Thread: Coolant Temperature and Power

I've been looking at coolant temperatures and what would be the optimal water temperature that has the least amount of thermal heat loss via Radiator while providing the best power and gas mileage.

For emission reasons they have increased the water temperature on today's cars to be between 200-212*F while cruising. On our cars the Fan temperature is set to 212*F, I'm sure there is a good reason for this.

So far I found these benefits that could be the result of higher water temps.
Advantages

• Lower emissions
• Having oil temperatures hotter then 212*F will have the water boil out and evaporate. Oil and Water temperatures are usually fairly similar. Claims say that oil should last longer too.
• Having water temperature at similar temperatures as the combustion chamber will increase thermal efficiency because less heat will be used to heat the water, and more heat will be used to push the piston down.
• Reduce cylinder scaring, metal expands with heat, if engine is too cold, it wont be the same shape as it will be when fully warmed up
  

Disadvantages

• KNOCK if temperatures get too hot
• Water boiling thus causing air bubbles
• Water temperature rising because it's not kept down as restrictivly
  

Ideal coolant temperature? - LS1TECH

Heheh. The basic issue is that we really don't want to lose ANY thermal energy resulting from combustion. Of course, since that's not practical in reality (because of physical limitations), we have the cooling system. Ideally, we would like to retain as much of this thermal energy as we can while avoiding things like knock and warped/broken parts.

This mainly means that the coolant inlet temperature to the engine would have to be increased (if we would like to retain more energy). You could do this by restricting radiator airflow for example.

The problem is, even if we were to "experimentally" increase the inlet temperature until the engine got TOO hot, and then backed it off slightly, that would only account for that one set of conditions, and you could start wrecking things left and right if the driving conditions changed and you had no factor of safety to make up for it.

The only real way to do this would be to have a control system regulating the engine inlet temperature at all times (to a pretty accurate degree). I don't know if just the radiator fans would cut it?

It's really a lot more complex than that if you were to go in and try to figure it all out. But I do agree that the manufacturers probably went the way they did for good reason. I think you'd have to do some serious calculations and modifications if you wanted to reliably increase power by trying to retain more energy.

Edit: BTW, look at the first guy who responded to that thread. 5.4 lbs/hp in a miata! That must be a fun ride!

Yup heat problems like soaking and warping is definitely a cation. But I was more of aiming at staying at stock heats, and not try so hard to keep the temperatures down.

Like right now most of us, turbo crowd at least, have overheating issues, to help prevent that we try to keep the temperatures all the way down with manual fan controllers, etc. etc. Now that I have an overall better cooling system (bigger rad, properly working fans, etc). I can now take my tune and set the water temperatures to stay at stock heats.

Thus I'm updating my tune to have the fans kick on at 212*F and will drive around and see what differences it will make, but the idea is not to go over those temps hehe.

there definitly are advantages and disadvantages even farther beyond what you listed, one of the problems i see that sucks is that with turbo engines like ours and where out turbo sits like many, it is near the radiator, when you incorperate a alumimum radiator into the situator that might be thicker the radiator may sit closer to the turbo like in my application for instance, so in return there are possiblities to have heat soak from not just engine tempatures, but from the turbo tempatures also, which would result in heat soak of the radiator thus rising tempatures. im not saying that there are other forms of radiators to be had as radiator are made of aluminum, however im just saying turbo applications can result in heatsoak of the radiator and not just the engine tempatures internally itsself

FYI: our engines are designed to operate at 76.5°C (169.7°F) exactly. Anything above that is considered "overheated". Anything above 212°F is catastrophic, which is why you'd want the fans to come on then. However, it's even better to have them come on around 190°F or even less if your car has a great cooling system.

For what it's worth, when my car was mostly stock, it never saw temps above 180°F unless I was idling and not moving at which point the temps would rise to 185°F where I had the fans set to kick on until it cooled 6°F or so.

Now with the turbo and Koyo radiator, coolant temps are rock solid at 170-175°F and only rise when idling.

Why is the stock thermostat a 180 then? I picked up a 170 one when I replaced my stock one.

But yes we do have to worry about heat soak like no other. Muffler mounted turbos FTW

Maybe it's different between years. For 1994 it's a 76.5°C thermostat. For B14s is might be higher. Why is it higher? There must have been some benefit Nissan found for running higher temps. The usual suspects are for cost reasons, emissions reasons, power reasons, reliability/longevity reasons (or a combination).

My point is, with few exceptions, the intended operating temperature is determined by the opening temperature of the thermostat. From what I've seen, that is never terribly close to the boiling point of water for obvious reasons. This talk of 200°F or more is serious overheating.
It doesn't take boiling temps to evacuate water from the crank case either. That's just silly. Water will evaporate just fine at much lower temperatures, and even lower still when a vacuum is present.

is that stock form or modified? to operate at those tempatures, surely there should be some kind of give considering modifications ?

They probably did increase the water temp gauges for low ports for emission reasons, to go along with the new low port system and smaller cams etc.

My car tends to stay at 180 on the highway on a cold night, if it's warm it will be at 190 with AC, but can go up to 200.

But the point I'm getting here is, stock 212*F is adequate enough for stock motor or NA build, but for turbo that's too high because of the risks right?

I have a 99 low port G20. Our fans come on at 203° and shut off at 195°. Also guys that went with the 170° thermostat that lived in colder climates caused the SES light to illuminate and I believe caused drivability issues.