octain at high aititute

[toydog]

I have been told that you can run a lower octain rating(85) versis (87) at this altitude,7300ft, without problems. It hase to do with lower oxygen
content I'm told. what's the true skinny? I have run 85 with no appearent knocking.

[smd3]

It's true. You might pick up a bit of power too, although it won't be noticable. I bet at that altitude the truck feels sluggish.

[chuff]

It's very true. There is less oxygen and the air is less dense, your engine can only suck in a certain volume of air so if it is less dense and has less oxygen you don't need the extra energy from 87 or higher.

In my experience your truck will feel less powerful.

[KDFrosty]

I run 86 octane, and am located just outside Albuquerque, NM at 6100' elevation. No knocking to report.
About the power loss. I have an '03 3.4L AC, so I dont own it for the V8 power anyways. Also, how would you notice the power loss if you didnt know any better?

Anyways, no knocking or pinging..

[jasbus]

You lose approximately 3% of your HP for every 1000' of elevation above sea level.
Guess it stands to reason that you could use a lower octane, with less oxygen, never really thought about it though..

[DJ]

When you simply drive down the road, you use only a portion of the maximum power the engine can put out. Compare what it takes to produce the same amount of power at higher altitude as at lower altitude.

Power is the rate of using energy. For the engine to produce energy at a given rate requires it to burn fuel at a given mass flow rate, which requires sucking in oxygen at a given mass flow rate, which requires sucking in air at a given mass flow rate. The air at higher altitude is the same as the air at lower altitude, i.e. 21% oxygen and 78% nitrogen. So, the only difference at higher altitude is that the throttle butterfly must be more open to suck in air at the same mass flow rate. Once the engine has pulled in the given mass of air and combined it with the given mass of fuel, it compresses the mixture to the same volume and so to the same pressure, all regardless of altitude, and therefore it has the same octane requirements to avoid preignition.

That's what is so nice about a fuel injection system that uses a mass air flow sensor -- it drives the same at all altitudes, needing only a more open throttle butterfly to produce a given amount power as the altitude rises. The only difference in performance at altitude is a difference in the throttle sensitivity. As altitude rises, you get less power at a given throttle setting, but you get the same power simply by pushing the throttle down further, right up until you hit full throttle.

The bottom line is that the only penalty you pay for altitude, up until you need full throttle, is a less sensitive throttle. You have the same fuel requirements at all altitudes.

I lived for a short time at 10,300 feet, then for a long time at 8,700 feet, all in the mountains of New Mexico. I used 85-87 octane fuel then, same as now. My Tundra and Sequoia have never pinged even once that I've heard. They drove beautifully at all altitudes, even when my Tundra pulled a 6,500 pound trailer.

[jasbus]

Actually, if I remember my science right, at 1 mile, or approximately 5000 feet above sea level, the oxgen content is 17%. Even in a fuel injected motor, there is a power loss. FI does compensate, but still there is a loss of raw power. A carbuerated vehicle can be manually tuned for altitude as well, but with the loss of oxygen, only forced induction will add the power you really wand or need...
I was riding bikes with a buddy last month in Durnago Co, he used to live down here in Florida with me. His R1 won't even pull a wheelie up there unless he pops the clutch..

[Ando140]

Air is the same mixture throughout the entire atmosphere. Dry, it is about 78% nitrogen, 21% oxygen and 1% other gases. Maybe that 17% number is referring to your lungs absorbing the same amount of oxygen at 5,000 ft as they would if the atmosphere was 17% oxygen at sea level? You lose approximately 1" of barometric pressure per 1,000 ft in altitude in the first few thousand feet of the troposphere (which is what we live it, it goes from the surface and the top varies anywhere from 25,000 ft at the poles to 65,000 ft at the equater, and it varies by season). So, as we know, the higher you go, the less dense the air is. Warm air is less dense than cold air, and humid air is less dense than dry air.
What happens with engines as altitude increases is, less fuel is able to be vaporized in the thinner air. I believe the computers on our trucks will automatically lean the fuel/air mixture, otherwise (as with older vehicles) you would end up with an overly rich mixture and risk fouling of the spark plugs caused by carbon. What you end up with is less fuel consumption and less power for any given rpm at higher altitude (that doesn't necessarily mean better gas mileage, you may have to run in a lower gear at higher rpms). The exceptions to this are, of course, turbochargers and superchargers which won't be affected until the altitude exceeds the amount of boost provided.
As for the octane thing, I would think you can go lower. Higher octane means the fuel can withstand higher pressure without detonating. Since there would be less pressure in the compressed cylinder for high altitude driving, it makes sense that you could use a lower octane fuel.

[cnold]

toydog,
I had the same question when I noticed that the octane number posted on regular unleaded pumps in Colorado was lower than what Toyota recommends. Looking to the Internet for answers, I found several reports on octane requirement tests. One test concluded that computerized engine management systems that adjust the octane requirement may also reduce the power output on low octane fuel, resulting in increased fuel consumption but no gas mileage test were none. My personal gas mileage tests indicate the opposite to be true but that may be due to the higher ethanol content of premium fuels sold in some areas. All of the tests showed that the octane required went down as the altitude went up. The number fluctuated between a 0.2 and 0.5 reduction per thousand feet gained in elevation. Taking the 0.2 number, your Tundra would do just fine with 86 octane fuel at Denver and would only need 85 octane at 10,000 ft.

[DJ]

Originally Posted by Ando140

Air is the same mixture throughout the entire atmosphere. Dry, it is about 78% nitrogen, 21% oxygen and 1% other gases. Maybe that 17% number is referring to your lungs absorbing the same amount of oxygen at 5,000 ft as they would if the atmosphere was 17% oxygen at sea level? You lose approximately 1" of barometric pressure per 1,000 ft in altitude in the first few thousand feet of the troposphere (which is what we live it, it goes from the surface and the top varies anywhere from 25,000 ft at the poles to 65,000 ft at the equater, and it varies by season). So, as we know, the higher you go, the less dense the air is. Warm air is less dense than cold air, and humid air is less dense than dry air.
What happens with engines as altitude increases is, less fuel is able to be vaporized in the thinner air. I believe the computers on our trucks will automatically lean the fuel/air mixture, otherwise (as with older vehicles) you would end up with an overly rich mixture and risk fouling of the spark plugs caused by carbon. What you end up with is less fuel consumption and less power for any given rpm at higher altitude (that doesn't necessarily mean better gas mileage, you may have to run in a lower gear at higher rpms). The exceptions to this are, of course, turbochargers and superchargers which won't be affected until the altitude exceeds the amount of boost provided.
As for the octane thing, I would think you can go lower. Higher octane means the fuel can withstand higher pressure without detonating. Since there would be less pressure in the compressed cylinder for high altitude driving, it makes sense that you could use a lower octane fuel.

Nope.

Consider what happens when you are rolling along at a very low throttle setting, even at sea level. Do you have any idea how thin the air is inside the intake manifold, where the fuel is mixed with it? Put a manifold vacuum gauge on it and watch it. At full throttle, the two are pretty close to each other, but at lower throttle settings where you normally drive, that low vacuum gauge reading tells you the air in the manifold is at very much lower pressure than the outside air. It's really thin. This ought to tell you that problems of "vaporizing the fuel in the thinner air" is a red herring. The fuel is vaporized quite nicely because it is sprayed though the tiny orifice of the injector at very high pressure. Literally, it comes out of the injector as a "fog" of fuel that is nothing but vapor.

The "mass air flow sensor" measures just what its name implies -- the mass flow rate of air into the manifold. The engine computer injects fuel via the injectors at a corresponding mass flow rate of fuel such that the mixture is the same at all altitudes and all throttle settings. It uses the exhaust gas oxygen sensors to "close the loop" and, in effect, continuously calibrate the injectors.

This is remarkably different from the operation of carbureted engines, and the "rules of thumb" of how carbureted engines behave at altitude don't apply to it. A carburetor uses the difference in air pressures within the carburetor itself to force fuel to flow through the jets. The lower air pressure at altitude causes the carburetor to work with lower pressure differences, and so fuel is pushed at a lower rate than the air mass flow rate would call for, and the lower pressure at the jets causes the fuel to be atomized more poorly, resulting in a slightly leaner mixture that is less well mixed. An engine with a mass air flow sensor, O2 sensors, fuel injectors, and an engine computer doesn't suffer from these problems.

Oddly enough, I once had an ultralight aircraft, which I bought new. It had a very nice two-cylinder, two-cycle engine of the type normally used in snowmobiles. In setting it up, we chose a jet (from a table provided by the manufacturer) for the carburetor based on the local altitude and the season. The mixture was sensitive to air pressure and temperature. But, remember that this was a carbureted engine, not a fuel injected engine with a mass air flow sensor, O2 sensors, fuel injectors, and an engine computer.

Yup.

You have less maximum power available at higher altitude because the maximum mass air flow rate into the engine is less. The fuel that is injected matches that mass air flow rate, whatever it is, at all altitudes. But, until you open it up to full throttle, you can get the same power at higher altitude that you get at lower altitude -- you simply open the throttle more to get it.

[DJ]

Originally Posted by cnold

toydog,
I had the same question when I noticed that the octane number posted on regular unleaded pumps in Colorado was lower than what Toyota recommends. Looking to the Internet for answers, I found several reports on octane requirement tests. One test concluded that computerized engine management systems that adjust the octane requirement may also reduce the power output on low octane fuel, resulting in increased fuel consumption but no gas mileage test were none. My personal gas mileage tests indicate the opposite to be true but that may be due to the higher ethanol content of premium fuels sold in some areas. All of the tests showed that the octane required went down as the altitude went up. The number fluctuated between a 0.2 and 0.5 reduction per thousand feet gained in elevation. Taking the 0.2 number, your Tundra would do just fine with 86 octane fuel at Denver and would only need 85 octane at 10,000 ft.

They are designed to do precisely that. If the engine computer detects detonation (i.e. pinging) via its knock sensor, it retards the ignition timing until the detonation goes away. Retarding the ignition timing reduces the engine efficiency, and so you get less power at a given throttle setting and altitude. Keep in mind that the engine works this way at all altitudes.

[Ando140]

Originally Posted by DJ

Nope.

Consider what happens when you are rolling along at a very low throttle setting, even at sea level. Do you have any idea how thin the air is inside the intake manifold, where the fuel is mixed with it? Put a manifold vacuum gauge on it and watch it. At full throttle, the two are pretty close to each other, but at lower throttle settings where you normally drive, that low vacuum gauge reading tells you the air in the manifold is at very much lower pressure than the outside air. It's really thin. This ought to tell you that problems of "vaporizing the fuel in the thinner air" is a red herring. The fuel is vaporized quite nicely because it is sprayed though the tiny orifice of the injector at very high pressure. Literally, it comes out of the injector as a "fog" of fuel that is nothing but vapor.

The "mass air flow sensor" measures just what its name implies -- the mass flow rate of air into the manifold. The engine computer injects fuel via the injectors at a corresponding mass flow rate of fuel such that the mixture is the same at all altitudes and all throttle settings. It uses the exhaust gas oxygen sensors to "close the loop" and, in effect, continuously calibrate the injectors.

This is remarkably different from the operation of carbureted engines, and the "rules of thumb" of how carbureted engines behave at altitude don't apply to it. A carburetor uses the difference in air pressures within the carburetor itself to force fuel to flow through the jets. The lower air pressure at altitude causes the carburetor to work with lower pressure differences, and so fuel is pushed at a lower rate than the air mass flow rate would call for, and the lower pressure at the jets causes the fuel to be atomized more poorly, resulting in a slightly leaner mixture that is less well mixed. An engine with a mass air flow sensor, O2 sensors, fuel injectors, and an engine computer doesn't suffer from these problems.

Oddly enough, I once had an ultralight aircraft, which I bought new. It had a very nice two-cylinder, two-cycle engine of the type normally used in snowmobiles. In setting it up, we chose a jet (from a table provided by the manufacturer) for the carburetor based on the local altitude and the season. The mixture was sensitive to air pressure and temperature. But, remember that this was a carbureted engine, not a fuel injected engine with a mass air flow sensor, O2 sensors, fuel injectors, and an engine computer.

Yup.

You have less maximum power available at higher altitude because the maximum mass air flow rate into the engine is less. The fuel that is injected matches that mass air flow rate, whatever it is, at all altitudes. But, until you open it up to full throttle, you can get the same power at higher altitude that you get at lower altitude -- you simply open the throttle more to get it.

I think you may have mis-understood me in the first part. I am a commercial pilot and I am very familiar with manifold pressures. As altitude increases the amount of fuel (as vapor) that can be supported in the air decreases because the manifold pressure for any given rpm decreases. That's why the mixture must be leaned, so less fuel is injected into the cylinders for any given rpm vs the same rpm at lower altitudes. Unless these engines work differently than piston aircraft engines... and I don't know why they would (other than things being electronically controlled).

[DJ]

Originally Posted by Ando140

I think you may have mis-understood me in the first part. I am a commercial pilot and I am very familiar with manifold pressures. As altitude increases the amount of fuel (as vapor) that can be supported in the air decreases. That's why the mixture must be leaned, so less fuel is injected into the cylinders for any given rpm vs the same rpm at lower altitudes. Unless these engines work differently than piston aircraft engines... and I don't know why they would (other than things being electronically controlled).

I don't claim to know anything about the carburetion or injection of piston/propeller aircraft engines, other than the 2-cycle engine of my old ultralight, but I have driven lots of fuel injected miles at high altitude in my Tundra and Sequoia.

[Dee Cab]

First of all, octane is a rating of compression. The higher the octane the greater the fuel/air mixture can be compressed before detonation, or "pinging" occurs. Octane is NOT a measure of power. However, a higher compression engine will produce more power and will require a higher octane fuel. But this extra power comes from the engine and not the fuel. Most all cars today will run fine on 87 octane. If pulling heavy loads or excessive hill climbing you may need 89. Corvettes, Vipers, etc usually have engines in the 11 to 1 compression and require 91. MOST piston aircraft engines require 100 octane fuel.

I believe ANDO140 has some training in my field and I have to go with him. An engine, any engine, will loose power as altitude increases. The exception being a super or turbo charged engine which will then lose power like any other once the critical altitude has been reached and the waste gate is fully closed. Any pilot can tell you this. The max performance out of an engine will occur at sea level. As altitude increases the pressure decreases (Standard is 1"/1000ft). The nitrogen and oxyen percentages remain the same but there are less molecules of each. For the optimum mixture, and power, you must reduce the fuel input to compensate for the "air" reduction. This is why all piston aircraft have mixture controls in the cockpit. You reduce your fuel with altitude to gain optimum power or fuel economy. This is done by leaning mixture with the Exhaust Gas Temperature gauge. The exception are newer aircraft with FADEC (full authority digital engine control) which does it automatically.

My Learjet has N1 DEECS (digital electronic engine control). Our max power is 3500 ft/lbs of thrust for each engine at sea level and 72 degrees F. This power decreases with altitude. We are not making 7000 lbs of trust at 49K feet. We have considerably longer runway requirements at Aspen then we do in Houston. (I'm ignoring the wing efficiency issue at altitude at this time.)

Now, we are also able to get the same Mach number, .78, at 20K feet that we can get at 49K feet. But we burn a whole lot less fuel at 49. 900 lbs and hour at 49 versus 3000 lbs an hour at 20. But, we are having to run a higher N1 (engine fan speed, similiar to RPM) to get the same speed at 49 as we do at 20.

So, at altitude, you will require more throttle to see the same speed, you will burn less fuel, and your power for a given RPM will be reduced. Don't worry about mixture, your computer will handle it for you. Unlike my old 79 Harley.

Rant over. Dee Cab

[Ando140]

Originally Posted by DJ

I don't claim to know anything about the carburetion or injection of piston/propeller aircraft engines, other than the 2-cycle engine of my old ultralight, but I have driven lots of fuel injected miles at high altitude in my Tundra and Sequoia.

Well, the only significant difference I can think of between an aircraft engine and the engines in our trucks is the transmission. So, you'll get the same speed for a given rpm in a given gear at any altitude in our trucks(assuming the engine can produce enough power to maintain that speed in that gear), which is not the case with an aircraft since there is no transmission. Manifold pressure for any rpm setting will decrease with atmospheric pressure (about 1"/1,000 ft at lower altitudes), so the air inside a cylinder is thinner for a specific rpm at higher altitudes than it is for the same rpm at lower altitudes, thus using less fuel if it is leaned. I believe modern vehicles use sensors to adjust the mixture as altitude changes, so that's why less fuel is used at a given rpm at higher altitudes.
I'm no engine expert, but its hard to get a pilot's certificate without learning the principles engines work on... If I mis-state anything, or am just completely wrong, I appreciate being corrected (but chances are I'll look it up if I can just to make sure). I'm reasonably sure everything I've said is correct.