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.