Quote:
Originally Posted by TexasChuck
Lower Octanes are used in higher elevations, because of the thinner air. Something to do with the fuel/oxygen ratio. I read this on Shell's site, after being use to 93 octane here is se texas, then being able to get only 91 in the mountains.
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The lower air pressure as you start climbing several 1000' above sea level reduces internal temps and also effective ("dynamic", I think...) compression, so less likely to autoignite the fuel.
Go up high enough, and the lower oxygen levels also mean you need less fuel to hit AFR/Lambda targets.
Of course, you also lose power.
Octane is just one of the many factors that helps control unintended combustion events (reflects resistance to "auto-ignition"), which result in knock (the sound made from an un-timed combustion event).
The force from untimed combustion events can break pistons (ring lands in particular tend to be weak points -- enough force produced on a timed even can do that too, BTW), break loose rods from their bearings (i.e., "spinning a rod"), or if you have a high enough temp produced from a flame kernel on the piston crown, it will look like someone took a welding torch to it and just melted through the center of the crown (google it -- it's kind of horrifying! Much nastier looking than a shattered piston.)
Extra fuel (i.e., "running rich") also helps cool down the piston -- that extra fuel is "wasted" to quench hot spots.
What causes fuel to ignite out of time (i.e., not during a planned spark event)? Too much in-cylinder heat (mitigated somewhat via clever valve timing or freer flowing head and EM as well as lowered intake valve temps -- that's part of why intercoolers or W/I is great on boosted engines), higher cylinder pressure, flame kernels that don't disperse quickly enough for various reasons (e.g., "hotspots" on the piston crown), uhh... I'm sure I'm forgetting something, but that's the gist of it, at least based on my modest understanding of engine tuning...