effeciantly so the output(bottom hose) will getter hotter until the fan kicks in. then when you get moving the rad will become efficient again so the water will come out the bottom cooler and the fan will switch off.

If you have it in the top i think it will end up running when it doesnt need to.

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I think the genral opinon on previous posts is in the bottom hose.

The coolant goes in at the top (hot) and comes out at the bottom of the rad cooler as the air cools it. So when traveling at A road speeds the cooling will be enough from the Rad alone and on the other end of the scale 2nd low going up a long hill at slow speeds the water coming out of the bottom of the rad will become hotter as there is very little air flowing though the rad which should then trigger the x-fan in to help cool the rad more.

If you put it in the top hose you may find it cuts in too often

Mine is in the bottom hose and rarely cuts in unless traveling at slow speeds offroad.

Dave.

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Really you are controlling the temperature of the coolant going back into the engine, ensuring it is low enough to cool it sufficiently -that's how I see it.... :)

Bottom hose for me, that's if I wasn't running Megasquirt and using the coolant temp sender ;)

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Really you are controlling the temperature of the coolant going back into the engine, ensuring it is low enough to cool it sufficiently -that's how I see it.... :)

[Devils advocate]

How do you know it's low enough to cool sufficiently unless you're going of the temp of the coolant coming out out of the engine?

Controling temp from the bottom hose makes certain assumptions about the temp increase through the engine and temp decrease through the rad.

If you go off the top hose you are adding extra cooling (ie switching the fans on) only when the coolant is getting too hot.

Surely it is the upper temp that is more important to control in the whole cycle?

[/devils advocate]

IMHO it doesn't make that much difference in practice if you use approprite temp switch in which ever location you choose, ie high temp in top hose and lower temp in bottom hose. But that's just my opinion and I know lots of people will disagree with it.

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Really you are controlling the temperature of the coolant going back into the engine, ensuring it is low enough to cool it sufficiently -that's how I see it.... :)

Bottom hose for me, that's if I wasn't running Megasquirt and using the coolant temp sender ;)

Yes, that's how I see it too - but at 88 degrees or whatever your fan is set to, it may not be.

Having MS control your fan is similar to having the switch in the top hose - controlling the engine temp, not the rad temp. I did try using MS but it didn't work and I couldn't be bothered to fault find. A separate fan switch also allows the fan to stay on when the engine switches off minimising heat soak etc.

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Yes, that's how I see it too - but at 88 degrees or whatever your fan is set to, it may not be.

Having MS control your fan is similar to having the switch in the top hose - controlling the engine temp, not the rad temp. I did try using MS but it didn't work and I couldn't be bothered to fault find. A separate fan switch also allows the fan to stay on when the engine switches off minimising heat soak etc.

Agreed, mine works very well TBH, on at 85, off at 83 :)

It is slightly better than it being in the top hose, as the temp sender will still heat up even if there's no water in the system, just through conduction through the inlet manifold.

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Most car rads have the switch about half way up, but newer models simply use the CTS and ECU.

Isnt that switch set rather low?

The stat in my V8 is 88c, and most engines will run at their optimal efficiency around 90c, and then having the fan kick in at 100c and back out at 95... My old astra would kick in at a slow speed at 95 then full at 100.

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I recently fitted my lecky fans and put the temp switch in the bottom hose as recommended. I have both fans connected to the lower temp (88-83)

I have become concerned though since the fans were not coming on anywhere near soon enough. When the first switch cut in my temp gauge (VDO one) was reading just over 100ºC. As a V8 owner I was quite worried that the engine had got this hot.

When the fans were running everything cooled down pretty quickly, so I'm reasonably confident the rad is not clogged inside.

I would imagine there may be a 10º difference between top and bottom hose temperatures, so if the switch kicks in at 88º, the top would be approaching 100º, which is pretty much what I have experienced.

I am currently using a manual overide switch to turn the fans on if the thermo switch doesn't cut in soon enough, but I need to sort out a proper solution. I'm thinking of moving to the top hose and using the higher (92-87º) option.

Surely it shouldn't matter which hose is used as long as the thermo switch used is of an appropriate temp.

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I recently fitted my lecky fans and put the temp switch in the bottom hose as recommended. I have both fans connected to the lower temp (88-83)

I have become concerned though since the fans were not coming on anywhere near soon enough. When the first switch cut in my temp gauge (VDO one) was reading just over 100ºC. As a V8 owner I was quite worried that the engine had got this hot.

When the fans were running everything cooled down pretty quickly, so I'm reasonably confident the rad is not clogged inside.

I would imagine there may be a 10º difference between top and bottom hose temperatures, so if the switch kicks in at 88º, the top would be approaching 100º, which is pretty much what I have experienced.

I am currently using a manual overide switch to turn the fans on if the thermo switch doesn't cut in soon enough, but I need to sort out a proper solution. I'm thinking of moving to the top hose and using the higher (92-87º) option.

Surely it shouldn't matter which hose is used as long as the thermo switch used is of an appropriate temp.

Been following this one with interest, as I am about to do this very thing.

If the switch is in the lower (radiator outlet pipe), the fan is controlled by the temperature of the water after the radiator. This temperature is equal to the temperature of the water at the radiator inlet, minus the cooling effect of the radiator. This cooling largely dependant on the airflow the radiator ( I appreciate there a some losses by radiation and conduction through the body work, but we'll ignore those 'cos it's far too hard <_< .

If the fan(s) (is/are) running, then the temperature drop across the radiator will be greater than if there was no airflow, allowing the engine temperature to rise whilst the outlet temperature remains constant (thanks to the switch and fan). However, this drop is entirely dependant on airflow and air temperature, both which vary.

In summary, I'm an "inlettist" as that is where the engine thermostat is ...

<edited to add actual point :lol: >

Edited by errol209
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Defiantly the bottom hose as it detects if the water entering the engine is too high.

Fitting the fan switch in the top hose wont make the engine run any cooler as the thermostat is the only thing that controls the engine temp other than the coolant in the rad being too hot.

If the advice is to fit in the bottom hose then fit it there as that is where the supplier has designed it to go.

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