hotter vs. colder thermostat

[mpapers22]

Ive heard from quite a few people that if you put a colder thermostat in your car it will circulate coolant sooner and keep engine temps down. However I was talking to a few old school road racing guys the other day and they were saying the opposite. Theory being that a hotter thermostat will open later but it will also give the coolant ample time to cool off in the radiator rather than just circulating it right through allowing heat sink. They both make sense to me which is why im a little confused. Anyone have any experience on this?

[oni jake]

the lower temperature the thermostat, the less likely you will overheat, but you exchange the ability to dissipate heat in an efficient manner. (works really well in drifting, but not so much in endurance racing)

OEM is generally the best because it balances heat dissipation and optimal motor performance. (Motors need to get to get an operating temperature, and lowering temperature will eventually increase friction which leads to wear and tear)

The point of a thermostat is to keep the motor at the proper temperature through variable environments and conditions. So I guess the question is, what is the application we're talking about here?

Edit: (came back to this a few hours later)

Flow rate has a lot to do with cooling efficiency. The longer coolant is in the radiator, the colder it gets; that shouldn't be a hard concept. Lower thermostats open at a lower range, this also means it has a lower max temp before opening completely. At this point, flow rate is only effected by the pump and this leads to a spike in temp. Whereas, during a spike, heat soaking temperatures are no longer controllable for the application. A higher temperature thermostat has a higher max temp efficiency and is able to deal with spikes better. The trade off for a higher temp thermostat is reliability of not overheating but adds more stress to oil. I haven't even touched how pressure effects the system as well.

Quote for Wiki sums up what's going on perfectly:

Once at optimum temperature, the thermostat controls the flow of engine coolant to the radiator so that the engine continues to operate at optimum temperature. Under peak load conditions, such as driving slowly up a steep hill whilst heavily laden on a hot day, the thermostat will be approaching fully open because the engine will be producing near to maximum power while the velocity of air flow across the radiator is low. (The velocity of air flow across the radiator has a major effect on its ability to dissipate heat.) Conversely, when cruising fast downhill on a motorway on a cold night on a light throttle, the thermostat will be nearly closed because the engine is producing little power, and the radiator is able to dissipate much more heat than the engine is producing. Allowing too much flow of coolant to the radiator would result in the engine being over cooled and operating at lower than optimum temperature. A side effect of this would be that the passenger compartment heater would not be able to put out enough heat to keep the passengers warm. The fuel efficiency (http://en.wikipedia.org/wiki/Fuel_efficiency) would also suffer.

[Mikester]

There is more to this than just a hotter or colder temp thermostat.

As stated above, the idea of a thermostat is to keep the engine operating within the prescribed operating limits... For road racing, drifting or drag, a lower temp thermo will keep the temps where you want them longer than an OEM unit... Also, if your car is tuned beyond stock, it's a good idea to upgrade the cooling system completely.

Albeit, the coolant mixture comes into play... you can put a lower temp thermo in there, but say, with an 80/20 coolant/water mix... it will function nearly the same as stock... the more water & less coolant, the more effectively heat is dissipated via the radiator.

A hotter thermo allowing the radiator to cool the fluid 'longer' makes no sense... The fluid inside the engine will eventually heat beyond operating temp... which pressurizes the radiator & opens the thermo... A hotter thermo opens later to keep the car WARM, not cool. The more the coolant FLOWS, the more effective it's going to cool as it is cycled thru the radiator... This is why a colder temp thermo is preferred in racing applications.

If you DD the car on say a 50/50 coolant/water mixture in PA, either will suit you nicely... I do NOT DD my car and have a fully upgraded cooling system... However, it's fully built & tuned so when I drive it, I DRIVE it if you know what I mean...

If you're really interested in kowing, mount a coolant temp sensor in your upper radiator hose with a gauge in the cabin... If your operating temp is right about 80-90, leave it alone.

[e1_griego]

I don't see the point of a lower temp thermostat.

If you're trying to keep the car from overheating, a lower temp tstat is just delaying the inevitable.

For most applications a lower temp thermostat is running the engine colder and not letting the oil get up to temp. I stick to OEM tstats and don't worry about it.

[codyace]

OEM Theromstat, and put the temp gauge on the head outlet, that way you can see what your car is actually doing.

[Kingtal0n]

OEM Theromstat, and put the temp gauge on the head outlet, that way you can see what your car is actually doing.

this.

And, for more info:

Many engines make best power around 200-212*F. Hotter engines are more efficient engines.

Keeping an engine cold all the time, under 180*F, has been shown to accelerate engine wear.

Adjusting thermostat opening point is only for adjusting the minimum temperature. it has little if anything to do with maximum temperature.
There are myths about "water flowing too quickly through a radiator"- phooey. The coolant system is closed, there is no flow of matter (negligible) in or out of the system.

[codyace]

Adjusting thermostat opening point is only for adjusting the minimum temperature. it has little if anything to do with maximum temperature.
There are myths about "water flowing too quickly through a radiator"- phooey. The coolant system is closed, there is no flow of matter (negligible) in or out of the system.

Yea I laugh at that too. Bigger inlet/outlet, bigger pump, better core, all help. You want to get that water in, out, and back through it all as fast as possible.

[shogun!]

Add to the discussion hot spots inside of the engine block when having massive amount of power. A colder thermostat help a bit.

[Kingtal0n]

Add to the discussion hot spots inside of the engine block when having massive amount of power. A colder thermostat help a bit.

think about what you are saying. A Hot spot- as you put it, implies there is an area that is "hotter" than the rest of the engine.

Well, that makes sense. The area of the engine, closest to the process of combustion, is always going to be "more of a hot spot" when compared to anywhere else. therefore, there is always a hot spot, yeah, the place where gasses are burning.

When we use the term "hot spot" we usually mean an area of the engine that has an excessively, dangerously high temperature when compared with the rest of the engine. If this happens, it is because the transfer of heat from one location to another is not fast enough, either because an area of the engine that should be exposed to coolant is not (neglecting the engine of water's high heat capacity) or an area somewhere else that cannot effectively transfer temperature given the current demand, such as a sharp corner somewhere around the combustion chamber.

In those situations, if the temperature rise is sufficiently excessive, it can warp an aluminum part, or cause pre-ignition.


Keeping your cooling system air free and sealed up will help with the first scenario. Do not let metal objects bounce around in your cylinders to help prevent the second.

A colder thermostat on the other hand, will help delay ultimate temperature rise, which has nothing to do with hot spots, and I think that is what you meant to say. If you demand maximum potential from an engine, as an example, and the temperature begins to rise uncontrollably, you may upgrade your cooling system or let off the throttle to correct that situation. It does not mean the cooling system necessarily needs to be upgraded; retarded ignition timing will push more heat into the exhaust system, for instance, than would otherwise be there. And in another case, upgrading your cooling system but neglecting to monitor oil temperature is also a mistake.

[shogun!]

I share my experience with you. I am well aware of hot spots, especially in combination with high power outputs (ie my +700hp ca18det). The coolant is boiling inside certain local areas of the engine due to reduced coolant circulation. During a short burst of massive power, a colder thermostat will help to surpress this local boiling (shifting the curve prior to boiling slightly upwards).

For a longer period of heavy throttle, this will not help since the heat build up is way faster than the transfer of heat to the water. The water will be heated over the boiling point faster than fresh water will enter the area. It is like a very fast event similar to a small explosion.

One way to reduce these problems is to redesign the coolant circulation through the engine block. Secure enough cold water to all areas. An example is to use tubing connected to the frost plugs in the block and circulate more and colder water to all areas with the help of an electric waterpump.

Another area to look into. I have removed the oem oil/water heat exchanger and replaced it with an air oilcooler. With this high power and boost, the oem heatexchanger started leaking due to spot heating causing local high pressures. This happened very rapidly.