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 **** Piper, director of Colorado's Division of Oil and Public Safety.