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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7306
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Fri Aug 23, 2019 7:09 am    Post subject: Reply with quote

The thermostat is controlled by the temperature of the coolant in the engine combined wuth the return flow of coolant from the radiator. Under low load conditions it will be partially open, so that some coolant is flowing through the radiators. It will control this flow to maintain a constant temperature (more or less) at the thermostat. If the returning coolant from the engine is at a low temperature, then less of it will be required to maintain the temperature, so the flow will be reduced. The following numbers are made up, but you can get an idea of the principles behind it.

2 radiators 15 degree temperature drop
Engine temperature 80
Coolant out 90 degrees,
Coolant return 75 degrees

3 radiators 25 degree temperature drop
Engine temperature 80
Coolant out 95 degrees
Coolant return 70 degrees

Lower coolant flow to maintain the engine at 80 degrees.

When I get my car together I'll look at putting thermocouples on it to measure these temperatures to see if this is happening in reality. My car will have a manually switchable centre radiator so I will be able try low load with and without the radiator.

MC
 
  
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Hertsdriver
Nürburgring


Joined: 12 Nov 2018
Posts: 469
Location: Hertfordshire/London

2004 Porsche 997 Carrera 2

PostPosted: Fri Aug 23, 2019 7:28 am    Post subject: Reply with quote

I must be missing something here as none of this explains Baz’s comment that it would make the engine run hotter.
A thermostat isn’t a regulator, it is a switch, it’s not sophisticated enough to regulate temperature, it will only open at a specific temperature and close at a specific temperature, how cold or hot the coolant is on the ‘other’ side of the circuit does not affect the way it operates.
To my mind adding a third rad can only result in a benefit in that when the section of the circuit that the rads sit in is called upon (by the thermostat opening) then the coolant mixing with the engine circuit coolant will be cooler than normal, resulting in a faster overall temperature drop on the engine circuit and therefore more protection around the engine. My car regularly has the fans on in traffic situations, indicating that the temperature sensor believes the system requires additional cooling, adding additional cooling area can surely only help to reduce the need for the fans?
I cannot understand how any additional cooling on one side of the circuit can result in the other side running hotter.
 
  
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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7306
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Fri Aug 23, 2019 7:39 am    Post subject: Reply with quote

Hertsdriver wrote:
I cannot understand how any additional cooling on one side of the circuit can result in the other side running hotter.


If the temperature at the thermostat stays constant. An increase in temperature drop on one side of the circuit must be matched by an increase in temperature on the other side.

Look at the numbers I posted above. More cooling with three radiators, engine out temperature is increased. The thermostat is not a straightforward switch which is on or off. It will usually be partially open to control the flow.

MC
 
  
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bazhart
Approved Trader


Joined: 20 May 2009
Posts: 984
Location: Bolton Lancashire


PostPosted: Fri Aug 23, 2019 8:08 am    Post subject: Reply with quote

Oh dear - this old once again - it is seemingly illogical but I am glad most of the contributors understand it and thanks for saving me a lot of time explaining it as we did when we were the first to mention it many years ago now and had to deal with countless similar disagreements.

If the thermostat is on the way into the engine then that controls the temperature of the coolant entering the engine.

During its passage through the engine it picks up heat so leaves the engine hotter than the thermostat temperature setting.

If the thermostat is on the way out of the engine it controls the temperature of the coolant leaving the engine.

Thermostats work by controlling the flow. If the coolant is too hot for the thermostat it opens more and allows more flow.

If the coolant temperature is lower than the thermostat setting it closes more and reduces the flow.

The faster the coolant flows through the engine the less time it has to pickup heat. The slower it passes through the engine the more heat it picks up.

An additional radiator increases the cooling potential of the radiator system so cools the coolant more than before it was fitted and this means the coolant reaching the thermostat is cooler than before. This in turn makes the thermostat close more to slow down its flow and this in turn allows more time for it to pick up heat as it travels through the engine resulting in the outlet temperature being higher than before the 3rd radiator was fitted. but that is OK because the extra cooling potential of the 3rd radiator then cools it more back to the lower inlet temperature again.

Only about 10% of the coolant goes into the cylinder block and 90% into the cylinder head after which it joins together before leaving the engine.

If the flow is slower then the coolant in the block gets hotter and so the oil in the cylinders gets thinner and then when loose particles of silicon become free from the cylinder wall and are trapped between the cylinder and the piston there is a thinner oil film gap and the potential for scoring increases.

You cannot adequately judge what is going on in the cylinders by measuring the outlet temperature because it mixes the 10% of cylinder coolant with 90% of cylinder head coolant and that is what you are measuring.

To test all this out about a decade ago - we fitted temperature sensors inside the engine in different places and recorded exactly what the above theory states.

We even fitted separate outlet thermostats to each bank to equalise the internal temperatures on our racing 996.

We also conducted tests at different seasons with the radiators partially blanked and found that in normal driving one small radiator would be sufficient however in contrast while racing up close behind another competitor even with 3 radiators temperatures rose above the thermostat settings and so it was necessary to drive slightly off line sometimes to get some air into the front of the car.

So fitting a 3rd radiator to a road going or track day car will increase the cylinder block temperature - which is not what you need to preserve the engine and help prevent it from scoring. However scoring occurs more when the revs are low and torque high so this does not really apply to racing or track use.

A thermostat on the way into the 3rd radiator set at a slightly higher temperature than the main thermostats allows it only to work when it is needed but for the 997 range there is little room to fit it easily and this is what we have been working on.

A lower temperature thermostat fitted to the original inlet side of the engine will lower all the temperatures inside the engine by the same proportion and is only ineffective in circumstances where the 2 radiators are no longer able to cool the engine at the desired temperature after which it makes no difference.

Everyone else trying to explain it different ways above is right - heat rise in engine = heat drop in radiators and delta "T" is greater if the cooling is also greater.


Baz
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Hertsdriver
Nürburgring


Joined: 12 Nov 2018
Posts: 469
Location: Hertfordshire/London

2004 Porsche 997 Carrera 2

PostPosted: Fri Aug 23, 2019 9:07 am    Post subject: Reply with quote

Hi Baz, thank you that explanation makes more sense Thumb So the 'sensor' side of the thermostat (that determines whether it opens or not) for some reason sits in the cooling circuit (containing the radiators) which if you add a third radiator would reach the point where the thermostat would close earlier due to the lower temperature within that circuit. What an absolutely $hit system! Floor

Im sure I remember somebody adding their own thermostat to the 3rd radiator linked to a temperature sensor...
 
  
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C4Silver
Spa-Francorchamps


Joined: 14 Aug 2019
Posts: 273



PostPosted: Fri Aug 23, 2019 10:08 am    Post subject: Reply with quote

AND There I was....... thinking I was doing something to preserve the longevity of the engine by increasing the efficiency of the engines cooling system !

Dont know
 
  
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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7306
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Fri Aug 23, 2019 10:46 am    Post subject: Reply with quote

Hertsdriver wrote:
Im sure I remember somebody adding their own thermostat to the 3rd radiator linked to a temperature sensor...


I am doing it in my car.

http://911uk.com/viewtopic.php?t=125503&postdays=0&postorder=asc&start=320

MC
 
  
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Hertsdriver
Nürburgring


Joined: 12 Nov 2018
Posts: 469
Location: Hertfordshire/London

2004 Porsche 997 Carrera 2

PostPosted: Fri Aug 23, 2019 12:32 pm    Post subject: Reply with quote

MisterCorn wrote:
Hertsdriver wrote:
Im sure I remember somebody adding their own thermostat to the 3rd radiator linked to a temperature sensor...


I am doing it in my car.

http://911uk.com/viewtopic.php?t=125503&postdays=0&postorder=asc&start=320

MC


Thumb thats the one! great thread, very inspirational Smile
 
  
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infrasilver
Fast & Furious
Fast & Furious


Joined: 04 Oct 2010
Posts: 8496
Location: East Midlands

2001 Porsche 996 Targa

PostPosted: Fri Aug 23, 2019 12:35 pm    Post subject: Reply with quote

C4Silver wrote:
AND There I was....... thinking I was doing something to preserve the longevity of the engine by increasing the efficiency of the engines cooling system !

Dont know


I totally agree with Baz and MC in regard to the way the system works, it was not a very well thought out design for a rear engined car and hard to get your head a round until it is explained fully but for reference I fitted my third rad many years ago, when Baz was recommending the third rad mod but before they did the research and testing with a thermostat at the third rad, I fitted it around 55k miles, I did fit it primarily due to doing a few track days and a lot of summer road trips where the engine got very hot on uphill alpine passes where you are not getting the airflow due to lowish speeds but still labouring the engine going up hill and doing tight turns.

I do mostly keep my revs high as that is the way I drive and I would guess I have done 70k + miles with this rad set up, I am just starting to get a little soot on my N/S exhaust tip but nothing major yet and I would say the third rad has been a good modification in my application and who knows if it has given me more miles without damaging the engine or if I have damaged the engine because of the third rad, I feel the former as I'm now on 130k miles.
I was thinking of pulling the engine apart to refresh the main bearings again and give it general health check but that is another story.
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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7306
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Fri Aug 23, 2019 1:14 pm    Post subject: Reply with quote

Hertsdriver wrote:
MisterCorn wrote:
Hertsdriver wrote:
Im sure I remember somebody adding their own thermostat to the 3rd radiator linked to a temperature sensor...


I am doing it in my car.

http://911uk.com/viewtopic.php?t=125503&postdays=0&postorder=asc&start=320

MC


Thumb thats the one! great thread, very inspirational Smile


If you look at the bits later in the thread about the instruments I am now driving an output from the built in coolant sensor, so no need for the external thermostat. That said, since I have done this now I'll stick with it on this car.

MC
 
  
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bazhart
Approved Trader


Joined: 20 May 2009
Posts: 984
Location: Bolton Lancashire


PostPosted: Fri Aug 23, 2019 1:43 pm    Post subject: Reply with quote

For the earlier 996 engines the thermostat housing we make is in stock and simply opens and closes the third radiator only when the other 2 can no longer keep the temperature low enough. It is a simple housing that owners must fit by using hose pipes and clamps (often moving the horns provides the room).

We supply a thermostat with it that is set slightly higher than the LTT we also supply.

The 997 has a short kinked pipe joining the off side radiator to the 3rd radiator and very little room to do a nice job.

We first made housings out of solid that worked well but would be too expensive - even using castings would cost a lot but now we have a digital - printed one under test that should be inexpensive and just pops in place of the original hose, using original clips and housing a thermostat.

Another way would be like my old Saab 96 that had a piece of string with knots in it pulling up a radiator blind and slotting the right knot into a vee notch when the temperature was running right.

An automatic digitally controlled blind would be good too or deflectors that alter the air flow into or out of radiators.

Just for clarity, thermostats don't know what the temperature is they just open and close when they get hotter or cooler. They move slowly and gradually find a setting where they stabilise (pre-set by the manufacturer).

If they get hotter they slowly open more and if they get cooler they close more - the problem is making a system that works when it is freezing and below zero outside and the owner is plodding along at 20mph and also preventing damage flat out on a race track when the difference in heat gain is magnified hugely.

A single thermostat system cannot adequately cover all eventualities accurately or reliably enough when the engines are extremely powerful because those engines generate little more heat at 30mph than a 1 litre family car yet also need the capacity to cool a full race car capable of over 150 mph and one simple system struggles to do all that in temperatures ranging from below freezing to a heat wave stuck in traffic.

Fitting an additional thermostat before the 3rd radiator is just like having a manual valve to engage it in hot conditions and isolate it in cold conditions except it can do so automatically.


Baz
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C4Silver
Spa-Francorchamps


Joined: 14 Aug 2019
Posts: 273



PostPosted: Fri Aug 23, 2019 3:46 pm    Post subject: Reply with quote

I’d like to say, that makes perfect sense now that it has been explained and the thermostat in front of the third rad to essentially control flow makes sense.

I do like the idea of veins in front of the rad to open shutters which would control flow

I’ll wait for the housing as I’m in no rush to fit the 3rd rad, thanks for the explanation.
 
  
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bullet-proof_biscuit
Newbie


Joined: 10 Jun 2019
Posts: 42



PostPosted: Thu Oct 24, 2019 2:32 pm    Post subject: Reply with quote

edit, I was talking jiberish
 
  
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