Here in Colorado we have 85 Octane as regular instead of 87. Owners manual says 87 Octane. Just filled up with 85 to see what happens. I have not noticed any pinging so far. Would anyone agree at this altitude, I don't think it will matter. Never has on any other vehicle I've had. Truck has 5500 miles on it. Anyone else in the area doing the same?

 

Well... the octane requirements for an engine do decrease with altitude, and there must be millions of people that successfully run 85 octane in the mountains because there are pumps all over the place. Having said that, my owner's manual (2002) recommends against 85 octane at altitude... naturally, without saying why.

 

Traveling through Colorado this past summer, driving our 99 Expedition EB 5.4 I decided to try the 85 octane. I never heard a ping, truck continued to run great, and even got better miliage than I normally do around home in SO. California. I am guessing California fuel is so full of **&^%%$#@@! that it just takes more to run things.

 

Hey Scott,

I've been running 85 octane in my 2001 with the 5.4 since I got the truck new last Feb with no problem. The dealer recommended that I not use any higher octane because the truck will get used to it and wont like anything else and would see no noticeable preformance for the additional cost.

DB

 

Dont want to run high octane at high altitudes...will damage engine...have to cut back on the octane..

Thinner air has to do with it...leaner condition...less octane

There are sensors to correct for the density of the thinner air...

Really not supposed to run premium fuel...

 

Simply and absolutely not true.

 

Maybe you are not getting any engine ping because the "knock sensor" is working overtime

 

Is it the gas or the altitude that changes the octane number?
Here in Arizona I have noticed that Chevron sell 92 octane in Tucson. But, get up in the White Mountains area and Chevron sells 91 octane.

 

Engine designers assign an octane number rating (ONR) to the engines they design and it is the octane number at which at engine will run without knocking. Many factors affect the ONR, including compression ratio, ignition timing, fuel/air ratios, cylinder head temperatures, and combustion chamber design. Compression ratio has the largest impact on both ONR and engine efficiency… higher compression results in a more efficient engine and a higher ONR. Retarding the engine timing, running either rich or lean, reducing cylinder head temperatures, and using swirl type combustion chambers all reduce ONR.

Other factors also affect ONR. Lower barometric pressure reduces ONR… this is why octane numbers for corresponding grades in the mountains are lower than at sea level. Increased humidity and lower temperatures decrease ONR. Deposits in the combustion chamber increase temperatures and pressures, and therefore, increase ONR.

Thus, at higher altitudes, your engine will have a lower octane number rating, and the gas companies respond by selling you lower octance gas, which of course, is cheaper to make.

 

I've been using 85 since I bought mine new, 30,000 miles ago. I haven't had any problems and average 14.5 - 16 mpg. I have only used 87 when traveling out of state and didn't notice any difference. 85 is expensive enough the way it is...

 

I drive in and around the Denver metro all day long and use 85 octane 24/7. Ocassionally I will through a tank of better (... read that as "when the price's go down") but have no problems w/ the 85.