Aftermarket thermostats
 

I'm toying with the idea of getting an aftermarket thermostat. Based on opinion the stock radiator has significant thermal capacity left to sink more heat, but most don't advocate changing the thermostat given reliability concerns. However all things considered, I wanted to address two particular potential benefits with the lower coolant temp on my setup:

1. Oil cooling. I'm on the Forester water to oil exchanger and having a larger coolant to oil temperature delta will further reduce oil temperatures.
2. Knock resistance - lower coolant and oil temps would influence cylinder head and bore temps, reducing the tendency to knock. This opens up further tuning potential.

There's several choices of thermostats available for our cars:

1. SARD 68C/82C
2. Mishimoto 76C
3. Billion 65C/72C/82C
4. NTCL 82C

Common sense will tell that the 82C and 76C ones are who want a drop in... it is very unlikely the stock radiator has enough cooling capacity for the 65C/68C, so those can be easily singled out. From owners' feedback in Minkara in regards to the SARD 82C thermostat the coolant typically settles around 84-86C, and as reference mine on OEM goes about 89-90C on a normal day and 92C on track, this is at night though.

The car is stock NA and I'm aware of the potential pitfalls - I'm familiar enough to address tuning related issues, but I wanted to find out from others' experience if it has opened further headway for more spark advance and knock relief. @tomm.brz @steve99 you are my usual go to guys... any thoughts on this? Based on the stock tune (A01G), having a thermostat with an idle warmed up coolant temp at 81C is enough to clear any warmup corrections/compensations, apart from the radiator fan temps. Based on this the Mishimoto seems to hit the spot; I'm aware of the quality issues though.

@JIM THEO seen in a previous post that you have the SARD thermostat installed - any feedback is appreciated.

For reference the stock thermostat opens at 88C and is fully open at 95C.

if you reach 92C for example, or more with your current setup, a thermostat that open earlier gives you a bit more headroom in time, but eventually everything will heat soak up to those temperatures as it doesnt aid in the cooling capabilities, it ust opens earlier

warm up will be slower, and temps cruising in highway at not so high load will be slightly colder ... but that s irrelevant really, you gain maybe a single step up in the timing map in some zones

I see some of your points, though I'm trying to understand the heatsoak part - if I'm on track and I see coolant temps of 92C, logic tells me that the radiator has capacity to spare if the thermostat fully opens at 95C. Since the lower thermostat is essentially transferring more heat from the block to the radiator, at least to the point where the radiator has not yet run out of thermal capacity - wouldn't heatsoak only be involved after it runs out?

Am aware of the reduced efficiency the colder the block is, hence I'm using the minimum coolant temp in the tune as a reference.

yes probably at the very same time, same situation but with lower thermostat, you could have lets say 90C... you keep then going and racing the car and it goes back up and the time you gained in this case it s what I call the headroom

If you don't usually overheat your coolant to at least 95 at the track, and your goal is to reduce those 92C to 89-90C for a fair more time, then I guess yes that could work for you
To me it's not so worthy mostly because personally i dont think you gain much about knock resistance and oil temperature but maybe wait for other opinions

An 82 degree thermostat is all the way open at 82 degrees. Anything beyond that is is unaffected by the thermostat. So if the radiator is only capable of cooling your coolant to 92 degrees at a given load/ambient temp. It doesn't matter if you have a 68, or an 82 degree thermostat, as they are both fully open. Any temp above thermostat opening that your coolant sees is a lack of cooling capacity in the radiator setup (core, ducting, etc). A 68 degree thermostat will expose your coolant to maximum cooling capacity EARLIER than the 82. But the radiator is still only capable of maintaining 92. You will still hit that temp. It will just take a few minutes longer with the cooler thermostat. If you're road racing, the cooler thermostat isn't going to help you because of the length of run time. If you're drag racing, maybe drifting (if sessions are short), or autoX then the lower starting temp might help keep you cool.

TLDR: it's not really gonna help you because you're still limited by radiator cooling capacity. Spend the money on ducting or a bigger core instead.

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Going to a lower opening thermostat will not get you more cooling. You will still end up at 92C stabilized temp once the whole cooling system heat soaks(stabilizes temp). The only way to try to cool more is to add more mass(coolant), or airflow into the system.

You will gain a marginal amount of time opening at a lower temp, but your stable temp will be the same because you haven't changed your cooling system.

Adding coolant will have a similar affect to cooler thermostat. Eventually it will all heat soak. Just take a little longer to do it

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Thanks for the input. I believe it's the other way around, where the thermostat starts to open at 82C instead of being fully open.

https://www.sard.co.jp/parts/products/cooling/thermo/ - see the note at the bottom of the page.

@Ashikabi @dragoontwo I see where you're coming from, but the fact that the OEM thermostat only fully opens at 95C yet I am on track at 92C, that implies that the radiator still has cooling capacity left to spare to have the engine running at a lower operating temp, does it not? From here one may assume that max capacity is reached at 103C, where both fans only turn on at that coolant temperature based on the tune.

It's not the additional cooling I'm after, a lower thermostat cannot bend physics nor magically give you more cooling than the radiator can provide - but if I could have the engine running at a lower temperature with more heat pushed to the radiator closer to its capacity and reap the assumed gains is my goal.

It means you haven't maxed out the stock capacity. You have overhead to put more heat into the system. You can not run the system cooler unless you move more heat out. Changing the thermostat will not do this.

Ahh, I didn't realize the stock thermostat was 95 degrees. If that's the case, then your radiator is indeed not at max cooling capacity. Then a cooler therm will affect your 92 degree number. Not sure how you could possibly be running at thermostat open temp on track though. Unless you're racing somewhere that is very chilly

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It should, no? A thermostat is essentially a variable valve which allows a partial amount of coolant to reloop the block to maintain its temperature. The more the thermostat is opened, the more 'hot' coolant is diverted to the radiator, which reduces the temps.
You mean someplace very warm? :)

But yeah that's the gist - the only variable now is that how much cooling capacity of the radiator is left. Probably can be calculated with some math but I'm going with the stock tune to infer - I added it to the previous post but both fans only turn on at 103C.

I find it very difficult to believe your stock radiator is keeping your coolant so cool that the thermostat isn't even all the way open on the track. Unless you're literally just cruising around at 2000rpm. When there are guys on big aftermarket units who are still over heating. Something is amiss, somewhere. Temp gauge is wrong? You're reading the wrong number. Something

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Where in the loop is the coolant temp being taken?



As said before, the 92C you are seeing is the temp your coolant is stabilizing at. Opening the thermostat earlier will cause it to take longer to reach 92C, but you will still reach it depending on how long your track sessions are.

I believe your question is directed at OP, not me. He claimed thermostat opens fully at 95 but coolant temp is 92 on the track. I think something is wrong here because that simply shouldn't be possible for the thermostat to not even fully open on the track. Almost sounds like he's looking at intake air temp or something.

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