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Old 07-25-2013, 01:29 PM   #1
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Octane of'regular' gas.

Being from the east I'm used to seeing regular gas as listed as 87 octane, in our trip out west I frequently see 85 or 86 listed as regular gas. I don't see a loss of performance or knocking using the lower octane fuel so I'm wondering what's up.
Thanks.
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Old 07-25-2013, 01:36 PM   #2
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85 octane is common in Montana for sure, check the rating on the pump before you buy. In areas with the low octane regular , 87 is available as mid-grade, 89 instead of 91 for premium.
Check your manual and buy the recommended fuel.
My 2 older Dodges 93, V-8, &00, V-10 , would not tolerate the 85.
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Old 07-25-2013, 01:36 PM   #3
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Yeah, 85 Octane is common in Colorado because of the altitude, it has to do with the air/fuel ratio, and since the air is thinner, it gets less compression. Lower compression means you need a lower octane fuel.

If you know you're leaving the state, fill up with 87 or higher octane beforehand. If you ONLY have access to 85 octane, then top off the tank with 91 or whatever you can get ahold of as early as possible, to try and get it as close to 87 or higher as possible.

Running 85 octane in lower altitudes can cause predetonation and engine knocking, which can cause damage if run long enough.
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Old 07-25-2013, 01:49 PM   #4
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Insofar as the owners manual on our 2000 coach with the v10 specs 87 octane, I usually opt for a higher octane fuel at stations where regular unleaded is rated at 85 or 86. May be wasting money but...oh well

Best of luck and safe travels
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Old 07-25-2013, 02:38 PM   #5
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With a normally aspirated engine, octane requirements decrease with elevation as air density, mass airflow and peak firing pressures decrease. Be careful, though. Many modern gasoline engines are turbocharged; in that case, the turbo will just spin faster as air density decreases with altitude to maintain mass airflow up to the limitations of the turbocharger; the octane requirement of these turbocharged engines does NOT decrease with altitude up to the operating ceiling of the turbocharger.

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Old 07-25-2013, 07:33 PM   #6
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Thanks for your replies. Interesting connection between altitude and octane, must explain why it's more common out west.
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Old 07-25-2013, 07:45 PM   #7
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I just checked the Workhouse manual, it says 87 octane minimum with no allowance for altitude. I tend to think in terms regular gas not octane but that might be technically wrong.
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Old 07-25-2013, 10:25 PM   #8
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That 85 and 86 octane dates back to when engines had carburetors and actually ran smoother at higher altitudes with lower octane fuel. This was primarily in the Rocky Mountain states and not in the coastal states. I'm not certain why it is even being sold anymore since there are so few carburetors around anymore.

It's good your manual specifically addresses the issue as I certainly wouldn't use it in a motorhome engine regardless.
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Old 07-26-2013, 08:16 AM   #9
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Octane varies by elevation. Because the gasoline is less dense the higher one goes.

87 octane gas would be 85 octane at a high elevation. It's EXACTLY the same gasoline but it tests lower at a high elevation. Thus the pump sign states 85 octane. That same gasoline would be marketed as 87 octane at sea level.

Octane rating isn't as important as it was years ago because your engine's computer will automatically adjust valve timing if it senses an early or late combustion sequence.

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Old 07-26-2013, 08:30 AM   #10
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Quote:
Originally Posted by Tom N View Post
...

87 octane gas would be 85 octane at a high elevation. It's EXACTLY the same gasoline but it tests lower at a high elevation. Thus the pump sign states 85 octane. That same gasoline would be marketed as 87 octane at sea level. ...

-Tom

Tom, could you provide a source for this information. My V10 dose not do well on 85 octane. If I have a tank of 87 and go to higher elevation, 10,000 feet or more, the engine runs just fine, even after sitting for a few weeks. Once I filled with the lower 85 at higher elevation and then came down to 1,000, and the engine did not run smooth until I mixed in some 89 to bring the octane level up to 87. My V10 calls for 87.

Just wondering.

H
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Old 07-26-2013, 08:42 AM   #11
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Tom N. I am a retired oil refinery worker. All fuel octane rating testing was done at the refinery. Where did you get the information that the gasoline sold as 85 octane was the same as 87 octane at lower elevations. I know that gasoline that left our refinery in coastal Texas would have different octane ratings for different parts of the country. That being said I have no problems using 85 octane when that is what is available.
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Old 07-26-2013, 10:04 AM   #12
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Quote:
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Octane rating isn't as important as it was years ago because your engine's computer will automatically adjust valve timing if it senses an early or late combustion sequence.
Actually, the ECM (ECU) will retard ignition timing when it senses detonation (knock). Retarding ignition timing lowers peak firing pressures and reduces the tendency for the fuel/air charge to autoignite, although at a loss of fuel efficiency.

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Old 07-26-2013, 02:38 PM   #13
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So if Octane availability is related to altitude, how come regular gas in CA, AZ and NV is 85 and we are only at 500' ASL?

And our Premium is only 91, back east it's 93
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Old 07-26-2013, 02:59 PM   #14
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So if Octane availability is related to altitude, how come regular gas in CA, AZ and NV is 85 and we are only at 500' ASL?

And our Premium is only 91, back east it's 93
I didn't say octane availability is related to altitude; I said octane requirement is related to altitude. The mountain state refiners obviously don't produce separate gasoline blends for low and high altitudes, so the low altitude folks have to live with/work around the high altitude blends.

Rusty
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