Altitude and a '97 Sierra

Question: My (135,000 miles) 5.7 (canadian built) that I drive on a regular basis from Los Angeles to San Diego averages about 14 to 15.5 mpg on the
highway (12 to 13 city). I just finished a camping trip in the Sierra Forest (altitudes ranging from 5k to 9k) in which I was driving the truck hard, passing often, steep hills, dodging deer and bear, and got 16.7 mpg!?!?! The obvious question comes to mind...what alttitude did the factory set this truck for????? I know how to change the needles and jets for my '46 Indian when I have it at a high altitude but I don't know how to change my trucks fuel to oxygen ratio. Have I been wasting fuel for over 135k miles? And please, please, don't laugh too hard at me for not knowing! Thank You.
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We have noticed the same thing when driving our old 84 B-350 Dodge van in the mountains..........its a 360 w. Thermo-Quad (the kind you can drive a GrandAm thru the secondaries and not scratch the paint)...........usual mileage is in the 14-15 range, get 17-18 in the mountains.........only reason I can think of is that it takes less gas to go up a mountain and coast down than it would to cover the same distance on the level.

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I thought of that...but the majority of that tank was uphill and hard driving...

truck
jets
over
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Well in aviation, the higher we go, the less fuel our Cessna's burn. In fact, we lean the mixture from cockpit for best performance (or min fuel burn) when we hit cruising altitude. Allegedly, the reason for this is that there is more oxygen near the surface. When you get up to around 5000', there's about ~20% less oxygen then at sea level; therefore, less fuel is needed to deal with the lower amount of Oxygen to reach best performance. There is less power, but also less fuel used. Not sure if this is the reason, or only reason, your seeing better MPG, but it's a hunch.

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wrote:

There is also less atmospheric pressure available to the engine as you gain altitude. In a normally aspirated engine (no boost) you cannot "adjust" for this the way you can with the mixture. This applies to land based vehicles as well as aircraft. The thinner air at altitude also offers less resistance to the aircraft, reducing drag.
DJ CSMEL
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that
wouldn't a MAF (Mass Air Flow) sensor type system do that ?
it meters fuel based on MASS of air passed
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On Wed, 11 Aug 2004 00:59:06 GMT, "TranSurgeon"

The Long Version
Correct, but this only adjusts the F/A ratio for the decreased "mass" of the air, not the ambient air pressure. This should meter less fuel at lower atmospheric pressures and lower air densities which will increase the fuel efficiency of the engine but it cannot compensate for the fact that less air pressure is "pushing" the air into the intake.
Our regular piston engines (no turbo boost or supercharger) depend on the difference in pressure created between the cylinder interior/intake manifold (lower) and the outside air pressure (higher). The piston goes down, creates a void, the air outside rushes in, in an attempt to equalize the pressures. Air will always flow from a high to a low. If we want to talk about weather we can explore why the flow around a high pressure system is always clockwise...
The lower the outside pressure the less the potential difference will be. Conversely, the lower the interior pressure, the higher the potential.
Take this to the extreme, how high will a piston engine run? Ignoring for the moment the lack of oxygen, in "space" at zero air pressure, there would be no difference in pressures at any point of the engine's cycle, no flow induced, no intake, no exhaust. At some altitude between the surface and "space", the normally aspirated piston engine will no longer be efficient enough to "run". It won't create enough power to overcome friction.
It stands to reason that if the above is true, then cramming more air into the intake will increase the power output. If I'm not mistaken, all a turbocharger or supercharger does is increase the ambient air pressure on the intake side of the system. Theoretically, you could accomplish the same thing with a cylinder of compressed air, it just won't last long. <G>
The Short Version
Now that I've used up all those words, simply put, the lower the air pressure available, the less cylinder filling you will get. Varying the fuel/air mixture, timing etc will make it more fuel efficient but will not compensate for this decrease in power.
DJ Gravity, it's not just a good idea, it's the Law.
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I have a "Granattelli" Mass Air Flow Sensor...took the stock one off about 4 years ago...

In
fuel
5000',
is
performance.
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Sooooo...... back to my original question...(sort of) on my 46 Chief with a Mikuni carb (40) I had to put tighter jets and thicker needles to lesson the fuel input in order to get maximum performance at high elevations. Obviously my truck's fuel input has been set for a certain elevation, unless-of course- it has been built to be auto adjusting(the basis for my original question). I did not feel any loss of power at the higher elevations. I assume the "chip" can be callibrated (I don't know a darn thing about "chipping" an engine), so, can anyone answer my assumation? lol Thanks

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Top post!!!!!!!!!! Chipping it may give you more slightly better performance but...I suggest you get a high altitude ambient air sensor....When you goto the mountians swap them out ...I live in deep south texas below sea level...We swap out ambient air sensor every year when the canadians come down ...And then put them back when its time to go back up there. It gives them better gas milage .Also in some cases being the reason for stalling . and nostart conditions...Along with other drivabilty conditions.But if your not having any problems why fix whats not broken? Steve C Re: Altitude and a '97 Sierra Group: alt.trucks.chevy Date: Wed, Aug 11, 2004, 4:40am (CDT+5) From: 97 snipped-for-privacy@earthlink.net (BobTruck) Sooooo...... back to my original question...(sort of) on my 46 Chief with a Mikuni carb (40) I had to put tighter jets and thicker needles to lesson the fuel input in order to get maximum performance at high elevations. Obviously my truck's fuel input has been set for a certain elevation, unless-of course- it has been built to be auto adjusting(the basis for my original question). I did not feel any loss of power at the higher elevations. I assume the "chip" can be callibrated (I don't know a darn thing about "chipping" an engine), so, can anyone answer my assumation? lol Thanks
Question: My (135,000 miles) 5.7 (canadian built) that I drive on a regular basis from Los Angeles to San Diego averages about 14 to 15.5 mpg on the highway (12 to 13 city). I just finished a camping trip in the Sierra Forest (altitudes ranging from 5k to 9k) in which I was driving the truck hard, passing often, steep hills, dodging deer and bear, and got 16.7 mpg!?!?! The obvious question comes to mind...what alttitude did the factory set this truck for????? I know how to change the needles and jets for my '46 Indian when I have it at a high altitude but I don't know how to change my trucks fuel to oxygen ratio. Have I been wasting fuel for over 135k miles? And please, please, don't laugh too hard at me for not knowing! Thank You.
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OK...here we go(now we are getting somewhere)! My truck is one of the Canadian built GMCs. Can you tell me more about the "canadian" chips that you have swapped out? "It gives them better gas milage" This leads me to believe that my chip is set for higher elevations and not auto adjusting, otherwise why would you have to switch them out.
Question: My (135,000 miles) 5.7 (canadian built) that I drive on a regular basis from Los Angeles to San Diego averages about 14 to 15.5 mpg on the highway (12 to 13 city). I just finished a camping trip in the Sierra Forest (altitudes ranging from 5k to 9k) in which I was driving the truck hard, passing often, steep hills, dodging deer and bear, and got 16.7 mpg!?!?! The obvious question comes to mind...what alttitude did the factory set this truck for????? I know how to change the needles and jets for my '46 Indian when I have it at a high altitude but I don't know how to change my trucks fuel to oxygen ratio. Have I been wasting fuel for over 135k miles? And please, please, don't laugh too hard at me for not knowing! Thank You.
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Oopps...I was reading too fast! Are you talking about a barometer? lol (honestly now)I do not know what a "h.a.a.a.s." looks like or where it hooks up. Or were you refering to swapping out chips?

Thanks
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Top Post.... Ok Hyper tech has a programmer which you can change and adjust you air fuel ratio...Its something ike hyper tech power programmer II.. Ok now we change out the ambent air sensr on most of them because our air is too thick down here which is idling it down too much to compinsate for AC usage...When they have thier sensors that are calibrated for high alt...When they get down here it will miss ,surge,and even die when the Ac is on...Whic gives them better gas and smooths out their drivability conserns...With the programmer you can adjust a-f ratio up and down...Only fall back is you can only use it on one truck at a time meaning if you put your vin into it you cant use it on another truck untill you put the program back into it and your vin is wiped clean...Also try cleaning the carbon out of your thrittlebody and intake to assure smooth air flow.. hope this was of some help.. Steve C
Re: Altitude and a '97 Sierra Group: alt.trucks.chevy Date: Wed, Aug 11, 2004, 4:56pm (CDT+5) From: 97 snipped-for-privacy@earthlink.net (BobTruck) OK...here we go(now we are getting somewhere)! My truck is one of the Canadian built GMCs. Can you tell me more about the "canadian" chips that you have swapped out? "It gives them better gas milage" This leads me to believe that my chip is set for higher elevations and not auto adjusting, otherwise why would you have to switch them out.
Question: My (135,000 miles) 5.7 (canadian built) that I drive on a regular basis from Los Angeles to San Diego averages about 14 to 15.5 mpg on the highway (12 to 13 city). I just finished a camping trip in the Sierra Forest (altitudes ranging from 5k to 9k) in which I was driving the truck hard, passing often, steep hills, dodging deer and bear, and got 16.7 mpg!?!?! The obvious question comes to mind...what alttitude did the factory set this truck for????? I know how to change the needles and jets for my '46 Indian when I have it at a high altitude but I don't know how to change my trucks fuel to oxygen ratio. Have I been wasting fuel for over 135k miles? And please, please, don't laugh too hard at me for not knowing! Thank You.
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Thanks Steve...I'll check into it! So you just reprogram the 'puter...it's not a different "chip"? Don't know much about these computer thingamajigs. Is there anyone else out there who has done this???
Question: My (135,000 miles) 5.7 (canadian built) that I drive on a regular basis from Los Angeles to San Diego averages about 14 to 15.5 mpg on the highway (12 to 13 city). I just finished a camping trip in the Sierra Forest (altitudes ranging from 5k to 9k) in which I was driving the truck hard, passing often, steep hills, dodging deer and bear, and got 16.7 mpg!?!?! The obvious question comes to mind...what alttitude did the factory set this truck for????? I know how to change the needles and jets for my '46 Indian when I have it at a high altitude but I don't know how to change my trucks fuel to oxygen ratio. Have I been wasting fuel for over 135k miles? And please, please, don't laugh too hard at me for not knowing! Thank You.
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Maybe its just the cooler temps in the mountains....Also, put an O2 sensor on it if its been a while....You could also visit your local GM dealer and see if there is an update for the computer. (programming)
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Speaking strictly on temperature differences
Cooler temps = denser air = more power = more fuel consumed.
It may be simply he was driving slower overall. Once you get a vehicle moving, drag is the biggest impediment, it squares with increase in speed. Double the speed and you get 4x the drag.
DJ
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Well.... I was putting my truck through it's paces...ie...passing everything in sight...accelerating uphill...barely any coasting downhill. In otherwords...I WAS NOT DRIVING SLOW! If I was to drive the same way here in town (Lost Angeles) I would probably get 8 to 10 miles per gallon. So this 16.7-16.8 mpg was radically different, otherwise I would not be posting this question. I am very aware of altitude to oxygen to fuel ratios as I know carburation/needles/jets. This question is really about the factory setting of Canadian built GMC altitude air/fuel settings. Sorry if I sound a little harsh...just wish someone knew...I would prefer to not talk to the dealer as I expect assinine answers (might be my personal problem...lol).
wrote:

over
sensor
and
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On Wed, 11 Aug 2004 19:56:17 GMT, "Bob Truck"

Not harsh at all, just that this situation is kinda opposite from conventional thinking. I feel your pain, I am a dirtbiker and, like you, I am used to being able to fiddle with three air/fuel circuits for three different throttle ranges with a handful of brass.
Maybe your truck just likes the mountains. <G>
DJ
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Not much help here, but....
In the summer of 2001 I borrowed a buddies motor home to take my father for his last big trip. We traveled from central Oklahoma to the Olympic Penninsula and back. Covered many high passes. The rig is a V10 Ford (FI) in a 32 foot class C Jayco. Not exactly lightweight. I noticed a marked increase in mileage, though probably not more than 3 mpg (from about 11 up to almost 14), when traversing the Rockies and such areas over what it returns here in the great plains states. Even going up and down major hills it improved noticeably. On the flat it was not quite so good, but still better than when at lower altitudes.
I wondered why too????
As for why.... I dunno! It just happened. But then it is a brand X so there is no telling.
George
Bob Truck wrote:

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