# Octane vs. altitude vs. mpg



## Robinlawson (Jul 30, 2004)

I just discovered something about my 2000 2.8L Z3 that I think could benefit everyone. I live in Salt Lake City Utah, altitude of around 4,500 feet above sea level. Ordinarily I put in 91 octane gas as the owners manual suggests, and reliably get around 25 mpg. Because of high gas prices, and our high altitude, I decided to experiment with a slightly lower octane. I put in 89 octane, and got 26 mpg under the same driving conditions! So, for those of you at high altitude, you might want to consider a lower octane. It will save you some money, and you may get better fuel economy to boot!

Now I know there are some people out there who will scold me for this. I understand that my car will be running a little hotter because of this change, and this may affect my engine life. But given all the variables that go into determining the life of an engine, I don't think a lower octane will be the deciding factor. And 89 octane is still above the minimum recommended in the owners manual. But feel free to make the argument either way, that's what the board is here for.


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## Pinecone (Apr 3, 2002)

a) Your car will not run hotter due to the octane change. But the DME system may be retarding timing to prevent detonation and this would reduce the power output. And you run teh risk of detonation if the DME doesn't react quickly enough to changes in operation.

b) At high altitudes you do not need as much octane due to the lower cylinder pressure. And engine is an air pump, it sucks in air, then compresses it. The final pressure after compression is what affects detonation. Since the air is less dense at altitude, the final cylinder pressure is lower, requiring lower octane. This is why the grade of gasoline are lower octane at higher altitude.

The potention problem with running lower octane at high altitude is if you drive down to lower altitude with the same fuel.

Most likely the change in MPG was due to a change in how you drove for that tank. As lower octane could produce reduced performance operating conditions to prevent detonation, which would tend to reduce gas mileage.


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## Robinlawson (Jul 30, 2004)

okay, now I'm confused. Higher octane fuels have a higher ignition temperature, so I understand it might be igniting at a lower temp with the lower octane. Hence lowered temp of the engine, but more risk of detonation. However, too high of an octane may not ignite fully because the compression plus spark may not be enough to reach the higher ignition temp (PV=nRT, temp rises as you compress a gas). This is why you want a lower octane at higher altitude (I believe Pinecone stated this, but in a different way) and would also cause a lower temp engine because of unburned air/fuel mixture. There is an optimum octane for any altitude and temperature day. Yup, I put in higher octane if I'm driving to a lower altitude. But 91 may be too high for my current altitude, and my increase in MPG would then be real. 

So why won't my engine run hotter? This assertion is related to the thin air at altitude, which has less cooling capacity. Mainly, I concluded the engine would run hotter because of the improved gas mileage. Higher efficiency generally follows peak engine temperatures. That's how I find the most fuel efficient mixture on an airplane for any altitude, by finding the peak in exhaust gas temperature. I know, it's barbaric to have the human make that decision, but airplanes are old technology in general.

Also, I don't believe the DME is retarding the timing. If it were, I would see a drop in fuel efficiency, not an increase. I've put in lower octane than 89 by mistake, and the change in mpg is way more than the 1 or 2 mpg from my driving habits. I've never seen efficiency from city driving above 25.5, so the 26.5 made me notice. Yes, the increase could be due to my driving, so I argue that the fuel economy is at least the same with the lower octane. Either way, I'm saving money, and others could be too. 

So do I have it all correct now?  Always looking to better my understanding of cars, so feel free to point out flaws in my argument.


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