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260z Overheating issues


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#21 Six_Shooter

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Posted 20 September 2012 - 08:27 PM

I've read this statement before on this forum, pretty sure it is incorrect. Increasing fluid velocity increases the convective heat transfer, which will improve the efficiency of the whole system, which results in lower average water temperature.


The statement is correct, which is why companies such as Moroso sell "water restrictors", that get installed in place of thermostats in race only engines.

The issue with no restrictor is that the coolant is allowed to flow so quickly, that it doesn't spend enough time in the radiator (and heater core, which is also a forgotten part of the cooling system), that the coolant doesn't spend enough time in the radiator to transfer the heat from the liquid to the metal (brass or aluminum) heat exchanger (radiator). It also works the other way, the coolant can flow so quickly through the engine itself, that the coolant has limited time to "pick up" heat from the engine itself.

Another issue is that the coolant can cavitate when allowed to free flow, which reduces the ability for the coolant to actually transfer heat.Air and air bubbles are a very poor conductor, and therefore have a poor ability to be able regulate temperature.

#22 stevo_gj

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Posted 20 September 2012 - 10:41 PM

The issue with no restrictor is that the coolant is allowed to flow so quickly, that it doesn't spend enough time in the radiator (and heater core, which is also a forgotten part of the cooling system), that the coolant doesn't spend enough time in the radiator to transfer the heat from the liquid to the metal (brass or aluminum) heat exchanger (radiator). It also works the other way, the coolant can flow so quickly through the engine itself, that the coolant has limited time to "pick up" heat from the engine itself.


That is a common myth. It is a closed system so heat in = heat out. It is not an issue of 'spending time' in the radiator but it is more about the equilibrium temperature reached. Restrictors in race engines are for increasing the vapour pressure as the water flows around the sharp geometry in the block so that it does not boil, as a steam reduces local cooling which can exacerbate already hot spots in the engine. Restrictors do not increase cooling efficiency at all.

I found this explanation the best: http://www.stewartco...Tech_Tips_3.htm

#23 Six_Shooter

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Posted 21 September 2012 - 12:10 AM

That is a common myth. It is a closed system so heat in = heat out. It is not an issue of 'spending time' in the radiator but it is more about the equilibrium temperature reached. Restrictors in race engines are for increasing the vapour pressure as the water flows around the sharp geometry in the block so that it does not boil, as a steam reduces local cooling which can exacerbate already hot spots in the engine. Restrictors do not increase cooling efficiency at all.

I found this explanation the best: http://www.stewartco...Tech_Tips_3.htm


It is not a myth at all.

If the coolant does not have sufficient time in a heat exchanger to actually transfer heat from the coolant to the heat exchanger, the cooling efficiancy is reduced. The pressure of the coolant is also a factor, which helps raise the boiling point of any liquid, but time spent in the heat exchanger is very important and wouldn't dismiss the need to control the flow as a part of that equation.

You also contradicted yourself, saying:

Restrictors in race engines are for increasing the vapour pressure as the water flows around the sharp geometry in the block so that it does not boil, as a steam reduces local cooling which can exacerbate already hot spots in the engine.


Then you said:

Restrictors do not increase cooling efficiency at all.


So which is it, do they help increase pressure to help increase the boiling point (which they as a by product), or don't they?



#24 Agno

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Posted 21 September 2012 - 12:35 AM

Just playing devil's advocate, but if the fluid has more time to flow through the heat exchanger/radiator it also spends more time flowing through the very hot engine. It all depends on where there is a greater temperature difference as to where the greatest heat transfer aka heating/cooling will occur.

#25 Linton

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Posted 21 September 2012 - 07:57 AM

So "260" have you fixed your o/heating problem , lots of debate here with great tips and you have disappeared, what was out the outcome  :-\

#26 stevo_gj

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Posted 21 September 2012 - 03:05 PM

It is not a myth at all.

If the coolant does not have sufficient time in a heat exchanger to actually transfer heat from the coolant to the heat exchanger, the cooling efficiancy is reduced. The pressure of the coolant is also a factor, which helps raise the boiling point of any liquid, but time spent in the heat exchanger is very important and wouldn't dismiss the need to control the flow as a part of that equation.

You also contradicted yourself, saying:

Then you said:

So which is it, do they help increase pressure to help increase the boiling point (which they as a by product), or don't they?


I did not contradict myself, because increasing static water pressure (to increase the boiling point) with a restrictor does lower the system efficiency by reducing the flow rate (if the pump does not change), and, it does prevent steam formation which is not so much about system efficiency as it is about having hot spots in discrete locations within the cooling water passages which can damage the block.

Total heat transfer of a closed cooling system increases with increasing convective heat transfer, and convective heat transfer is proportional to the fluid velocity.

Having water temperatures as high as possible without starting boiling as it passes around sharp geometry in the block would give you the maximum total heat transfer for your system without causing damage.

If you combined a restrictor with higher flowing pump then you would actually have a lower efficiency system (efficiency is heat transfer divided by work of pump, and the pump would have to work hard thanks to the restrictor) that had a higher heat transfer as it would be possible to run with a higher temperature of water in the block at the same flow rate as before. What you have to consider is that having hotter water does not result in higher heat transfer through the system for a given unit of time if the flow is slowed down as well.

Regardless, that "enough time in the heat exchanger" idea is a myth. I will concede the point if you can find reasonable links to an engineer who explains why "increasing time in the heat exchanger" will increase the total heat transfer capability of a cooling water system.

Update: Wow I just tried googling 'too much flow in radiator myth'. This discussion has been had on at least 100 other car forums.

#27 MaygZ

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Posted 21 September 2012 - 04:31 PM

The system uses both convective heat transfer and conductive. 

I personally agree that if fluid is running through the system at an alarmingly fast rate and only in contact with the heat source for a very short time, it won't absorb (or dissipate) the heat.  However, that would need to be a very fast flow considering the areas of heat source and exchange in an auto engine.  The radiator and the inherent restrictions within the cooling system (block design/radiators/heater cores/hose sizes/etc) will slow the water down enough to work effectively. 

In most cases it is the lack of water flow that causes over heating!  Do what you can to speed water flow (a larger and more open radiator/bigger water pump/removal of shape edges and casting burrs/ increase hose sizes where possible and remove any kinks/etc) and engine cooling will improve.

You don't fight a bush fire with slow moving water  ;)

Certainly in my case I'm not looking for maximum (expensive) efficiency, but rather economically effective.

Many years ago on my old track car (which was also an Italian daily) would over heat very quickly in stop start traffic if the outside air temperature was above about 27C.  I removed the thermostat completely (and yes it was operating correctly when I tested it).  This completely solved the over heating problem.

It also had a subjectively measured side product.  The engine idled better and I suspect it revved out a little quicker.  This is quite possibly due to the reduced load on the engine due to the reduction of back pressure on the water pump.  Before we start flaming me, this was a subjective perception and not proven scientifically or statistically.

Again my opinion, if you are running high octane fuel and/or higher compression and don't do very short trips on very cold mornings,  remove the thermostat completely and allow your engine to warm before pushing it - life will be good.

#28 Zedman240®

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Posted 21 September 2012 - 05:43 PM

From experience, without a thermostat, the coolant takes a long time to heat up as the system as a whole is working too well with a fast velocity of the coolant but when eventually the water gets to a higher temp it keeps climbing as when said the coolant doesn't have any time to get rid of the heat in the radiator. The thermostat varies the flow depending on temp. Closes off to heat up and opens more to allow more coolant to flow through the radiator.




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