High Altitude Issues January 20, 2005, 06:04:33 pm Yahoo Message Number: 49696Hi, Yall!Let me say first that I am not a mechanic, nor particularly knowledgeable about automotive mechanical issues. With that disclaimer out of the way, I will say that I am an older Jeep hobbyist (meaning carborators) and also, that I live at 8500'. I am a member of several Jeep clubs, and the accepted wisdom is that for every 1000' you gain in altitude, you lose about 3% in power. This includes Fuel Injected engines. With older non fuel injected engines, this 3 % holds true as well, IF the engine timing and tuning are set for higher altitudes. If not, you lose a good bit more, as it uses too much fuel, not enough oxygen.With newer fuel injected engines, my guess would be, if you live at sea level and drive over the Rockies, you will see MORE than 3% loss, because the computer will compensate, but I think (not sure, but think) that the computer needs some time to adjust to changing conditions. However, if you stay at altitude awhile, you should gain back some of the lost power, as the computer compensates.How I didn't murky the waters even more.Lee
Re: High Altitude Issues Reply #1 – January 20, 2005, 06:36:47 pm Yahoo Message Number: 49700Lee: You lose the 3% power, but your engine management computer uses an air mass sensor to add the correct fuel for that air density (altitude). The benefit is that you use less gas per mile. You may not see it on your V-10, but when we moved from Los Angeles Bas
Re: High Altitude Issues Reply #2 – January 22, 2005, 11:00:05 am Yahoo Message Number: 49760Lee, Gus is correct on all accounts. I'll add that when I go down to sea level, I notice the increase in power and worse milage.Another thing, the decrease in HP does not effect turbo or super/charged vehicles as much. This is why the cars that have the best times on the Pikes Peak Hill climb are turbo charged.OT: I use to have a Jeep J20 and belonged to a Full Size Jeep club. That group of people remind me a lot of this group. Nicest and most giving people. I can't tell you how much I fooled around with that carb and points. :-) Although, it was super simple to work on.-Victor '91 MB Longmont, CO
Re: High Altitude Issues Reply #3 – January 28, 2005, 03:40:10 pm Yahoo Message Number: 50021Quote Lee:You lose the 3% power, but your engine management computer uses an air mass sensor to add the correct fuel for that air density (altitude). QuoteThe benefit is that you use less gas per mile. You may not see it on your V-10, but when we moved from Los Angeles Basin to 6100 feet, my Honda mileage went from 29 to 32 MPG and my Acura went from 29.5 to 33.5.Since I didn't see anyone mention WHY you use less gas per mile at high altitude, I thought I would bore you with the totally useless reason for this interesting phenomena.Bottom line reason is reduced engine pumping losses. If, for example, it takes 10 hp, at the rear wheels, at sea level, to run a midsized car at 50 mph, on level road, it will still take 10 hp, at the rear wheels, at 6100 ft, to run that same midsized car at 50 mph, on level road. (The reduction in air friction due to the reduced density of the air at 6100 ft is negligible compared to that whopping 10% increase in fuel economy.) The hp at the rear wheels is what is left over from what is produced by the pistons after all the power losses in the system are overcome. Those losses are: 1) friction in the rear axle, 2) friction in the transmission, 3) friction in the engine, and 4) engine pumping losses (how hard the engine has to work to suck in the air it needs to produce a given amount of hp.) Friction losses (1-3) are essentially independent of altitude, however, pumping losses (4) atre not. At sea level there is lots of oxygen in the air so the engine doesn't need much air to produce 10 hp (in my example.) This means the throttle plate doesn't have to be open very much to allow the engine to suck in the required amount of air. But since the throttle plate isn't open very much, the engine has to suck real hard to get the air through that itty- bitty opening, and the power it takes for each piston to suck real hard has to be supplied by all the other pistons that are making power at that particular time. Now at 6100 ft the air is much less dense, so there is a lot less oxygen (or anything else) in that air and the throttle plate has to be opened way farther to allow the engine to suck in the much larger amount of air required to produce 10 hp. Since the throttle plate is open a whole bunch (relative to sea level,) it takes a lot less power to suck in the required air through that big opening. Consequently, each piston has to produce noticably less power to deliver 10 hp to the rear wheels at 6100 ft than at sea level. And as mentioned previously, less power required means less heat required, which means less fuel burned.Another way to look at it is, altitude tends to cause the pumping losses of a throttle plate controlled gasoline engine to approach the pumping losses of an unthrottled diesel engine (which are almost zero all the time) and that, for those of you who are still awake, is where that fuel economy increase comes from at altitude. (Additional useless information: Those nasty EGR gasses do the same sort of thing, i.e., they cause the throttle to have to be opened wider which reduces pumping losses, which results in a fuel economy gain of about 1 mpg on cars in the 30 mpg range -- and that's in addition to reducing smog -- how much better can it get?)I'm sure many of you are wondering how you have managed to live meaningful and productive lives all these years without having had a firm grasp of this phemomina. :-)
Re: High Altitude Issues Reply #4 – January 28, 2005, 05:08:53 pm Yahoo Message Number: 50025"Bottom line reason is reduced engine pumping losses."Thanks for the detailed explanation. So if I have it right, as you keep on gaining attitude you will eventually get to a point where you can't move but you will get incredible fuel economy...right?Larry
Re: High Altitude Issues Reply #5 – January 28, 2005, 07:11:04 pm Yahoo Message Number: 50035QuoteI'm sure many of you are wondering how you have managed to live meaningful and productive lives all these years without having had a firm grasp of this phemomina. :-) I wouldn't go quite that far but I very much enjoyed your post. :-)I'm curious. At what altitude would there be so little oxygen that your hypothetical car could no longer run? At one foot less than that altitude, what sort of mileage could I expect compared to sea level?Ed
Re: High Altitude Issues Reply #6 – January 28, 2005, 09:39:16 pm Yahoo Message Number: 50049Quote "Bottom line reason is reduced engine pumping losses."Thanks for the detailed explanation. So if I have it right, as you keep on gaining attitude you will eventually get to a point where you can't move but you will get incredible fuel economy...right?Larry I think there may be another reason too, at least on fuel injected engines and those with altitude compensating carburetors. As you go up, or hotter, the air gets less dense. So with a normally aspirated engine, the fuel/air ratio will be kept more of less constant to ensure proper combustion. Thus, at altitude, you are burning less gas but also making less power.In my Mooney (4 passenger aircraft with fuel injection), as you go up, all the above applies, at least to a point. You have to adjust the mixture manualy, but at 8000 feet, the air is less dense, so provided less resistance to the aircraft. I see 180 to 190 mph while burning about 9 gallons per hour. The aircraft will go a bit faster down low, but burns more gas to do it because of the denser air.all the best,bumper
Re: High Altitude Issues Reply #7 – January 28, 2005, 09:47:00 pm Yahoo Message Number: 50053QuoteI'm curious. At what altitude would there be so little oxygen that your hypothetical car could no longer run? At one foot less than that altitude, what sort of mileage could I expect compared to sea level?Ed The difficult part is building a road that high . . .bumper
Re: High Altitude Issues Reply #8 – February 02, 2005, 04:04:09 pm Yahoo Message Number: 50280Quote I'm curious. At what altitude would there be so little oxygen that your hypothetical car could no longer run? At one foot less than that altitude, what sort of mileage could I expect compared to sea level?Ed Ed, I suggest you direct that question to bumper. Since he called it right when he pointed out the difficulty of laying blacktop at that altitude, let's refrase your question: Bumper -- How high can a piston engine aircraft fly?As far as what the mileage would be compared to sea level, the politically correct answer is: Much better. :-) The politically incorrect answer is: Beats the snot out of me. :-( Thus proving it is more fun to be politically correct, but more useful to be politically incorrect.One other point, before I go back to watching for the groundhog to come out of his stupid hole (been watching for over 9 hours now!) -- I believe bumper mentioned less dense air requires less fuel. Not exactly. That's why I prefaced my original dissertation with the example of the car at sea level and at altitude. The amount of fuel burned is not directly dependent on the density of the air (except for the pumping loss junk) as long as the engine can suck in sufficient air to get the oxygen it needs to make the required power. The required power is what directly determines how much fuel you burn, and given the same vehicle and same speed at sea level and altitude, the required power is the same, so the required fuel is the same. Only the required air is different and that is where all this bologna about pumping losses comes in, i.e., same fuel requires more (less dense) air so throttle plate has to be opened farther which makes it easier for engine to suck,...hey, this is all starting to sound familiar -- didn't I read something about this on this website recently? :-)Hope this helps (with the mileage and road height.)
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