I moved to Colorado from Kentucky a few years ago and have been wondering why the gasoline grades out here are 85, 87, and 91. I thought I read somewhere that I could actually get away with using 85 octane instead of 87....
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Lower octane at higher altitudes?
- Thread starter zeke1985
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In theory you can get away with it because thinner air leads to lower cylinder pressures. Probably less of an issue with NA than FI. Besides, with O2 and knock sensors it wont make a difference anyway..
imo its a scam against us coloradoans. other places have high elevation and dont have to pay more for 87 octane. my owners manuals say nothing about using 85 octane being acceptable
It depends on how old your car is. Modern engines can compensate for the thinner air, and in fact most newer cars require at least 87 octane regardless of altitude.
85 octane was usable back when carburetors were still a thing.
85 octane was usable back when carburetors were still a thing.
Try it. Some cars will and some cars won't like it... Only way to know is to try
It is a rip off really. If it costs less to produce the 85 octane, that cost savings should be passed along to the consumer. I don't believe that's the case though.
85 octane works fine in most normally aspirated engines, fuel injection or carb, at higher altitudes. Ran it for years when I lived there, and still run it when I visit family now. If your car doesn't like it, then step up.
If turbocharged, stick to manufacturer recommendations. My ecoboost gets premium out there since the turbos can compensate for the thin air some...
If turbocharged, stick to manufacturer recommendations. My ecoboost gets premium out there since the turbos can compensate for the thin air some...
I noticed same thing moving to mountain states. At first I was concerned about 85, but it works fine. Late model vehicles are designed to run on varying quality gas.
i get slightly better MPG on 87 octane. My guess is the computer enriches the mixture or retards timing to protect the motor with 85. The loss in MPG is very small, compared to the extra cost of 87, so stick with 85.
(Even though it doesn't bother me; I suspect the high altitude thing is [censored]. It's just an excuse to sell cheap 85 as regular and 87 as mid grade.)
i get slightly better MPG on 87 octane. My guess is the computer enriches the mixture or retards timing to protect the motor with 85. The loss in MPG is very small, compared to the extra cost of 87, so stick with 85.
(Even though it doesn't bother me; I suspect the high altitude thing is [censored]. It's just an excuse to sell cheap 85 as regular and 87 as mid grade.)
The less dense air has the same effect as lowering the engines compression ratio. Cylinder pressures at TDC are lower. This lowers the octane requirement of any NA engine. No need for the car to "compensate" from any 87 octane tune status.
New Mexico and Colorado have been running 85 octane Regular and 90 Octane premium for decades. No problem other than lower HP (due to thin air) at the same pump price or, higher, than lowlander Texans!
New Mexico and Colorado have been running 85 octane Regular and 90 Octane premium for decades. No problem other than lower HP (due to thin air) at the same pump price or, higher, than lowlander Texans!
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Gasoline composition is highly regulated by Federal and state environmental regulations. Octane requirements are reduced by two points for areas in the Rocky Mountain area above about 4000 feet elevation. Meeting all these requirements for various areas at different times of the year with the various pipe line distribution systems must be difficult. See the following summary from ASTM Specifcation D4814. / 1.2 This specification describes various characteristics of automotive fuels for use over a wide range of operating conditions. It provides for a variation of the volatility and water tolerance of automotive fuel in accordance with seasonal climatic changes at the locality where the fuel is used. For the period May 1 through Sept. 15, the maximum vapor pressure limits issued by the United States (U.S.) Environmental Protection Agency (EPA) are specified for each geographical area except Alaska and Hawaii. Variation of the antiknock index with seasonal climatic changes and altitude is discussed in Appendix X1. This specification neither necessarily includes all types of fuels that are satisfactory for automotive vehicles, nor necessarily excludes fuels that can perform unsatisfactorily under certain operating conditions or in certain equipment. The significance of each of the properties of this specification is shown in Appendix X1.
Originally Posted By: MNgopher
85 octane works fine in most normally aspirated engines, fuel injection or carb, at higher altitudes. Ran it for years when I lived there, and still run it when I visit family now. If your car doesn't like it, then step up.
If turbocharged, stick to manufacturer recommendations. My ecoboost gets premium out there since the turbos can compensate for the thin air some...
Boost is supposed to be relative to atmospheric pressure, so theoretically it might still have a lower demand at high altitudes. Still, it sounds like 91 is still pretty much available. Seems a bit odd though.
I'm thinking that it's just a way for the refiners and pipeline companies to use up the low octane rating fuel streams without having to sell it specifically as something else. There's only so much demand for camping fuel, and there are limited means of distributing small engine fuel.
Now I suppose one of the first important applications for turbos was in aircraft engines, since they had issues with high altitude performance. General Electric had a series of ads where they showed historical uses of their technology. Apparently they tested one of their first turbocharged engines by hauling it up to Pikes Peak. With the turbo it was able to produce about what it could at sea level. Back then they typically called it a "turbo supercharger".
85 octane works fine in most normally aspirated engines, fuel injection or carb, at higher altitudes. Ran it for years when I lived there, and still run it when I visit family now. If your car doesn't like it, then step up.
If turbocharged, stick to manufacturer recommendations. My ecoboost gets premium out there since the turbos can compensate for the thin air some...
Boost is supposed to be relative to atmospheric pressure, so theoretically it might still have a lower demand at high altitudes. Still, it sounds like 91 is still pretty much available. Seems a bit odd though.
I'm thinking that it's just a way for the refiners and pipeline companies to use up the low octane rating fuel streams without having to sell it specifically as something else. There's only so much demand for camping fuel, and there are limited means of distributing small engine fuel.
Now I suppose one of the first important applications for turbos was in aircraft engines, since they had issues with high altitude performance. General Electric had a series of ads where they showed historical uses of their technology. Apparently they tested one of their first turbocharged engines by hauling it up to Pikes Peak. With the turbo it was able to produce about what it could at sea level. Back then they typically called it a "turbo supercharger".
This is nothing new and original. I lived on the front range of Colorado for nearly 12 years starting in '77. The 85 octane thing was identical back then as it is now. Never was a issue for carbureted, non computer controlled engines back then, so not sure it means much either for the newer computer controlled engines. My '73 El Camino SS 454 had not problem with the stuff, nor my '76 Mustang Cobra II 302, nor my '79 Ford Bronco with a 351M. Same for the wife's '86 Camaro. The only issue I could even see might with the newer turbo, force fed engines, but probably the ECM would compensate. While 91 is available there, 93 is available readily in lower elevations like Iowa where I live now.
It relates back to old carbureted engines before electronic controls. The thin air with its low oxygen content made the 85 AKI gasoline work fine, subject of course to the 3% power loss per 1000' of elevation due to the lack of oxygen in that thin air. Modern engines will do best with the specified AKI (87 or whatever) at all elevations.
Here's more:
"The posted AKIs are lower in the Rocky Mountain states. Altitude gasolines have historically
provided the same antiknock performance as higher-AKI gasolines at sea level. The
octane requirement of older-model engines decreases as air pressure (barometric pressure)
decreases. Barometric pressure is lower at higher elevations.
"Since 1984, vehicles have been equipped with more sophisticated control systems,
including sensors to measure and engine management computers to adjust for changes
in air temperature and barometric pressure (see page 69). These vehicles are designed to
have the same AKI requirement at all elevations, and the owner’s manuals specify the
same AKI gasoline at all elevations."
www.chevronwithtechron.ca/products/documents/69083_MotorGas_Tech_Review.pdf
Here's more:
"The posted AKIs are lower in the Rocky Mountain states. Altitude gasolines have historically
provided the same antiknock performance as higher-AKI gasolines at sea level. The
octane requirement of older-model engines decreases as air pressure (barometric pressure)
decreases. Barometric pressure is lower at higher elevations.
"Since 1984, vehicles have been equipped with more sophisticated control systems,
including sensors to measure and engine management computers to adjust for changes
in air temperature and barometric pressure (see page 69). These vehicles are designed to
have the same AKI requirement at all elevations, and the owner’s manuals specify the
same AKI gasoline at all elevations."
www.chevronwithtechron.ca/products/documents/69083_MotorGas_Tech_Review.pdf
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