There was a question in another thread about whether being in a higher altitude means you can have lower octane fuel...
I found some interesting info online about the subject, so I thought I would start a new thread to discuss it here.

Here's something I found on a Toyota Tundra forum for the Rockies:

"I grew up in Colorado and the theory is at higher elevation there is less air so less octane is required. With that in mind, I would never run less than 87 given the opportunity. I did fill my Camaro SS in Colorado a few times and it required 91 octane and I didn't have any problems with the 89. It still ran like a monster and got great mileage."

"Another thing to consider is that the computers can handle lower octane than recommended, but they will adjust the timing and reduce the performance. The Acura dealer told my friend that he could run regular instead of premium in his RSX Type S, but that he should pick one and let the computer set itself so the motor would run well. Another example would be in the Camaro racing world. Guys will install mods to allow them to lean out the motors for more power, but if they leave it alone the computer will adjust until it ends up being essentially stock."

Hmmm...I'll post more info below from other online sources too...
L-

 

Here's what 7News said in an interview in July of 2001:

"But not only are prices higher in Colorado -- our regular gas has a lower octane.
In Colorado and a couple of other mountain states, you are used to seeing 85 octane regular gas. Mid-grade is 87, and premium is 91.
But when you cross the border to Kansas or some other neighboring states, the prices are lower and regular grade is 87-octane. Why?
Bennitt says with less air pressure, Colorado cars run well with lower octane.
Bennitt: Throughout Colorado, Utah, Wyoming, Montana -- because of high altitude -- the octane has been 85. It works!
Bennitt: At this altitude, if your car doesn't ping, if you don't get pre-ignition under normal conditions, why not?
Steve Larsen has been working on Jeeps for 20 years. He doesn't recommend 85 octane because most every owners manual on late model cars recommends 87 octane.
7NEWS: Is there a fuel-injected engine that you work on that recommends 85?
Steve Larson, Mechanic, John Elway Jeep: I don't believe there is.
Larsen: You can start getting pre-ignition, or ping, as the public knows it. It causes cylinders actually firing before it is supposed to, causing a knocking sound to come from the engine. Uh, it can cause engine damage.
But mechanics at other dealerships say 85-octane is no problem.
Bennitt: It depends on the quality of the engines. It depends on their own skills. I would listen to my mechanic. I listen to mine."

 

...one other one....

On another forum someone said this:

"The reason for lower octain in CO is due to the altitude. The 85 octane will help cars with carburaters run better with less Oxygen. However, most cars made since the mid 1980's are fuel injected. Lower octane will cause the engine to do what is called "Knocking". This knocking over time will do harm to your engine. The recomended octane for most fuel injected engines is 87. As for 91 octane, this is reserved for high compression engines (sports cars, high performance engines, etc...). Most normal cars will not last any longer or preform any better with a higher octane than 87."

 

Great Info! Thanks for the research.

 

great info.
the only thing I noticed that I disagree with was the rsx type-s. this is really high compression like an s200, or a forced induction car. colorado and other high altittude states have a lower octane than if you were at sea level becuase yes, you don't need the high octane because fuel is more stable here. for example california has a premium level of around 95-97 octane. colorado has a premium level of 91 octane.

so on rsx-s and s200, and wrx, and evo ect. the gas door states premuim fuel only. you should still run premium fuel becuase becuase the higher octane the better on these cars. so if your car states premuim fuel you should use premium fuel. thats what I was taught and its just what I go by.

the last thread explains this a little more detailed. yes the scions becuase they are naturally aspirated and mild compression you can run 85 and be completly safe. sea levels low grade fuel is useally around 87 because thats the min. octane rating for most vehicle.

if you do go turbo, supercharged you should run the highest octane available on the streets.

agian great info, thanks for the tips.

 

91 or 97 octane does not mean "better" gas
91 just means 91% of oct (meaning and 9% hep (meaning 7) hydrocarbon chains
therefore
85 means 85% of octane(8 hydrocarbon chains) and 15% heptane(7 hydrocarbon chains)

octane = 8 hydrocarbon chains
heptane = 7 hydrocarbon chains

8 hydrocarbon chains are more stable under compression and therefore will not explode in the compression stroke of an engine BEFORE the spark plug gives spark

"you can compress octane a lot and nothing happens."-howstuffworks
--on the other hand it is the heptane that causes the pre-igniting combustion
"It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously."-howstuffworks

some misconceptions are V8s need 91 and 4 bangers can run whatever
that is false because the engine will knock depending on engine compression NOT how many cylinders there are.

that is why some V8 engines will not knock running 85 because their engine compression is not HIGH enough to EXPLODE the 15% HEPTANE

researched have shown 91% octane and 9% heptane chains is a good combination for
production sports cars that have high engine compression
ratios
for example BMWs and other performance vehicles

the high altitude lower air pressure has nothing to do with the compressed stroke pressure inside a cylinder. I don't know where they come up with that.

source: http://auto.howstuffworks.com/question90.htm
tells you all about it there

 

I would believe howstuffworks over what people say on forums or 7news interviewees on the street

But newer cars are known to run well on lower octane rating gas even with engine compression ratio of 10.5:1 (in my opinion pretty high, when my 98 camry is only 8.5:1) because of engine management computers program timing so that the 13% heptane would not spontaneously ignite before spark hits.

By the way Acura RSX base model engine compression ratio is 9.8:1 and Type-S is 11:1

So 10.5:1 of Scion xB and xA you be the judge

 

"better" meaning that it is more stable, so it is better on high compression cars. and I don't much about the xa and xb. I drive a tc and its at 9.6:1. so 85 is ok for me.

 

lucky

 

just because I can doesn't mean I do!!!!!

 

"the high altitude lower air pressure has nothing to do with the compressed stroke pressure inside a cylinder. I don't know where they come up with that. "

Here's how:
Compression itself is the compressing of air. If your engine was put in space (in a near vacuum), your piston would have a zero compression rating no matter what because there would be no air to compress. Air has a different density at sea level than it does in the Colorado mountains at 8000'. When your cylinder draws in air at higher altitude, the density is less and therefore the piston has less to compress, effectively lowering the compression. (The atmosphere that your engine sucks in at higher altitudes is not the same density as the air it sucks in at sea level). Because the air compressed at the top of your piston (at TDC with all valves seated) is less dense at altitude then at sea level, the mixture is slightly less explosive. With a high-octane gasoline, the mixture would remain not entirely combusted after ignition occurs and the byproduct would be excessive carbon. This is why, in the old days, at 10,000' in Leadville (an altitude that so few people live at that no oil companies will formulate a low-octane gasoline for) every head you removed had excessive carbon buildup. Today's engines simply readjust the timing (via a knock sensor and a digital controller) themselves to compensate for this effect and more efficiently burn all of the combustible mixture. Nonetheless, using high octane gasoline at Rocky Mountain altitudes in your street vehicles is a waste of money and reduces efficiency. Your owners manual tells you which octane will cause peak efficiency as tested AT SEA LEVEL. In Denver you need to reduce this by a point or two and in the mountains another point or two. (This has been researched and documented by engineers).

For example, my 4Runner will reach peak performance and efficiency using 87 Octane at Sea level which is 85-86 octane in Denver and 84-85 octane in the mountain towns. The gasoline companies already do the formulating for you. You buy Regular at sea level and you buy the same in Colorado/Wyoming or the (relatively) lower states of Utah/Montana (Salt Lake City is a thousand feet lower than Denver and btw Utah has no towns as high as Leadville, Breckenridge, etc.).

The only time Premium will make your engine perform better is if you drive, for example, a Ferrari, a German sports car, or have a turbo/supercharger (or a modified engine, of course). Everything else is just a placebo effect. Too high of octane can give you poor performance and octane boosters are a scam that can sharply decrease engine life. The automobile manufacturers have smarter petroleum engineers than the goop-scammers and they do extensive tests to find out what makes your engine perform best. Then they tell you in your owner's manual.

Fuel additives have changed due to stricter regulations. They're just kerosene or alcohol because that's all they legally can be. Kerosene (jet fuel) that doesn't have lubricating properties will burn up fuel pumps and destroy valves. Alcohol based additives have other problems such as causing dirt your fuel filter has trapped to be released into your injectors or confusing your on-board computer into changing your engine timing and therefore reducing your efficiency.

The gas companies have a bit of a scam going by calling higher octane "Premium" which falsely leads people to believe it is somehow cleaner or more powerful. 85 Octane could just as logically be called "Premium" and 93 octane called "Slow Burning".

Some people will tell you that there vehicle pings going up to Eisenhower tunnel and then claim the ping goes away with 91 octane. Don't believe them: The tunnel is at 11,000' and the math doesn't support the claim. There are either other problems with their engine design or, more likely, almost all engines will experience ping due to the gain of 6000' while driving form Denver through the tunnels. Modern engines will compensate but second-guessing with high octane is a bad idea because when you coast down the other side, you're creating too much carbon. Another thing you should keep in mind is that your average dealer mechanic does not hold a doctorate in petroleum engineering and 7news does not interview the right people.

Any highly knowledgeable petroleum engineer will tell you that the automobile manufacturers and gasoline refineries have already created the most efficient situation without the customer trying to second-guess with octanes and goops.

 

AHHH! A Zombie Tread! It's back from the dead!!!

The funny thing is I was just thinking about where I saw this information. Good call on bringing it back. I needed a refresher on this.

 

"zombies ahead! RUN!"

 

use a turbo in higher altitudes

 

Sorry to bring up old post but this one has info I've been searching all day for.

So Tempinator or anyone else that knows, does temperature affect compression and gas type and elevation. I was wondering if this is different in summer and winter, seem to get mixed opinions, also I live at 7300' with car 10.7:1 ratio, no turbo or supercharger, manual suggests 91, what grade gas should I get, from post above 89 or 88 sounds like the one, thanks
J

 

Yeah the combustion is a function of the compression ratio of the engine and how much air gets into the cylinder so at high altitudes there is less air to compress so less total peak pressure. The ignition of the fuel happens due to the TEMPERATURE of the air / fuel mixture that get HOT when it is compressed. The WARMER the air fuel mixture before it is compressed the higher a temperature it will reach when it IS COMPRESSED therefore with cold intake temperatures (I.E. intercooled turboed or supercharged air) the less preignition. In any case if you don't use heavy throttle you should not have a problem since light throttle means less peak pressure from reduced air fuel mixture volume. You should be ok at lower temps at 7300 feet. Another thing about California is that it does get HOT out there so higher octane is needed for that reason also.

 

bored, reharsh some old info without reading the post:

"in the Rocky Mountain (high altitude) states, 85 AKI is the minimum octane, and 91 AKI is the maximum octane available in fuel. The reason for this is that in higher-altitude areas, a typical naturally-aspirated engine draws in less air mass per cycle due to the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI is available [1]. In parts of the Midwest (primarily Minnesota, Iowa, Illinois and Missouri) ethanol based E-85 fuel with 105 AKI is available [2]. Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI. California fuel stations will offer 87, 89, and 91 AKI octane fuels, and at some stations, 100 AKI or higher octane, sold as racing fuel. Until summer 2001 before the phase-out of methyl tert-butyl ether aka MTBE as an octane enhancer additive, 92 AKI was offered in lieu of 91."

 

i run 105 all day every day got to love my grain fed car keep getting bent over by the man with your gas lol go ethonal

 

I would be interested in what your ECU runs for ignition timing running that high an octane. I see 34-35 degrees BTDC with regular on the highway.

 

im boosted i dont retard factory timing till about 7 pounds of boost love me some alcohol lol