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Sixth topic part two: ICE behavior, elevation & fuel octane

 

Going south to California we really start climbing in elevation after passing through Kansas, Oklahoma & Texas. I believe that in Oklahoma the gas was still 87 octane. However, in Albuquerque the gas was 86 octane. We drove on the 86 octane gas south from Albuquerque up through the mountains and then down to Phoenix. Then we filled up with 87 again in Phoenix. However, none of those tanks were full tanks. Coming north we filled up with 87 octane in Phoenix and were running that up the mountains to Albuquerque.

 

I noticed that the ICE behavior changed running 87 octane at higher elevations instead of 86. With 86 octane the spark advance would drop down to only about 5-8 degrees under a heavy load. Running 87 octane it stayed at 30 degrees under heavy load and 40 degrees under light load. Thus I decided to fill up with 88 octane today in Albuquerque instead of 86 octane. It was only $0.10 per gallon more so it cost about $1 extra to try this experiment.

 

So far this morning I'm seeing similar behavior. The spark advance is about 20-25 degrees at 35+ kW and 30-35 at 25 kW and 40 at 16-20 kW.

 

I've tried to research the interaction between octane and spark advance but I'm not fully understanding it. Why would the higher octane at high elevations lead to more spark advance? Is that a good thing or bad thing?

 

Higher octane is slower burning, so in the higher elevations they use lower octane fuels to compensate for the lower oxygen and pressure. Advancing the spark with lower octane causes detonation, so the knock sensors adjust timing to compensate. Average cars run fine on it in higher elevations, but some run better on higher octane. It was 85 Octane in CO at the Kum and Go. Yeah I LOL when I saw the first one. My F150 took a big hit in MPG on the way through there with the 85 octane, but ran just fine all the way up, in high hear.

 

 

You may be wondering how I'm able to get elevation data while driving. The answer is simple, the car knows. Tap on the gears on the MFT screen and go to "Help" and go to "Where Am I?". The car will then tell you your current latitude & longitude along with the elevation.

Thats where it is, I completely forgot about that, I found it in the F150 when I went through CO to Chino.

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Non Nav MFT has the upper right quadrant as information instead of navigation, maybe they are referring to that screen.

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Interesting read. I have also found the wind direction/speed to be the single biggest factor in MPG variability. I have no scientific evidence to prove this but it seems like a 10 MPH headwind has a bigger impact on MPG than driving 10 MPH faster. Like I said, no way to prove that (only a gut feel) but, if true, it seems odd that it works out that way.

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On days that were in the upper 70s the car would let the HVB get up to 96.8 before turning on the fans to cool it.

Can these fans be heard when they turn on? I don't think I have ever detected/noticed mine turning on and I usually notice a different noise right away.

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