Hotter thermostat for FE
 

I made a ~250 mile trip last month, and got 24 MPG on one leg, and 30 on the return. The elevations were not too different (maybe 30 feet), weather was similar, and weight was a little heavier on the return leg.

While I was vacationing, I swapped my 160* thermostat for a 195* one. (hey! I have to relax somehow!)

There ya go, engine temps really do play a role in efficiency.

...with carburetors, a higher operating temperature helps ensure better fuel vaporization which leads to better fuel economy.

...but with todays' fuel-injection systems, this "vaporization" benefit is basically negated and overshadowed by injector pressure, which is now the major (but not the "only") factor.

I agree with all that, but my fuel injection system is not quite like today's systems. My TBI is pretty much an electronic carburetor, more or less squirting the fuel in there with less-than-optimal atomization. Looking down the throttle body, you can see the fuel hitting the top of the throttle plate (the injector is in the middle, over the plate, shooting straight down) and some sitting on top of it.

That may very well explain the change, as the intake manifold is actually heated by some coolant passages.

As you said, that is not the only factor. It could also be tolerances tightening up with slightly warmer parts, could be the warmer oil having less resistance, could be the better vaporization. I don't claim to know all the reasons, those were my results.


On a side note, maybe if the better vaporization is the major factor in my increased FE, it would seem as though an injector cleaning/replacement could be in order.

Keep in mind, you were also bringing the operating temperature closer to what the TBI system was designed around (probably 180* or 195*) and expects to be working with. It probably doesn't do much adjusting for colder temps, other than cold start enrichment.

I just got a big jump in economy by replacing my thermostat. It was running coolant temperature at 165F with the worn out part. Now it runs 192F. Completely fooled the ScanGauge, I was running normal tank mileage around 40mpg, filled up today and I actually had 46mpg. And I calibrated my pump nozzle just to be sure, I was so surprised. I filled the tank the same way too, but if it didn't really fill, I will catch the drop next fillup.

In theory, higher coolant temps means less heat transfer (energy loss) to the coolant system and more energy available to do work on the piston.

from what i understand the biggest help to FE when it comes to the engine temperature is that it vaporizise the fuel better cause the hotter the engine the hotter the intake manifold which heats the incoming air, i checked the temperature of my intake manifold at 180 degrees and it was 269 degrees at the ports(my engines a I6 with the intake right next to the exhaust too) i was thinking of adding a inline fuel heater to get the fuel up to 180 degrees before injecting it into the engine so it vaporizes instantly and if it vapor locks then drop to a 165 t-stat and 0w30 oil or something(also dropping the engine temps allows for higher compression ratios with the same fuel octane forthermore the lower intake temperatures also is equivelant to higher fuel octane)
(8 degrees engine temp drop = .25 CR increase/for every 10 degrees intake temp drop = 1 octane point increase of fuel equivelant)

You're better off keeping the higher temp t-stat and cooler fuel to avoid vapor lock. Try warming the fuel, but not enough to vapor lock it.

You're missing the point here. Dropping the engine temp only increases the ping threshold a little. Hotter/cooler air and fuel have a much greater effect. Ideal for high compression is reverse cooling flow (heads first, then cylinder walls), a 180* or 195* t-stat, and cold air coming in. For FE, the same equation with warm air would be good (although not as high compression). Running the engine too cold is counter-productive, and also increases wear.

Going to have to make that my first mod this weekend, got scangage couple weeks ago and water temp is running 165.

...195ºF would certainly be better.

30 mpg with a ram 2500 and a 5.9?

wow

edit: confused, is this thread regarding the truck in your garage profile? I see you mentioning having TBI in the thread but the truck has a cummins?

i looked at the pressure of fuel online at 140 it says it will hit 29 psi but if u have a pressurized system the fuel will stay liquid tell injection and thats all i was saying, and from what i read lower temps only increase wear if u try it in a used engine thats broke in at higher temps with lower tolerences if you build a engine for the lower temp and higher CR then it's "SUPPOSED" to work i guess

True, if it's built with lower temps in mind, then it's ok (although ring wear is still a little faster, as cylinder wall lube isn't quite as good). However, the 4.0 wasn't built for that, and isn't meant to run that cold. They're toasty engines by nature.

It's tough to vapor lock an injection system, but given enough heat, certainly possible. There have actually been some issues with vapor locking the 4.0 in hot weather. Newer ones had a heat shield over the exhaust manifold runners where they connected to the head, as well as some shielding for the injectors to avoid this.

i'm also planing on ceramic coating the exhaust and nansulating the intake cause the intake gets to almost 300 degrees at idle and full temp right at the last 3 -4 inches of the ports so if i do though's two things then i'm hoping for cooler intake temps and better power and FE with the fuel heater also, from what i understand colder intake is better then the warm air intake as long as the fuel is also heated for better atomization and more power so you can hit lower rpms on cruising if your geared right anyway

NanoBoost liquid applied automotive parts coating for thermal insulation

Quoting myself here, but I filled up last night, and confirmed a huge increase in fuel mileage with my thermostat change. That's two tanks in a row, one B33, one B65 where I got 46mpg, 6mpg better than previous tanks. My worn out thermostat was regulating 165F before, now 192F.

That's a 15% gain!!

One of the best of the hypermilers here, palemalasian, gets good economy in part due to living in a warm climate, and if I remember correctly he uses a block heater too.

There may be some small change in fuel atomization with warmer coolant, but it is nothingcompared to the gains possible when less energy is used to warm coolant. With cooler coolant, more energy is used to warm the engine that would have otherwise been used to push the piston down with more force.

So I would say it is less theory and more like a proven fact that a warm engine is more efficient thana cool engine, and less efficient than a hot engine. I've read an article here describing a crazy high efficiency engine that was severely insulated and heated the gas to a vapor. Will find the link when I get to the comp, using my phone right now.

Since I put on my grill block at the same time as I put in my SGII, I really can't say how much warmer my car is running or how much mpg was gained by the grill block alone. However, it does have a 180f thermostat and now routinely runs between 192-202 with routine excursions to 213f when climbing long, continuous grades. At the same time, I'm getting 25-30% better mpg this winter over last winter, using the same snow tires and same winter gas. I was up over 40% until winter got nasty over the last three weeks. While I believe the overwhelming majority of that gain has come from improved hypermiling technique, I have to believe that at least some is coming from the higher operating temperature, too.

I know, there's no science in any of that, but until warmer weather returns so I'm challenged with overheating issues, I'm not going to do any A-B-A testing.

I always run a 195 in all my cars. Also change them every few years, I don't wait for them to fail.

What would be a good target temp for the fuel line? if you warm the fuel will it be able to evaporate from the tank easier building pressure and getting past the fuel tank cap? I'm thinking of my veggie burner and that tank got fairly warm but we vent to the outside?

what about temps on the intake for the air? I have a geo with the factory cold air intake system which could be easily modified to draw warm air.

in my opinion the warm air intake is heating the wrong thing for example the temperature where i'm at is 38 degrees at night and thats how cold the fuel is in the morning so ya you can heat the air but the thing your trying to improve is the evaporation of the fuel that really sucks at 38 degrees. i have no return fuel line so i'm trying to see what happens when you heat the fuel line with the coolant line from what i've read so far i should be able to hit 180 degree fuel without vapor lock in the line.

Changing the thermostat to the 195* F version seems to be a popular mod for the VW TDI group for better FE as well. Looking into doing that soon on my TDI, now that I have a few tanks under my belt.

On my car the operating temperature range 194 to 225. The cooling fans with the air off won't come on until the temp reaches 220 degrees then turn back off at 197 degrees.

stat temp
 

I think my new stock stat is a 180f so I'll boost it to a 195f. I installed a wink mirror in the car last night so there would be no blind spots driving around with my mirrors removed. So many tweaks to do.

Replaced my T'stat Saturday, what a pain compared to the old V8's, running temp was 165-170 via SG, now 195-197. No obvious pickup in MPG via SG, this morning commute was pretty much average 31.5. Will take a few tanks to really be able to tell. My SG isn't callibrated prefectly, but going to leave it alone for now.

I can't tell you why, but my scangauge completely missed the mileage improvement with my thermostat change. It was hard work to keep my tank mileage at 40mpg, and I barely made it when I filled the first time after replacing the stat. And the scangauge was way off, as calculated mileage showed a 6mpg difference.

...just remember, the boiling point of 50%/50% mixture of water/antifreeze with a 15 psi cap is something like 245ºF, well above normal 212ºF boiling point of water alone.

...water/antifreeze mixture alone is good for about >225ºF

...each pound (1 psi) of cap pressure is another +3ºF.

^ Yup. Most engines are perfectly happy up to 220* or so, with no adverse effects. Some are good to 230*. Taking a look at factory e-fan kick-on points (where applicable) can give you some idea of how hot you can push it safely.

New best on drive home today, 35.4 SG mpg, falling snow had me driving slower and my foot's getting better at letting up going up hills.

Evans Engineering coolant
 

The contrary actions are so funny to me. Here people want to put in higher temp Tstats. On the "performance" forums people put in lower ones (ie 180*). Trading HP for FE.
My RAM has a 193 Tstat, and that seems to work fine. The main issue is the boiling point of the water/coolant mix. The pressure is what pushes the boiling point above 212. That said, if you switch to Evans Engineering coolant...you get a 370* 0psi boiling point. If I got that swapped in, I'd want a 250* Tstat! Then the issue would be reprogramming the ignition to not cut timing....

I was contemplating a WAI for my truck...by ducting air that came through the radiator fins into the filter box. Figured I should do some A-B-A testing for any results.

Econoram - At 250*, you have to start worrying about head gasket integrity, and particularly issues with cracking or warping heads. They just weren't designed to run that hot. 210* - 230* is the hottest running temp most modern engines will hold up to on a continuous basis.

As far as temp, a hotter t-stat would actually make more power. However, on many performance forums, they'll drop from a 195* to a 180*, as it is an easy way to drop the intake temps, particularly on V8s like mine that run coolant through part of the intake manifold. The intake temp drop overcomes the reduced efficiency from lower coolant temp when power is the only factor. If they could get the intake temps down while keeping the 195* t-stat, they'd get even more power.