Octane Ratings and Elevation

[javacontour]

I was reading the thread where someone was trying premium fuel to see if there was any/enough fuel economy increase to justify such fuel.

There were many quotes from owners manuals about fuel grades.

Which reminded me of thoughts I've had before, but never really did the research, so here goes.

In places like Denver, CO, the "regular" grade gas isn't 87, but 85 octane (R+M)/2.

I don't recall ever seeing an owners manual give adjustments for octane at higher elevations. Yet I suspect most folks fill with 85 in locations where that's the regular grade.

I understand the theory behind it. I may not have the vocabulary correct, but effectively, since the air is "thinner" there is less likely hood for pre-ignition at higher elevations, so a lower octane fuel is sufficient.

But is it really OK with respect to the car makers? Since their recommendations are for 87 or 91 octane, depending on the car, what is the position on using such fuel in higher elevation environments?

I've driven out there before and usually try to fill up with the higher grades when I leave so as I drop to lower elevation, I'm not sitting on a tank of fuel that is 85 octane.

But I doubt most folks really think about it, and their cars manage to survive.

Any thoughts?

 

[fsskier]

You, are correct, your octane requirements drop dramatically at high altitude! Newer cars mask this effect, knock sensors automatically set the timing back and make it difficult to determine how much octane is enough.

Altitude - pressure, 14.7 psi at sea level, 12.2 at Denver (5,000 feet) 10.1 psi at 10,000 feet (many Colorado passes are 11,000 feet) 8.3 psi at 15,000 feet (Colorados highest roads are just over 14,000)

I have gone to the mountains annually for about 40 years for snowskiing, vacations, etc - always experienced this with the older cars - no knock sensors.

Typical experience, 78 Zephyr Wagon, 2300 CC 4 cyl, stick shift, heavily loaded, and in later life a .040 overbore! Great ski vehicle, lots of room, very good on gas (interpret, more money for ski trips) but VERY pingy after the rebore on octane 87 fuel. Usually needed 89 on bluesky high barometer days in Iowa to avoid pinging.

From Denver on up into the mountains (5,000 feet through 12,000 feet) octane 85 could be run at full throttle with no pinging ever!! Once, when skiing in Aspen I spotted some octane 83 fuel (in the town of Basalt) and I filled with that and could never get a ping ever, the mountain valleys there are 7,000 plus feet.

One year, skiing Keystone (9000 foot base elevation) I started home with a full tank of 85, dropped down through Denver and on into Nebraska, the pinging became severe and I could not even maintain cruising speed without pinging. Of course the 4Cyl engine sipped so little fuel that the tank was still more than half full, even a fill with Octane 92 did not fully cure the problem - but was good enough! Yes, I would recommend refilling with higher octane before returning to the flats!!

 

[Drew99GT]

Pretty much everyone runs 85 octane here for the regular cars, and they're not being killed from it. I usually step up to 87 octane in summer. I notice in summer many cars pinging HORRIBLY, especially cars requiring premium. One thing you have to remember as well is since the air is thinner at high altitude, you have less power and less cooling ability, so that combination, especially on steep grades, can possibly lead to more pinging with 85 octane from increased engine temperatures. On a hot day here in Colorado Springs, we can get density altitudes of 15,000 feet! Meaning the air pressure/temp would be like operating at 15,000 feet. On steep grades at high ambient temps, I hear some cars pinging so bad it sounds like they're going to explode.

 

[Benzadmiral]

Yes, I used 85 octane in my '94 Olds, which required 87. The corollary to this, I was advised by Mercedes Club people in Denver, is that our premium-requiring cars could be safely run on the mid-grade, which there is 87 or 88. I put that in my '86 420SEL and had no pinging or other problems for two years, even driving in the mountains to Leadville (10,000 feet).

When I left CO to move to The Swamp (Dear Lord, what was I thinking???), I made sure to fill with 91, the highest octane available at 5000 feet, so that after I came down the Raton Pass in New Mexico and headed out across the flats to Amarillo, the car would run fine. It did.