Highway MPG

[theholycow]

Isn't the thermostat a pretty simple electrical output? Why not just use a common potentiometer? Instead of adjusting the temperature the thermostat reads, just adjust the electric signal it outputs...

If you're willing to braze, just put in a hole for a second thermostat and use a toggle switch to choose.

[GasSavers_RoadWarrior]

You're thinking of the fan thermostat or ECU coolant temperature sensor.

[theholycow]

Ah, I see we've gone past my knowledge. Time for me to learn something new. What is this thermostat, and how does it work?

For all the theory and experience I have, there are still some huge gaping holes in my knowledge of this stuff.

[GasSavers_RoadWarrior]

It's a mechanical flap valve device typically implemented with a spring and sealed wax pellet that closes off the radiator to the coolant flow when the engine is cold, and opens to let the coolant flow to the radiator when the engine requires cooling.

See... http://en.wikipedia.org/wiki/Thermostats and http://en.wikipedia.org/wiki/Wax_thermostatic_elements and http://auto.howstuffworks.com/cooling-system8.htm

[palemelanesian]

I believe the thermostat is a thermally activated mechanical valve. When it reaches the set temperature, it opens and allows coolant to flow between the engine and the radiator. Until it reaches that temperature, the radiator is doing nothing, and the coolant is just held inside the engine.

Edit: RW beat me to it, and with a more complete answer too.

[dkjones96]

People report getting over 22mpg in everyday city driving in their vettes.

Imagine the P&G that sucker probably does? Pulse for 3-4 seconds to 45 and glide for 20-30 seconds down to 30-35 mph.

[dieselbenz]

Originally Posted by theholycow

Now, I have no clue how they expect anyone to get 28 highway out of it with silly low gearing. A 2.5l 5 cylinder at 3000 rpm can't be very efficient, but it sure is lively!

My GF's 12 year old 180k mile volvo 850 wagon with annoyingly short gears (3k @ 70mph in 5th) and a 170hp dohc 20v 2.5 liter 5 cyl engine (sound familiar?) does 30mpg @ 70mph, 32mpg @ 65mph and 36mpg @ 55mph. There is no direct correlation between displacement and highway fuel economy IMHO. I suspect the Golf can beat those numbers in the real world.
In city driving the volvo returns 20-24mpg depending on who is driving.

To the author of the thread: I'm willing to bet given an identical driving cycle the corvette will return better fuel economy than your wrx. Not trying to knock the wrx, its one of my favorite cars. But the GM LSx series of engines are extremely efficient at part load by comparison to any other engine of similar power or displacement. This is due to reduced friction from its 16 valve single cam design.

[GasSavers_RoadWarrior]

It's those stupid auto journalists again, you know the ones who complain about 10 second 0-60 times being "too slow" for an econo car... I expect that they whine like slapped and denied stepkids when they test a car with an actual overdrive 5th gear, saying stuff like "passing performance abysmal" because they couldn't figure out how to shift down to 4th, or didn't want to...

[Mayhim]

My 98 Z28, 350hp, auto, 3:43 gears will get 20-21mpg in daily driving with the A/C on. 24mpg on the open hiway.

Probably get better if I drove it like I drive the CRX. Would certainly get better if I have a 5-speed. And the fun factor for those days I feel like smoking the tires...better than a Focus.

[theholycow]

Originally Posted by tjts1

My GF's 12 year old 180k mile volvo 850 wagon with annoyingly short gears (3k @ 70mph in 5th) and a 170hp dohc 20v 2.5 liter 5 cyl engine (sound familiar?) does 30mpg @ 70mph, 32mpg @ 65mph and 36mpg @ 55mph.

Extremely familiar. That describes my VW's engine and gearing perfectly, except you didn't mention torque and I don't know if the VW is dohc.

There is no direct correlation between displacement and highway fuel economy IMHO.

I'm not sure I'd agree 100%, but given appropriate gearing, I'd say the difference would be minor. I was actually thinking about something similar this morning. Imagine a 5 liter V8, and a 2.5L I4 made exactly the same - same bore, stroke, even the same head, just appropriately different cam and crank to keep the firing order even (unlike 1970s Buick V6 oddfire). At any given RPM, the V8 would have to make approximately double the power; and at any given amount of power, the V8 should run at half the RPM. So, given twice as tall gearing, the V8 should use approximately the same amount of fuel to cruise at the same speed.

I know there's MAJOR stuff I missed there, and I only thought about it for a minute or two while in the shower, and that idea has no practical effect on anything, but it was an interesting thought process.

To the author of the thread: I'm willing to bet given an identical driving cycle the corvette will return better fuel economy than your wrx. [...] the GM LSx series of engines are extremely efficient at part load

In addition to engine efficiency, the Corvette's drag coefficient is .286 vs. the WRX's is .33 to .35 (depending on trim level).