The Stovebolt Site Forums The Shop Area Engine Shop cAltitude and economy

I was shocked to find out that my fuel economy increased while taking a trip through the mountains. I can understand it now but my question is would the economy increase if I had a carburetor? My vehicle is fuel injected.

I got 37.83, 37.5, 35.8 on my trip. The last tank at 35.8 was as I came out of the mountains and I can't say what the elevation was, but I was steadily getting lower.

This was driving from 77-83 mph. I thought this was excellent economy considering the speed.

What are you driving. I have found that mileage increases in hilly/mountaines driving. I figure you gain more going downhill then you lose going uphill.

I am driving a 2004 Neon 5 speed. I have gotten 39.06 on a long drive in Nebraska, but I was only driving around 60-65.

I figured the economy increase is due to more throttle opening in the thinner air, thus not pulling as much of a vacuum. The thinner air would also have slightly less wind resistance, but I think this would contribute so slightly that it is not important.

Since virtually all modern injection systems compensate for altitude, there should be little, if any difference in performance. Not so with a carburetor, since the thinner air at higher altitudes tends to make an engine run richer than it will at sea level. An aircraft engine with an altitude-compensating carburetor, or with a pilot who's good at watching cylinder head temperature and adjusting mixture manually will get better fuel efficiency at altitude.

With either a MAP (manifold absolute pressure) sensor, or a MAF (mass air flow) sensor, the fuel injected automobile should run at least as well at altitude as it does down lower, and you'll get slightly better mileage because the injection is running leaner to match the air density. Couple that with an oxygen sensor and a computer clock frequency that allows mixture adjustments up to 90 times a second, and you've got an extremely efficient system!
Jerry

When I drove my 1984 Ranger through the Mountains of Colorado I actually got pretty good mileage, right around 30 mpg with a v-6. There was one particularly long down hill stretch that I averaged 34 mpg. In the flat lands of Wisconsin I averaged low twenties.

With the electronic engine controls, you probably get more spark advance at elevation, which helps improve engine efficiency at altitude. So the combination of all that the engine controller is doing keeps all the variables at the best point for the driving conditions.

The thinner air at altitude would make a difference in principle, but I don't think you would notice any difference at the altitude and speeds at which you were driving. Air density and the associated drag makes a difference with airplanes, which is why pilots like to climb as quickly as possible.

The pumping loss/improvement resulting from lower manifold vacuum is another point that is correct in principle, but I don't think you would detect any improvement from that alone.

Harvester

The theoretical ideal air/fuel ratio is 14.7:1, by weight not volume. Since a carburetor meters volume by flowing liquid gasoline through a main jet, as the altitude goes up, air density decreases, but the volume of fuel remains the same. At about 3,000 feet altitude or so, carbureted cars experience a noticeable drop in power due to the mixture richening enough to get way out of the stoichiometric band. The higher you go, the worse the performance gets. My 59 Burb with the 1970 307 engine would barely pull Sonora Pass in California at 10,600 feet, and that was with a 4.56:1 rear axle ratio. It ran great down in the San Joaquin valley, but a fishing trip to Leavitt Lake above the pass was a real adventure. Yes, altitude makes a measureable difference, something that electronic injection is constantly compenasting for with mixture, timing, and air management to the catalytic converter.
Jerry