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Lucas
2004 Forester XT Manual
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2,824 Posts
Discussion Starter · #1 ·
Hi,

I am going to replace the radiator on my 2004 2.0 FXT with the Mishimoto aluminium one. Together with that, I am puting new SAMCO hoses and fresh cooler. Have any of you guys replace thermostat? I believe it is pretty simple thing. The question I have is about the sport thermostat, it will open at 64°C versus 76.5°C from the factory. Is it worth to do it? Is it going to affect the fuel consumption or power?
Mine is working fine, but it is 10 years old, and I have a really bad experience with radiator and thermostat on my old Legacy few years back, when I lost my head gasket :( So as a precaution I want to change it.

Cheers,

Lucas
 

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Premium Member
2019 Crosstrek 2018 Forester XT
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15,109 Posts
A lower temp thermostat decreases efficiency and at higher loads and summertime..it doesn't keep the engine temp lower. That's a function of radiator size alone. Bad idea to replace IMHO (speaking as a Mechanical Engineeer)
 

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Registered
2008 GRB 20th anniversary 6 MT
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6,597 Posts
As adc says, internal combustion engines operate more efficiently when the water temp is within a certain range.

If you want to replace it replace it with an OEM one.
 

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Lucas
2004 Forester XT Manual
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2,824 Posts
Discussion Starter · #6 ·
Thank you all. You just confirmed my thoughts. I will replace it with a stock. I've got 103k miles on the clock so I will do it just in case.
 

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Registered
99 UK S-turbo
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8,566 Posts
A lower temp thermostat decreases efficiency and at higher loads and summertime..it doesn't keep the engine temp lower. That's a function of radiator size alone. Bad idea to replace IMHO (speaking as a Mechanical Engineeer)
I wouldn't employ you given the rubbish you just stated I'm afraid, it has a temp rating, clearly posted above and yes that low temp will decrease effcicinecy, speaking as a powertrain engineer of 24 years experience working for various OEMs and consultancys, that is why modern cars with electronically controlled stats run them at circa 120C at light load for best efficiency.

If the temp was a function of the radiator we wouldn't need a thermostat!
 

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Lucas
2004 Forester XT Manual
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2,824 Posts
Discussion Starter · #9 ·
I wouldn't employ you given the rubbish you just stated I'm afraid, it has a temp rating, clearly posted above and yes that low temp will decrease effcicinecy, speaking as a powertrain engineer of 24 years experience working for various OEMs and consultancys, that is why modern cars with electronically controlled stats run them at circa 120C at light load for best efficiency.

If the temp was a function of the radiator we wouldn't need a thermostat!
Hi Rookie,

Hmm. Thanks for the clarification. Now, I am mechanical engineer, but with no real experience on automotive engines. Although with a lot of theory on the back :) Now from what i remember from Uni. is that combustion engine will obtain efficiency equilibrium at certain temp. Obviously that doesn't mean it will be linear function hotter=better (After reaching some temp level I think it will be less effective). Now what I don't really get, why then on the market you have all this 'fancy'thermostats, saying that there are sport one? If the obviously decrease efficiency? Is it one of those marketing stuffs??

Thanks in advance,

Lucas
 

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Premium Member
2019 Crosstrek 2018 Forester XT
Joined
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15,109 Posts
I wouldn't employ you given the rubbish you just stated I'm afraid, it has a temp rating, clearly posted above and yes that low temp will decrease effcicinecy, speaking as a powertrain engineer of 24 years experience working for various OEMs and consultancys, that is why modern cars with electronically controlled stats run them at circa 120C at light load for best efficiency.

If the temp was a function of the radiator we wouldn't need a thermostat!
At above a certain load ] both TStats are full open. And equilibrium steady state temperature is a function strictly of the radiator cooling capability. Also equilibrium temp (at a higher load) won't be a functioin of cooling medium (percentage of Glycol) or even water pump flow. These things only vary time to equilibrium conditions.

Believe what you want..I could care less. :cool:
I got my M.E. degree 45 years ago, but I don't think Clasical Thermodynamics has changed much.
 

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2015 Fiat van Man stick
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4,808 Posts
It's no use to this thread but I've always used distilled water only in my bikes so I can see if they boil up (clouds of steam) and water is great for moving engine heat to radiators. They have always lived in garages though.
 

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Premium Member
2019 Crosstrek 2018 Forester XT
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15,109 Posts
It's no use to this thread but I've always used distilled water only in my bikes so I can see if they boil up (clouds of steam) and water is great for moving engine heat to radiators. They have always lived in garages though.
True..water has the greatest heat capacity and will move more heat from the engine...but works against you in the radiator. Unless the radiator is very large, heat from the water (remember high heat capacity) is discharged slower out the radiator. But again its irrelevant. The heat capacity of the liquid only affects how quickly the engine heats up.

Sorry this thread got contensious..I was only pointing out facts. I saw a computer simulation many years ago which showed that heat capacity didn't affect the final temp, only the speed to heatup. I didn't believe it..so I wrote my own. They were right. Also its likely that water may undergo nucliate /local boiling which may make water less suitable (although I don't knows this).

Well..I'm done. PM me for further discussion so this thread stays civil.
 

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2005 Forester 2.5 STI Manual
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312 Posts
Bit of a thread bump as googling it but if your running more power than standard, think mines running 350/350 (or was according to a rolling g Road map print out), would it be best upgrading the thermostat?

The previous owner apparently did but when the water pump failed on me coming up to 3 years ago now, I changed the thermostat back to oem so just wondering if it's better to have an upgraded t-stat or just stay oem?

Thanks
 

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Registered
2005 Forester 2.5 STI Manual
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312 Posts
Hello

Can you explain for a simpleton what you mean by over temp issues?

And if there is issues, then what?

Sorry, should say I'm running a mashimoto radiator and not oem

Thanks
 

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Premium Member
2004 forester sti
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2,708 Posts
As far as i know just make sure the oem is working as it should.
If you take it out and place it in cup of boiling water you should see it open, if it doesnt its faulty
Ive only ever run a oem and standard rad. Take from that what you will.
I did replace the rad on mine a few years back.
 

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Registered
2021 Forester Limited
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442 Posts
Hello

Can you explain for a simpleton what you mean by over temp issues?

And if there is issues, then what?

Sorry, should say I'm running a mashimoto radiator and not oem

Thanks
The hot engine warning light.
Since Subaru doesn't have a regular temperature gauge.

Unless you have an actual gauge installed.

The Mishimoto radiator shouldn't be an issue. The thermostat will dictate how cool it gets and how warm, under normal warmed up operating conditions.

With air flow through the radiator in my Impreza, it would stay a little below the thermostat temperature. About 170°F.

With the car stationary, and relying on the fans to kick on, it would hit 210°F before they turned on.

I installed a higher temp Subaru thermostat, and it ran about 180°F, and still 210°F for stationary idling. Which took a while.

This was a N/A engine. A turbo under boost will make more heat, but unless you are constantly boosting a substantial amount, you should be fine. (Given the tune is correct).
 

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Don, King of the parts diagram
MY11 WRX STi Sedan 6MT
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5,984 Posts
OEM Thermostat 21200AA072
OEM Thermostat with seal 21210AA030
 
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