Help me understand lower temp thermostats

[Bmaughan]

I’m trying to understand what the real world benefit of a lower temp thermostat would be.

To my logic it just seems like it wouldn’t do very much except slow down the initial warm up time... but once the engine is at operating temperature any thermostat would pretty much just remain open.
And if you reduce engine load and let the engine cool...the cylinder heads would cool more than would be optimum for combustion.

What am I missing?

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[sigintel]

Some “lower temp” like Reische 170 also have a larger opening cross section when above the target temperature.
This decreases restriction to flow vs stock when open.

Any lower temp tstat decreases combustion efficiency and increases carbon buildup at “daily” driver engine loads.
Advantage is increased thermal headroom during track or extended “mexico” sessions or continuous asshattery in traffic in the Texas summer w a Whipple or other FI.

If you run lower temp tstat, it is advisable to regularly produce engine loads over enough time to hit 215+ CHT and insure removal of carbon.
A low temp tstat with 12 months of granny driving will likely result in excess carbon in cylinders that can increase knock when the engine is pushed ++225 CHT.

 

[markmurfie]

The air fuel ratio is the main determining factor of optimum combustion, not engine operating temperature. It can get too hot and you can get preignition during the compression stroke, but unless the fuel isn't evaporating fast enough to achieve the proper AFR, things are not too cold. Think E85 cold starts, which even that can be fixed without increasing the temperature, just more liquid fuel so you get the proper air to "evaporated fuel" ratio(the thing that actually burns). You pretty much can't make engines run too cold in terms of wanting to produce more power output. Fuels evaporate fast even at below freezing temps. The calibration just has to be setup for the colder temperatures and account for less evaporation. Even at high operating temps this evaporation rate still needs to be accounted for, as fuel doesn't always completely evaporate and you get valve and wall wetting, which can actually help clean carbon build up off the cooler surfaces.

I don't think the thermostats operating temp rating has as much of an effect on warm up time as people make it out to be. Just cause its a lower temperature doesnt mean its slower getting there. My case for this is, if the fan settings in your calibration are not changed to come on sooner, when the cars not moving the thermostat doesn't do a whole lot to reduce engine temperatures. The air flow over the radiator, taking the heat out of the water, makes more of a difference than the water flowing through the block. You think your cars not warming up fast enough, park it and raise the RPMs to have more fuel burned and produce more heat. Turn the AC off, as that can enable the fan to cool the condenser. The defrost usually enables the AC compressor to dry the air and the fan can turn on for that as well.

For a negative you could say the tolerances of the engine at lower temperatures are looser than they were designed to be, leading to increased premature wear, allowing oil to have an easier time getting past the piston rings and valve seats. I've run 170* tstat in my car for four years now, and have only had the positives come from it. Any oil consumption was undetectable before it, and has remained undetectable after it for all these years. Back of my valves and inside of my cylinders all have minimal carbon build up even at 65k miles. I do a fair mix of hard track driving and driving the car for long periods between the hard driving at normal low loads. I also have a hard time even getting the CHT above 205*f unless the belt breaks or comes off. I change my oil around 3000 miles or after a track day/weekend. I change all other fluids, spark plugs, Maf sensor and oxygen sensors regularly as well, and try not to wait for negative symptoms to arise before I change them. Maintenance can not be neglected, when you are expecting a certain performance/ reliability from the car. I can't stress that enough, as being more important than running the engine 15-20* warmer throughout its life.

 

[Bmaughan]

Thanks to both of you... very informative and helpful! Thanks for taking the time to explain your thoughts...

 

[sigintel]

The air fuel ratio is the main determining factor of optimum combustion, not engine operating temperature.

yes, “air fuel ratio”, but all the way down to the molecular level. Unvaporized (still liquid) fuel is insulated by neighboring fuel molecules from achieving the desired air fuel ratio.

I should have clarified, I meant the pure technical “combustion efficiency”, not the colloquial “efficiency” which we may all think of in different terms but generally with consideration of desirable performance.

Could be wrong here, but pretty sure that ”desirable” combustion from a hp/performance perspective in gasoline and ethanol performance engines is deflagration.

https://www.google.com/search?q=deflagration+vs+detonation&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari

In regards to pure technical “combustion efficiency” detonation is superior to deflagration.

In regards to pure technical “combustion efficiency” within the conditions of deflagration: vaporization of hydrocarbon fuel and mixing of that vapor with oxygen containing air are improved with higher fuel and oxidizer starting temperatures.

Temperature is a measure of how hot or cold something is; specifically, a measure of the average kinetic energy of the particles in an object. Kinetic Energy is a type of energy associated with motion. More molecular motion = better mixing + better fuel vaporization.

 

[markmurfie]

Charge density > homogeneity in a spark ignition engine.

Proper stoichiometry is king in any engine that operates by a combustion reaction.

 

[EFI]

It slowsdown the engine getting to max temp basically. It's mainly for those who are looking for to eek out every single horse out of their engine (for 1 run) so that they can get have a nice printout of their power on a dynosheet.

Ok that may be a little bit harsh, but either way there's very little to gain from these in the real world. If you're driving hard (whether racing or just having fun) this won't reduce your engine temps. Yeah they may flow a little more than the stock one, but it's not that big of a difference to really reduce the overall max temp of the engine.

 

[Whitedevil95]

Coming from many years of Terminator life before this coyote the main need for a lower temp thermostat is in blower applications. Engine temps directly effect the air charge temps that typically dictate timing and overall performance. So by hopefully keeping your car running in the 160-190 range vs 200-225 range you can improve overall performance and drop charge temps. So from a blower point of view it can be helpful. I know whipple and other usually send a lower temp thermostat with their kits.

 

[sigintel]

Charge density > homogeneity in a spark ignition engine.

Proper stoichiometry is king in any engine that operates by a combustion reaction.

Think we agree?
Charge Density for max horsepower.
Homogeneity for Prius like combustion efficiency(mpg).

 

[Bmaughan]

Some “lower temp” like Reische 170 also have a larger opening cross section when above the target temperature.
This decreases restriction to flow vs stock when open.

Any lower temp tstat decreases combustion efficiency and increases carbon buildup at “daily” driver engine loads.
Advantage is increased thermal headroom during track or extended “mexico” sessions or continuous asshattery in traffic in the Texas summer w a Whipple or other FI.

If you run lower temp tstat, it is advisable to regularly produce engine loads over enough time to hit 215+ CHT and insure removal of carbon.
A low temp tstat with 12 months of granny driving will likely result in excess carbon in cylinders that can increase knock when the engine is pushed ++225 CHT.

What you said makes perfect sense to me...regularly hitting 215+ also seems like very sound advice.

I guess what I don’t understand is why people say that they have lower engine operating temps after installing a lower temp thermostat..when at normal operating temps, I would assume both the stock and lower temp stat would both be fully open.

Does the increased flow really account for that much change?

I think I can see how sitting at idle with the tune altered for increased fan activity would yield lower temps but outside of that, it seems like little difference should be noticeable.

I’m in AZ, so with ambient temps consistently reaching over 115, I feel like I’m most likely maxing out the thermal capacity of the cooling system itself, not the thermostat.

Let me know if any of my thinking is wrong.

Thanks again!

 

[Stymee]

Wouldn’t fan settings need to be changed for optimal efficiency

 

[Stephen@lethal]

Wouldn’t fan settings need to be changed for optimal efficiency

Thats correct.

 

[Avispa]

To the OP's last question, if the cooling system is working normally, and AZ in the summer sun may be at the upper end of "normal", a thermostat isn't usually open all the way. Once it's open all the way, if the cooling load increases, the water temp will increase and the thermostat temperature won't matter. The thermostat set temperature might matter if it's so dang hot outside that at lower thermostat temps the outside air temperature differential isn't enough to cool the water - meaning a 170 degree stat would be open all the way in some conditions but not a 195. That said, Ford designed these things so that the radiator has more heat transfer capacity than the maximum expected cooling load - yes in AZ on I-10 going 100 mph when its 120 degrees outside and you have the AC on.

For a boosted motor, the risk of higher water temperature is detonation, not heating intake air in the heads. Yes, you'll lose a few HP because of the water temp, but not much. Like Whitedevil says, it's the charge air temperature that matters most. Higher cylinder head temps aren't going to matter as much as the intercooler. That's what makes all the difference. If your intercooler system is too small for the job, a 170 degree thermostat isn't going to help you.

 

[markmurfie]

Lowering the operating temperature of the engine does help your intercooler and air charge temps. Not as significant as say phenolic spacers between intercooler and heads, but to some small degree the 20-30* lower operating temp means less heat going into the supercharger intercooler/ heat exchanger system allowing it to cool the air charge more. When the supercharger is completely heat soaked is when it would matter the most. The spacers just prevent this heat soak from happening, and is why they work so well. They make it so, most of the heat in the system is only from the supercharger and little from the higher engine temperatures.

You guys might be surprised, but Whipple supplies the 160* thermostat, and doesn't change the fan settings in the calibration. Mainly because they are trying to remain 50 state emissions compliant, and at stops/ low speeds this raises the operating temp to when the fans turn on. When the car moves faster than 25-30 mph, the airflow over the radiator is more than the fans create and the operating temperature of the engine would then drop and be controlled by the thermostat. If the thermostat wasn't there it would drop to as much as the cooling capacity of the radiator could remove heat, which would be highly dependant on airflow over it, not just its size.

Im attaching a picture to show how well the base radiator with a transmission cooler, AC condenser, and heat exchanger w/ entire stock grill in front of it works at 95* in 85% humidity. Basically WOT at 16-17 PSI using high octane gasoline for 10.5 seconds. 0-145+ MPH at the start its at 185*, Where I let off its at 197*, it peaks at 204*, and 10 seconds later returning down to 40 mph, after letting off its already back down to 194*. On the stock thermostat it would do the same thing, just 20* higher every where. Rate of temperature change wouldn't change. With higher starting temps because of stock fan settings it would negate the purpose, unless you had a decent part of a run that allowed the lower temps that the stock Tstat wouldn't get to. I have done a SCCA events in 90* high humidity using 12-13 psi, and in 45-50 second runs, tight turns low speeds, starting temps at 185*, my peak ETC temp was also only 205* briefly. Could a larger radiator and more openings for airflow to it keep the temps even lower by reducing the rate the temperature rises? Sure, but the base cooling system does very well even with being choked and under extreme conditions relative to what it was designed for. A lower temperature thermostat may not improve the capacity of the cooling system, but it doesn't limit heat being removed until a lower temperature, and thats the entire point of using one. The two thing to keep in mind are the coolant flow between heat source and radiator/exchanger and the airflow over the radiator/ exchanger. You need both, more flow rate of water inside and air outside and more heat will be removed resulting in lower temperatures.

 

[Avispa]

That is a great post, Mark. It would be really cool to see a similar log of IAT2 with the two different thermostats. No doubt lower water temp would make some difference, just not sure how much or whether it matters enough to deal with the downside of a low temp thermostat for street driving. It's clear from the discussion that full throttle at 12+ pounds of boost makes more waste heat than the cooling system was designed to handle. The advice is still good: Keep the stock thermostat if most driving is on the street. The downside is bigger than the upside. It might be a benefit to change it out for track day. That wasn't a good option for a Terminator. Changing those things was a royal PITA. Haven't had to do that on a Coyote yet.

95 degrees, 80% RH - Houston in July. I don't miss it for a second.....lol

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