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High Octane Requirements

Some auto engineers have determined that 91 octane,or higher, is necessary. I believe that requirement was set for sea level. We who live in high altitudes can not buy octane of 93 as is available on the east coast. I am told by petroleum engineers that the higher octanes are not necessary nor perform as well in higher altitudes. What is your esteemed opinion on this?

If you’re high up enough that stations sell 85 fuel (like Denver) then you can use 91 in place of 93. You car manual says 93 required?

As a Mechanical Engineer if your car says use 91, then use the highest you can find. Your vehicles knock sensor will retard the timing to save your engine from failure if you use lower octane fuel, but it’s not bulletproof, nor recommended. You will have lower performance with lower octane fuel. If you have a supercharged, turbocharged, or high compression engine, then treat it right and give it the high octane. I doubt it’s a cheap or low-end car/engine, so why pay a lot for a car then cheap out .30 a gallon for fuel.

Your understanding is correct.

High octanes are used to prevent preignition from the heat generated by compression. Matter generates heat when compressed. At high altitudes the air is less dense, so excessive heat is not generated (compression in PSI is less after the compression stroke because there’s less matter in the cylinder) and high octane is not necessary to compensate.

Thanks. I suspected that altitude had a lot to do with octane requirement since back in the days of carburation one had to change or adjust jets for high altitude driving such as in Colorado or Utah.

Yeah, the change in jets was to adjust for the less oxygen per volume of the less dense air. Today’s systems automatically adjust.

True, but this fact ONLY applies to normally asperated engines.

Turbo charged engines routinely bleed off extra boost to not go over a certain PSI range. At altitude the turbo does have to gather more source air to make the same boost, but that doesn’t mean that it can’t hit the same target PSI range.

That is normally not a problem while not at wide open throttle in a modern engine, since the O2 sensors provide mixture feedback, and timing is pulled to compensate. But at or near WOT, the engine runs off a programmed map - one for the specified octane. If you have something less in there, you have a good chance of destroying a piston and/or valve under WOT.

So for turbo engines, make sure to ALWAYS use the same, or at least highest available octane as specified by the manufacturer.

Would it be fair to say, considering that air is heated from intake temps due to compresion (which is why high compression ratios require high octane), that one could probably do better “skimping” on octane in cold weather than in hot weather?

That is what I was told by one Acura dealer. The one problem that I found with using lower octane is that the transmission does not shift smoothly or at proper shift points because the transmission is linked by computer to the timing determiner.
Maybe it is time for me to dump the Acura and get a good Ford.

So many people expend so much energy to get around the octane requirement set by the manufacture.

I agree. Although sometimes (like here), we just can’t get 93 - only 91. :frowning: