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Lower octane at higher altitude... #5169033 07/23/19 03:43 PM
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ZZman Online Content OP
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Visiting my daughter in Colorado and just realized they sell 85, 87 and 91 octane. In Michigan they sell 87, 89 and 93 octane. Had to look up why.


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Re: Lower octane at higher altitude... [Re: ZZman] #5169036 07/23/19 03:47 PM
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I think I thought the 85 octane was E85 the first time I drove in Colorado...visited there years ago but was either just tagging along with friends or being driven by salesmen.


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Re: Lower octane at higher altitude... [Re: ZZman] #5169043 07/23/19 03:53 PM
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Yup, I brought this up several years ago on here and from what I remember was told that (essentially) the air being thinner due to the increased altitude allows for a lower octane to be run. I remember even seeing a few stations with 85.5 octane, threw me for a loop.


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Re: Lower octane at higher altitude... [Re: ZZman] #5169044 07/23/19 03:53 PM
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Lower air pressure at altitude so less air to compress in the cylinder...less heat from compression . Therefore lower octane is what is used.


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Re: Lower octane at higher altitude... [Re: ZZman] #5169053 07/23/19 04:09 PM
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Thats all fine except when your owners manual says 87 min.. or you are changing elevation daily.


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Re: Lower octane at higher altitude... [Re: ZZman] #5169058 07/23/19 04:21 PM
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One time I drove from Nebraska to Denver and by the time I got out of state I was having problems with vapor lock. I assumed that it was something to do with gas for low elevations having boiling point problems in high elevation, hot weather.


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Re: Lower octane at higher altitude... [Re: Rand] #5169074 07/23/19 04:40 PM
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Originally Posted by Rand
Thats all fine except when your owners manual says 87 min.. or you are changing elevation daily.


Do manuals ever recommend a minimum lower than 87? I can't say I've ever seen this personally.


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Re: Lower octane at higher altitude... [Re: ZZman] #5169089 07/23/19 05:07 PM
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For a long time it was 85/87/89 here. It was raised when the higher performance engines required 91 no matter what the altitude. Pretty much same goes for 87 here now on many if not most modern cars. My 2000 Blazer runs no different on 85 than on 87. The 2015 Trax it makes a huge difference.


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Re: Lower octane at higher altitude... [Re: ZZman] #5169106 07/23/19 05:32 PM
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Don't turbos make the effect of altitude a moot point?

Re: Lower octane at higher altitude... [Re: Kestas] #5169115 07/23/19 05:41 PM
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Originally Posted by Kestas
Don't turbos make the effect of altitude a moot point?


Based on personal experience, I would say not entirely. I lived in Colorado Springs for many years (6500 feet) and also in the midwest and New England. I drove my '09 Caliber SRT4 back and forth to Colorado quite a bit (lived in IL at the time) and despite being turbocharged there was a noticeable loss of power at higher elevation. Also owned a supercharged vehicle & plenty of N/A cars that I've driven back and forth. Of all, the supercharged car ('00 Pontiac Bonneville w/ 3.8L S/C) suffered the worst at high elevation, the N/A cars are seriously noticeable too, and the turbo only mildly noticeable. Just got back from visiting CO last weekend. I'm always amazed at the difference in power-- I can cruise around at 2-3k RPM effortlessly here in almost any driving condition, but there requires frequent pulls to 4.5k for "normal" driving.

All owners manuals I've ever come across say 87 minimum. I recall seeing one or two that stated an exception for high altitude use, but not all have this. I had always run 85 in my N/A vehicles and never noticed a difference. If I'm traveling out of state to lower elevation, I fill up with 87 before leaving. The switchover (going east) to 87 octane occurs shortly after the Kansas border, around 4000-4500 ft. elevation.


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Re: Lower octane at higher altitude... [Re: Kestas] #5169136 07/23/19 06:20 PM
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Originally Posted by Kestas
Don't turbos make the effect of altitude a moot point?


Many turbos make a max boost (say + 13 psi) but this is 13 psi above ambient (atmospheric pressure at your location). Which might be 14.7 in Houston...but at my house only 10.4 psi (7000ft asl)


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Re: Lower octane at higher altitude... [Re: ammolab] #5169367 07/23/19 11:29 PM
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Originally Posted by ammolab
Originally Posted by Kestas
Don't turbos make the effect of altitude a moot point?


Many turbos make a max boost (say + 13 psi) but this is 13 psi above ambient (atmospheric pressure at your location). Which might be 14.7 in Houston...but at my house only 10.4 psi (7000ft asl)

Sure. As a combination of boost and normal air intake it's not going to be as much total air. Also turbos should spool up to the max boost, but won't it take a bit longer to reach that pressure?

Still - one of the most important applications for a turbo is in aircraft engines, where they're obviously going to be dealing with higher altitudes and thinner air. I remember the General Electric ad touting their testing of a turbocharged aircraft engine on Pikes Peak back around 1918. This article mentions that test:

https://www.airspacemag.com/history-of-flight/hill-climb-2023375/?page=1

Apparently the problem was that it worked too well at low altitude and during testing they failed because of detonation and spark plug failure. But the real environment would be at altitude and Pikes Peak had a road to the top to haul it. Must have been before knock sensors and electronic controls.

Re: Lower octane at higher altitude... [Re: ZZman] #5169426 07/24/19 05:40 AM
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Originally Posted by y_p_w
I remember the General Electric ad touting their testing of a turbocharged aircraft engine on Pikes Peak back around 1918. This article mentions that test:
https://www.airspacemag.com/history-of-flight/hill-climb-2023375/?page=1

Apparently the problem was that it worked too well at low altitude and during testing they failed because of detonation and spark plug failure.

Must have been before knock sensors and electronic controls.


All they had to do was monitor their manifold pressure.... Of course, they figured it out eventually


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Re: Lower octane at higher altitude... [Re: ZZman] #5169498 07/24/19 08:12 AM
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I remember reading of high altitude car & truck engines that had higher compression ratios . And carburetors being adjusted differently for high altitude .


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Re: Lower octane at higher altitude... [Re: ZZman] #5169501 07/24/19 08:13 AM
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Not sure why but I ran the Trax when new on 85 for a solid year. I re-read the manual and decided to try to 87. My wife, who is the primary driver, noticed quickly how much difference it made, and I didn't tell her I did a thing. When I drove it, well the computer must have been pulling a lot of timing, turbo or not, because it changed that engine for the better. 91 makes no difference that I can tell.


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