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Old 11-09-2012, 01:47 PM   #1
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91 Octane

Where I live, the highest octane I can find is 91. Do any of you experts know if that is high enough or do I need to add some booster to each tank? The car seems to run fine but I don't know if higher octane gas would make a difference in the performance. Will octane booster hurt anything in the long run? Any thoughts would be appreciated.
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Old 11-09-2012, 01:52 PM   #2
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In CA 91 is the best available & works ok. I don't think pour in octane boosters would be worth the hassle & cost. Try to find 91 octane with the least alcohol content.
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Old 11-09-2012, 02:00 PM   #3
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93 octane fuel tests at (and is rated at) 91 octane at high altitude. I assume you're well over sea level in your part of Utah. You don't need any octane booster.
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Old 11-09-2012, 02:08 PM   #4
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Originally Posted by blue2000s View Post
93 octane fuel tests at (and is rated at) 91 octane at high altitude. I assume you're well over sea level in your part of Utah. You don't need any octane booster.
How does the altitude alter the fuel octane rating?
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Old 11-09-2012, 02:36 PM   #5
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How does the altitude alter the fuel octane rating?
It's a matter of air density. Because there are fewer molecules of air and fuel to rub against each other in a combustion chamber at altitude, the tendency to auto-ignite is lower. Thus the lower octane number because it's correct for the altitude where it is sold, and the lower octane requirement.

Last edited by blue2000s; 11-14-2012 at 11:04 AM.
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Old 11-09-2012, 03:14 PM   #6
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Cool 91octane or 94

To put in an observation here in Toronto we have a 91 octane and a 94 octane available, and every now and then i alternate using either one..
Not much difference observed, i figured as long as your Box is tuned properly, and mechanically sound would be very minimal difference...
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Old 11-09-2012, 06:19 PM   #7
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FWIW, Grassroots Motorsports magazine has a test of different octane and ethanol content fuels in this month's issue. They dynoed for HP and torque. Pretty interesting read.
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Old 11-10-2012, 04:29 AM   #8
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Air and fuel "rubbing against each other" is one way of explaining what's happening...

As pressure increases, the molecules in the combustion charge will vibrate, increasing the temperature of the charge.

The concern about increasing temps and pressure is detonation.Detonation is caused by high peak internal cylinder pressure.

High octane fuel is less volatile and able to burn evenly at higher pressures and temps than lower octane fuel. The octane needs of almost any normally aspirated vehicle will go down as altitude increases. This is because the atmospheric pressure drops with altitude. This equates to lower intake manifold pressure which means less air going to the engine. This is why power drops with altitude.

This is the reason why the pump octane number of gas at altitudes above 5K is 2 points lower across the board.

If you only have 91 available, I have to assume you live ~5K or above.

If the ECU has retarded ignition timing to prevent knock, I believe it is logged and should be accessible with a durametric computer. If durametric shows that the ignition timing has been unaltered from the stock timing map than higher octane is a waste of money under the current conditions.

Last edited by shadrach74; 11-12-2012 at 01:13 PM.
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Old 11-10-2012, 04:37 AM   #9
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Local PCA guys along with FCA guys use Shell 91 octane, zero ethanol exclusively... so that's what I use.

BUT if you have a FI car you must use 94 octane or else it'll ping...
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Old 11-10-2012, 08:46 AM   #10
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In Cali, 91 octane is the best that we can get (at a regular service station). Everyone uses it and it works fine. I wouldn't worry about it.
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Old 11-10-2012, 10:22 AM   #11
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Great info. Thanks for the help.
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Old 11-10-2012, 10:43 AM   #12
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Quote:
Originally Posted by blue2000s View Post
It's a matter of air density. Because there are fewer molecules of air and fuel to rub against each other in a combustion chamber at altitude, the tendency to auto-ignite is lower. Thus the lower octane number because it's correct for the altitude where it is sold, and the lower octane requirement.

The 91 fuel at the pump in Colorado IS the same fuel as the 93 octane at seal level.
If fuel rated at 93 octane at sea level is used at high elevation (2,500 ft & higher) it may increase the octane rating, but it definitely will not lower the octane rating.
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Old 11-10-2012, 11:50 AM   #13
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Retracted statement

Last edited by blue2000s; 11-10-2012 at 03:01 PM.
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Old 11-12-2012, 01:11 PM   #14
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Originally Posted by BYprodriver View Post
If fuel rated at 93 octane at sea level is used at high elevation (2,500 ft & higher) it may increase the octane rating, but it definitely will not lower the octane rating.

It really should not do anything to octane rating. The only reason some engines are fine to run lower octane at higher altitude is due to reduced manifold pressure. A turbo charged engine that needs 93 octane at sea level would do well to have it in Denver as well. You could likely get away with running lower octane in a turbo car at altitude, but not without some performance degradation due to the timing being retarded.
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