Why is gasoline octane lower in Colorado?
 

This has baffled me for some time and I never remember to ask here. Their regular is 85 octane an mid is 87, everywhere else 87 is minimum.

I would think they'd need higher octane there with the mountains, what gives?

At high altitude you have less air going into the cylinders- it is like lowering the compression rate. Less pressure at ignition. The lower the compression rate the less octane you need to keep the 'knock' away...

There is less air inhaled with every gasp. Engine can be human or mechanical, doesn't matter. So less power made equals less actual compression needing less octane.

Also, engines last longer here if not started outside in negative temps/driven up mountain passes heavily loaded etc, as they are stressed less all things being equal.

Fox beat me by a minute. More concise as well.

Ah, she isn't just a PARF know it all - she actually does know something about cars... ;)

Which is why you should strafe the tech section more (insert appropriate icon here).

...or living in Colorado.

I used to - however, when someone claimed that their string and masking tape method of aligning a car was going to give them more accurate results than a laser aligner - I sort of gave up. (I know alignment really, really good ;) )

Don't let one bad apple ruin the bunch. Alignments concern many variables that most, even trained alignment techs, will miss.

85 here in Montana as well, although Costco regular is 87.

From what I understand, the lower the octane, the more horsepower. Win!

nope

They used to water down the beer as well ? :(

Coors light IS water...

Fuel with lower octane rating does have a greater heating value than higher octane according to the Bosch book, but "gasoline" varies a whole lot, especially from continent to continent- like Europe to US to someplace like Africa.

However, if you use lower octane in a car that needs high octane it will change ignition timing and maybe even valve timing if it is new enough and that will decrease efficiency and counter the extra energy you get so that your milage doesn't get any better. the higher compression ratio allowed by higher octane will make up for the lesser energy density (unless you use something like Methanol which is about 1/2 the energy density of average gasoline).

Not entirely correct. Higher octane stuff is more resistant to uncontrolled combustion events, ie detonation. More fuel economy might be had using lower octane but each engine might be different in what it "likes."

I believe that is an insult to water.... :)

Poppycock!

Conventional Gasoline - about 87 octane

116090 BTU/GAL

"White Gas" aka Naptha, about 70 Octane

116920 BTU/GAL

MTBE

93540 BTU/GAL

ETBE

96720 BTU/GAL

Ethanol

76330 BTU/GAL

Hydrogen Data Resource Center: Hydrogen Calculator

petroleum refining : Octane rating -- Encyclopedia Britannica

Gasoline,
Regular: 0.720-0.775 kg/liter 41.2-41.9 MJ/kg
Premium: 0.720-0.775 kg/liter 40.1-41.6 MJ/kg

Bosch Automotive Handbook, 7th ed. page 331

@red-beard

Yes, but you are misconstruing things a bit. More ethanol content or octane percent allows one to run higher compression all things being equal.

Ethanol is an octane enhancer, but an octane rating is an octane rating. If you have 87 octane pure gasoline you will have greater energy content and therefore better milage than 87 octane E10 but the engine will still knock at the same compression ratio and therefore will have similar thermal efficiency.