85.5 gasolene?

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1,981
Location
Greatest Earth on Show, UT
I ran into 85 octane (no 85.5 though) on a recent road trip through UT, WY, NE, and CO, but always filled up with 87 because I knew I would be going to lower elevations and not wanting to be stuck with too low octane fuel when at a lower altitude.
 
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1,462
Location
East Mountains, NM
I live at 7200' elevation. Regular is 85 or 86 octane, depending, with mid grade and premium being 88 to 91. In a naturally aspirated engine, octane requirements drop a point every couple of 1000'. The higher you go, the less octane required. I only worry about it when I go downhill. If you have a TC of SC motor, you will still need the same octane you needed at sea level.
 
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4,931
Location
Lima, Ohio, USA
run into this same thing when we vacation out in S ID to visit Mom's Family. I can't get dad to understand that 85 octane, and E-85 are not the same thing, and in fact are very different products.
 
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3,061
Location
usa
Only on older cars. Modern engine management can adjust for the thinner air, thus they require 87 regardless of altitude.
 
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1,848
Location
Laramie, WY
Originally Posted By: tommygunn
Only on older cars. Modern engine management can adjust for the thinner air, thus they require 87 regardless of altitude.
really? are you 100% certain? Wife just started pharmacy college in University Of Wyoming, and I told her to do 85 octane because of the elevation, If 87 is a must please confirm for me. Hers: 01 Celica GT
 
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Messages
2,053
Location
Sequim, WA
Originally Posted By: tommygunn
Only on older cars. Modern engine management can adjust for the thinner air, thus they require 87 regardless of altitude.
Unless the engine has forced induction, the lower atmospheric pressure at altitude results in lowered effective compression, reducing the octane rating required to prevent detonation. Normally aspirated cars only adjust the fuel to maintain the proper air/fuel ratio given the air available. Ed
 
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6,429
Location
San Francisco Bay Area
Originally Posted By: edhackett
Originally Posted By: tommygunn
Only on older cars. Modern engine management can adjust for the thinner air, thus they require 87 regardless of altitude.
Unless the engine has forced induction, the lower atmospheric pressure at altitude results in lowered effective compression, reducing the octane rating required to prevent detonation. Normally aspirated cars only adjust the fuel to maintain the proper air/fuel ratio given the air available. Ed
Possibly true to some degree, but modern engine controls can probably advance the timing, which might make up for the deficiency in normally-aspirated air intake pressures. My understanding of high altitudes and older engine technology was that performance was typically degraded, but fuel economy was about the same. For newer engine technology, it can probably advance the timing and then back off when it detects the onset of knock. Frankly, manufacturers no longer say that lower octane is OK for higher altitudes, even though that's what's available. It used to be in the fine print of many owners manuals (I remember it), but they've since backed off. And even so, they still get lower octane regular.
Quote:
http://www.denverpost.com/latin/ci_4503169 Colorado and several other Rocky Mountain states have minimum octane levels of 85 for regular gas, while most states with lower elevations have a minimum level of 87. Research several years ago from the American Petroleum Institute showed that lower air pressure at higher altitudes allows vehicles to perform as well with 85 octane as they would with 87 at lower altitudes. But a 2001 study by the Colorado Legislative Council, the state legislature's research arm, concluded that the altitude difference may apply only to older cars. "Research findings indicate that newer vehicles manufactured in and after 1984 are equipped with sophisticated electronic control systems that minimize this altitude effect and may perform better using higher-octane gasolines," the report said. ** ** ** But it was only last week that ASTM International officials determined that the newer automotive technologies that apparently have rendered 85 octane less useful warranted a discussion about possible changes in Colorado. Many car manufacturers recommend that motorists use at least 87 octane. "They used to put in fine print that 85 octane was acceptable at higher altitudes, but mostly the manufacturers no longer agree with that," said Dick Piper, director of Colorado's Division of Oil and Public Safety.
 
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8,051
Location
Michigan
Turbocharged vehicles can definitely compensate for altitude because the ECU controls the wastegate to achieve a required abolute pressure in the intake manifold. Turbocharged engines I have worked on in the past were able to compensate up to 8000 feet, then the turbo hit the operating limit for shaft speed, and had to allow MAP to decrease above that. Supercharged vehicles may be able to do altitude compensation if they have a bypass that is normally opening at lower altitudes to regulate MAP. I know the old supercharged Cobalt did this, but don't know if current supercharged engines do. The bleeding bypass at WOT on the Cobalt cost 30HP at sea level, so it wasn't such a wonderful thing. Naturally aspirated engines have no way of making up for the loss of atmospheric pressure with altitude, but they can get a little performance back with extra spark advance. To reliably do that, the control system would need to have a barometric pressure sensor to detect loss of ambient pressure.
 
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6,388
Location
Washington St.
01_celica_gt, modern engines do OK on 85, and they do better on 87. If your wife makes repeatable drives where she can compare the gas, it would be interesting to see her actual cost per mile for three tanks of 85 vs. three tanks of 87. Modern engines (since the mid-80's) with their computer controls do better with their designed octane gasoline. The thinner high elevation air has less mass, and the mass airflow sensor measures the mass of the air that enters the engine, so the system automatically corrects for the elevation and provides the correct fuel air ratio and timing the engine can use the best. I don't know why 85 octane has persisted, except maybe it is adequate while being cheaper to produce with higher output per gallon of crude oil.
 
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2,035
Location
Ontario, Canada
Originally Posted By: The USA
85 octane
This is why vehicles bound for North America are so detuned. This is we can't have nice things. 85 octane in a 1st world country - only someone suffering from altitude sickness and low O2 concentration could dream up and implement such a bad idea.
 
Messages
2,035
Location
Ontario, Canada
Originally Posted By: A_Harman
Naturally aspirated engines have no way of making up for the loss of atmospheric pressure with altitude, but they can get a little performance back with extra spark advance. To reliably do that, the control system would need to have a barometric pressure sensor to detect loss of ambient pressure.
Yes, most (non-MAP) cars since the inception of fuel injection have baro sensors. They used to be externally mounted in the 80s, and then went internal to the ECU (Denso, at least) in the 90s. New VVT cars with high static compression ratios can also run the intake cam more centered, if not just outright centered.
 
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