A thermostat is like a valve that opens when it reaches it's desired temperature. It is located in the cooling system line of most cars in the market. When it is closed, it allows little(close to no) water to pass through along it's cooling line.
Okay, thermostats have certain "degrees" or opening temperature. For example,
OEM : 82degrees Celsius, perhaps 75 for some.
Sard for Toyota 4e : 68deg C
TRD for Toyota 4e : 71deg C
Okay, most importantly, WHY is the thermostat there?
Here's my answer: your thermostat is there for a reason. Sure, you can remove it and assume that there will be no harm to your car. It may be half true. Cold start takes a longer time, takes forever to heat up since water is always passing through the radiator. The affect in this situation is minimal as the car is not moving yet, and taking into consideration that your air cond fan is not running, aka aircond off. No natural airflow going through the radiator yet. The car will take forever to heat up to operating temperature though if you start moving immediately.
Secondly, I was once young and stupid. My charade used to be a 1.0 3cylinder carb engine. I was on the way down from Genting and took the Karak Highway back to KL. No thermostat. The engine temperature dropped to below the bottom dot of C on my temp meter. Idling was 1.5k as the car was recognised as cold start. With a thermostat, temperature would've been regulated. This is also noticeable on EFI cars.
Thirdly, piston ring clearance and engine oil. When the car is THAT cold, ring clearance gap increases as the rings have not yet expand. Engine oil temperature may still be cold and unable to lubricate or protect the engine fully, this is debatable though. We once race a race porsche that was smoking like a pot on cold start. Once it's achieved 80deg Cel, compression was perfect and the car was no longer churning blue smoke. I repeat, blue smoke, not white. This theory is supported by a situation of when you are bulding an engine; you have ring end gaps for expansion allowance. Crucial, very crucial especially on forged piston sets where they expand way more than OEM pistons.
F1, for example. They pre-boil their engine oil before pouring it into an engine before race.
Fourth, some engine magements especially standalones are so tunable that we can tune maps according to IAT, engine temperature, oil temperature, coolant temperature. I need not elaborate on this.
Fifth, for rally or race cars that run without thermostat, this is understandable though. They take extra precautious steps in ensuring engine protection. This shouldnt matter to them as they will rely heavily on engine cooling. My highest respect goes to these guys for taking things a step higher. If I'm not mistaken, Lotus cars by Aleyzo even run without radiator fans in Dubai and achieved 4deg Cel cooler in terms of operating temperature compared to when they have fans. The had proper ductings though. They'd have one guy holding a blower towards the radiator when the car is in the pits.
Sixth... back to RACE THERMOSTATS, they open and close at a lower temperature. If you have a heavily modified engine that heats up so fast, an oem thermostat may just be the culprit of your car not being able to cool down itself in time, hence, an aftermarket thermostat. They open earlier, allow a fair amount of time for the coolant to start passing through to cool down just before it heats up dangerously.
Sard Thermostat
-This is available in two models if I am not mistaken, 68 and 71deg. One of them has a hole on top, jiggle valve removed.
TRD Thermostat
This is a TRD Thermostat. Opens at 71deg Cel. Closes anything below that
Opening time is normlly written on the thermostat itself
Seventh, you will notice a jiggle valve. Most mechanics do not care on the position of this during thermostat installation. Please note that the valve is to be located on top, nowhere else. The jiggle valve allows little water to pass through. Fact is that perhaps less stress on the waterpump, like a bypass valve. Air or air bubbles is also allowed to pass through if the valve is positioned on top. Less chances of airlock and hotspots. I am unable to verify this though.
Eighth, oh my... my brain cells are fying. I have a couple more reasons and facts but my brain just stopped working, sorry :).
Conclusion :
Thermostat has its function. But not as important as the reason it is there.
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