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I was talking with my boss today who happens to like tuning vehicles and he told me that in his adventures he discovered that if you get a thermostat that turns on at a lower temperature you can get slightly better gas mileage and overall better engine performance. Does anyone have any input on this idea?? He also mentioned that it might help provide a denser air charge.

I'm not really sure it would yield better gas mileage, but it would be a sure fire way to keep underhood temps down. I believe that the factoy thermostat is designed to open at 172 degrees. A company called Crucial Racing makes a colder one that opens at 160 degrees. Its my opinion that it would be more benefical to cars in a warm climate constantly. Some people have said that they can cause problems when winter rolls around, however that might just be their coolant mixutre.

It's a "widely believed fact" among a lot of gearheads -- especially ones from the older school -- that a cooler thermostat improves performance. My take: I wouldn't do it. I'll give you my reasons.

Motor oil is designed to work best at he correct operating temperature. So you'll likely put more wear on the motor.

All the moving parts in your engine have been broken in to work best at the dimensions they have at normal stock operating temperature. So you'll likely see slightly poorer sealing and accelerated degradation of lubricant.

When coolant temperature is lower, fuel mixtures tend to be richer. If it was cold enough, you'd also probably have the IAC valve opening more than usual. So you'd likely see a decrease in economy and an increase in emissions.

If driving under conditions where you experience low vehicle speeds and/or high ambient temperatures, you normally rely on the radiator fans to make the radiator actually work. These fans don't kick on until coolant reaches its standard operating temperature. So under these conditions, you'd get temperatures bouncing around.

I could imagine that some cars might run better with a colder thermostat, but I'd bet most of them either have some other problems that this masks, or are some kind of special case.

It's true that the intake manifold would be a little bit cooler and so it would heat the intake charge less, but especially on turbo models I don't think it's a meaningful difference. If you really want to prevent the manifold from warming the charge, use phenolic spacers between the manifold and block.

This comes from many many cars not designed to handle temperature produced in a performance situation. So you need the lower t-stat to keep from damaging stuff.

I wouldn't worry about the cooling system in a Subaru. If the car does not start overheating on a hot day in slow traffic, then it should be alright for an occasional romp. If you plan on really running the car, then a lower t-stat shouldn't hurt because your operating temps will be higher.

It won't do squat....and will probably make mileage worse.

You have two things to control engine temp. The t-stat and the ECU. It's pretty much a dual type setup.

When temps are below 200 deg F, the ECU just reads coolant temps and adjusts operations of the engine accordingly.

The tstat remains closed until like 170 something.

So under normal conditions, the car will/should operate in the 170-200 deg F range. The tstat being the lower limit of your control envelope, and the ECU switching on the fans at your higher limit.

During around town driving, your temps will almost never drop below 170 deg, so essentially, your tstat will be taken out of the loop of temperature control, and you basically flushed your money down the drain.

When driving on the highway, and having a constant flow of air across the radiator, temps may dip low enough where the tstat may start to close slightly, choking flow and allowing temps to be maintained. I have not confirmed, this, but I could probably easily do it by watching the select monitor on a long trip. I may try it on my trip to austin.

The exceptions to the above rules I can see might be when driving around town with the a/c on. The fans are running all the time. Again I can't say this for sure, but could probably watch the temps on the select monitor. Other scenario might when ambient temps are extremely low, like if you live in alaska or something.

Beyond the control point of things, you're really not gaining anything on a relatively stock car since the ECU is mapped to run a/f ratios at a certain point. Running slightly colder may give you slightly more power, almost certainly will increase fuel usage, and may keep you from going into closed loop. On a heavily modified car, the added heat will almost certainly keep you from utilizing the lower end of control that the t-stat is providing. So again, I feel your wasting money.

On the older cars that were carburated & didn't have electric fans, you pretty much only had the t-stat for temp control, assuming you had enough cooling capacity in your radiator, your tstat would keep your engine temps around its opening point, and yes the engine would run cooler if you swapped to a cooler t-stat.

Sorry about the book, but this is one of the things I feel strongly that there's a lot of misinformation out there, even from the so-called "tuners"

if fitted with the proper engine control, a hotter engine will produce a higher efficiency, and thus more power and mileage for the amount of gas produced. It is simple thermodynamics. If all the metal is kept hotter (closer to combustion temps), less of your energy is thrown away as heat.

In fact some of the future hybrid engines under development will have operating temps around 200F, and there has been a trend to higher thermostat operating temps already.

I think I read somewhere that it can be good for squeezing out an extra MPG or two. The trade-off is that certain plastics used around engines start having problems, so you have to use more expensive materials, and of course the margin of error before making steam is lower.

I ran the select monitor on my trip to Austin. Temps at cruise were pretty much consistent at 181-183 deg F. I was driving about 90 mph or so most of the way.

So without knowing where the t-stat starts opening, and where it's fully open, I can't say whether those temps were being controlled by the t-stat, or if those temps were the result of the amount of air flow through the radiator and engine speed (coolant flow)

The specs for the OEM thermostat are to start opening at 169 to 176 degrees Fahrenheit and to be fully open at 196 degrees.

With a healthy cooling system, your highway temperatures are pretty directly controlled by the thermostat, aren't they? They can't get hotter than the thermostat's setting since there's always airflow to the radiator, and they can't get colder since there's always heat from the engine.

It's only at low speed that the radiator fans become an issue.

With those specs, yes highway temps are pretty much solely controlled by the t-stat.

In what way does it depend on the specs? As long as the radiator isn't undersized for the application, shouldn't what I said hold?

I ran for about 7-9 months with NO thermostat. No problems to report other than it is REALLY cold in the winter. And defrosting the windshield is a bitch.

In regular weather, the engine temp would only register regular temps in long stop and go traffic.

I finally reinstalled the thermostat in early January (I think) but since I have had problems burping the system, I drilled a 1/16th" hole in the thermostat to allow air and water to pass through so I didnt have to burp the system. This small hole though takes the system about an additional 5 minutes to warm up to operating temp. Then the sytem is the same as normal.

vrg3 wrote:In what way does it depend on the specs? As long as the radiator isn't undersized for the application, shouldn't what I said hold?

yeah it should.