I've been told now by every Radiator shop to be careful not to cycle coolant too fast through the radiator as the coolant won't have time to cool down. This doesn't sit well with me as I can't understand why moving coolant faster would have a negative effect, I am standing firm on this as I think it doesn't make a differnce how fast coolant flows obviously high revving engines will require hi-flo rates but why can't the flow be say 150L per minute all the time.

P.S. I thought this would be a good healthy debate topic.

Hi Muzza,

Dont fall into that trap...

Faster is definately not better,same for to slow...there is a happy medium, it is all relevant to the time in which it takes for heat to displace from the coolant to the radiator tubes and fins and then from the tubes and fins to the air travelling through them. In a conventional cooling system, the thermostat provides the restriction to flow when coolant is cold by closing, but when fully open it is still providing a restriction to flow due to the cross sectional area of its opening. The only time where more is better is when you are talking cfm of air movement through the radiator.

If you have ever had a car where the thermostat was removed...you will probably have noticed how long it took to warm up on the guage...what was really happening was the bores and combustion chambers were wickedly hot because the coolant was travelling to fast through the block to be as effective as it is capable of being when travelling at a slower pace.

To solve your cooling issues the answer is in the cross sectional and cubic area of the radiator(and its efficiency) and the airflow through the radiator...As long as the pump and thermostat are in good working order[;^)]

One of the guys who first fitted the FJ to your car ran an FJ20DET in a 120Y successfully for many years without cooling issues...the engine had the std water pump and thermostat.

Cheers,

Phil.

Just looking at your engine bay shots...I cant tell how high the top of the radiator is compared to the head, if the radiator is to low with a fill point on the radiator, you might be getting cavitation in the head. A similar story to the RB30 in the Skyline and Commodore...Commodores cracked heads and had overheating issues, Skylines didn't...it was all relevant to the radiator position in relation to the head...Skylines were easy to bleed the cooling system and would self bleed, Commodore's weren't and wouldn't. The answerfor your car if this is the case would be a header tank or a fill point on the high side of the head

Like this

Thanks for the reply Phil! I was thinking about this one night and it kept me awake for ages, Let me throw another angle at you.

Engine - Creates Thermal energy
Radiator - Dissipates Thermal energy

Above we have a balanced equation

In creating good thermal heat dissipation is to get the radiator as hot as possible (closest to engine temp) then flow maximum ambient air through it thus creating the highest heat transfer. (Radiator temp - Ambient temp = differential temp) The higher the differential temp the better cooling of cousre suface area plays a big part too. Now I can't see why flowing coolant at high speed would lower the radiator temp BECAUSE once coolant leaves the block it is already starting to cool.

If the coolant moves to fast through the radiator, the majority of heat will stay in the coolant and travel back into the engine. The radiator and coolant need to be able to perform their heat transfer as in the 'cold engine no thermostat' example. If the coolant travels to fast, you will have the same effect but at the opposite end of the process, the heat exchange(coolant to radiator) part of the process cant occur effectively and the heat is retained in the coolant and therefore in the cooling system.

quote:

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Originally posted by muzza



Engine - Creates Thermal energy
Radiator - Dissipates Thermal energy

Above we have a balanced equation

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It is only balanced if both are equally efficient

quote:

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Originally posted by muzza



In creating good thermal heat dissipation is to get the radiator as hot as possible (closest to engine temp) then flow maximum ambient air through it thus creating the highest heat transfer. (Radiator temp - Ambient temp = differential temp) The higher the differential temp the better cooling of cousre suface area plays a big part too.

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Agree'd...this is why aircon systems run higher pressures on a hot day, in order to create a temp/pressure differential. Your cooling system operating pressure also comes into it

All of the above is only relevant if the heat transfer can occur...which will only occur efficiently if the coolant flow is travelling at the correct speed...to fast is as bad as to slow. To fast and the heat tranfer between the coolant and the radiator structure is hindered, to slow and the transfer between the radiator structure and the atmosphere cant keep up. Its a no win situation for to fast or to slow...it needs to be just right.

The 'just right' speed of flow will be very hard to identify and will change as the ambient temps change. The Davies Craig electric pump is most effective at maintaining the correct flow as it is monitoring the heat stored in the coolant.

A good example of to much flow being bad is what happens if a factory waterpump is spun to fast...I had a Sigma years ago with a modified 2.6L and it kept blowing up heater cores...the fix was to fit a larger water pump pulley to slow the pump down...Danny's N/A YB is also a good example of this...the revs it see's demand's a larger waterpump pulley to slow down the water pump

ok I believe that cavitation occurs when the impeller spins at high speeds and creates occsilations rendering the impeller inefficient.

But consider this, remove cavitation from the equation, What you are saying is that it takes time for the coolant to radiate heat into the rad core. But if this takes time then the theory of the faster the coolant flows thru the core the cooler the radiator will get cause the coolant won't have time to transfer heat, but imagine coolant flowing say 50000 litres per minute this would mean the radiator would stay the cool cause heat transfer would be way inefficent, but I say the core would = the source temp.