Thermostat: why you need it

I have been meaning to do the write up for this, among other things I have wanted to write about, for a while now. So let's get started in this. To be honest, this topic is new territory for me having only learned this whild trying to sort out a cooling system problem in our Turbo Diesel Hyundai Starex. I'm an now applying knowledge learned to my '79 Lancer project car. Previously I did not believe in using a thermostat in a hot tropical country like the Philippines. There are many arguments against it. It is commonly believed that the thermostat is only used to keep the engine hot in cold countries, something we don't need here. We normally thing that it's best to keep the water temp as low as possible. Bottom of the temp gauge is best to keep the engine as cool as possible. Thermostats also have the tendency to get stuck up as they get old, more often than not they get stuck in the closed position leading to overheating problems.

To start with, let's have a little looks into how a thermostat works and how it relates to the cooling system as a whole. 

This is a thermostat, the cooling system thermostat of course not to be confuse with the A/C switch that control temperature that is also called a thermostat...

The thermostat comes in all sorts of shapes and sizes but more or less they look the same. The cylinder at the bottom contains a calibrated amount of wax designed so that when it is exposed to hot water, the was will expand and open the thermostat at its set temperature. Usually more or less 90 deg. Celsius. The one I got for my car is 86 deg. The spring will close the thermostat as the wax contracts when it's exposed to cool water.  

The UFO shape let's in fit into a housing somewhere in the car's cooling system. This is usually a round area where the upper radiator hose goes into the engine although I have seen it also at the entrance of the water pump in some engines. 

The orientation of the thermostat is important. The wax end of the thermostat has to be facing the inside of the engine and the other side towards the radiator. This makes sure that when the water in the engine is hot the thermostat will open to let the cold water for the radiator in. Get that reversed and the thermostat will not open until the water coming in from the radiator is at the thermostat's pre set temp, so it will be already hot when it goes into the engine. 

The common belief is that all this thing does is make sure the engine is at operating temperature be staying closed as the engine is still cold then opening. But actually it does a lot more than that. It is never always open once the engine is at operating temperature. It opens and closes at needed to control the flow of water in and out of the engine and radiator. 

With out the thermostat the water freely circulates, the water pump pulls cooled water from the radiator then pushes this through the water channels in the engine block into the cylinder head where the water absorbs heat and then is pumped back into the radiator. The higher the engine speed (RPM) the faster this happens. Unless you have a really huge radiator, the problem is that at really high engine speeds the water is moving so fast that it does not spend enough time in the radiator to be able to cool enough. This is the reason why the water temperature goes up when running prolonged high speed if the car is not equipped with a thermostat- do that long enough and that could lead to overheating. 

With a thermostat installed, the water in the system gets halved. Half stays in the engine to cool it, the other half stays in the radiator to get cooled. When the water in the engine gets hot enough it will open the thermostat causing the cool water from the radiator to flow into the engine and the hot water in the engine will flow to the radiator. The cool water in the engine will then make the thermostat close allowing the water inside the engine to absorb heat and keeping the hot water in the radiator, which of course is a heat exchanger, allowing it more time to dissipate it's heat before going back into the engine. This exchange happens at the calibrated temperature of the thermostat, so even at prolonged high engine speed the water will always have time to stay in the radiator and the temperature will stay constant all the time. 

Constant temperature. The thermostat will maintain water temp at more or less half of the temp gauge. In the past I had always thought that it is best to have the water temp as low as possible. That's why my car was equipped with a huge 3 row radiator, high pressure cap and a huge electric cooling fan plus a second auxiliary fan. It was only when I started to delve into the finer points of engine tuning that I realized how wrong that setup was. 

There are a lot of variable that affect engine tuning. Some of them we cannot control, like outside air temperature and air density changes due to changes in elevation (when driving up a mountain) so it is best to control and keep constant the variables that we can control in order to keep our tune more constant. Be keeping water temperature constant and at just the right heat, the thermal expansion of the metal in the engine will be constant. this is an important consideration when breaking in a newly rebuilt engine as this gives the piston rings and bearings the opportunity to expand to their ideal running clearances and most importantly stay there.This will also mean that the temperature in the combustion chamber will be constant so our mixture setting for the carbs will not have to be varied as much since the engine will always be at the same optimum engine temperature for the settings.

One fear I had when setting up the twin side drafts carbs for my car were the old stories that cars equipped with twin side carbs would struggle to get up Kennon road going to Baguio City without having the change their mixture setting (or in some stories even the jettings) to something much leaner about 3/4 the way up the mountain. I later learned from more credible sources that the reason for this was not only because of the altitude change, but rather because most of those engines were no longer equipped with thermostats they would have likely been running cold, giving less than ideal conditions for fuel atomization and combustion. A properly tuned car with twin side drafts and an engine at the ideal temperature should have no problems making the climb. 

The only disadvantage to running a thermostat is that it could get stuck closed when it ages. This though is nothing that good old fashioned preventive maintenance can't take care of. Thermostat units are pretty inexpensive so it won't hurt to change it to a new one every two or three years. It is also a good idea to learn how to monitor your gauges. The thermostat will maintain temperature at around half, that is quite a long way from 3/4 of the gauge and even longer to the point of the gauge that's red where the engine is overheating. So even if you did forget to change the thermostat before it got stuck up there's no reason for you not to notice that the temperature is higher than normal and pull over before the water completely boils over. 

Some pics!

This is where the thermostat usually lives. A round housing where the upper radiator hose meets the engine;

Inside, there is usually a groove where the thermostat sits in. 

This is what the water coming from the radiator 'sees' :P

And lastly, if you are reading this blog, chances are you own a Mitsubishi 4G32/33 engine. If I have convinced you of the merits of running a thermostat and you wish to install one, you will find that it is rather hard to find an original one for a 4G3x (at least in the places I looked it was). You will be happy to know though that a thermostat for a Mazda 323 will work just fine. That's what I used. Here's the box with the part numbers; 

P.S. always use coolant with your water, and always used distilled water because both of those prevent the engine from rusting on the inside and prevent it from having mineral deposits. This also prolongs the life of the water pump. Upgrading to a higher pressure radiator cap (1.2 or 1.4 Bar vs the stock 0.9) is also a good idea for a high performance engine. High pressure makes the water harder to boil. A radiator cap from W123 Mercedes Benz will be a nice upgrade for your old-school Japanese car since this is a high pressure cap and it's only a fraction of the price of performance rad caps from Ralliart, TRD or Spoon, not that those come in sizes for old-school cars anyway.

Credit to my friends from SMT Competion Headers for showing me the right way. Otherwise I would still be ignorant to the importance of this. :)