K04/RS6/What Octane is best in the city Q&A

[R-Audi]

Re: K04/RS6/What Octane is best in the city Q&A

They even sell 85 Oct fuel in the US. now thats really asking for problems

But isnt that only in States like Colorado with a higher elevation??


Here is one quote I found:

"Ambient air pressure is very important in determining the octane needed to prevent knocking. Normally aspirated gasoline engines (those without a turbo-charger) experience a drop of about one octane for each 1,000 feet increase in elevation above sea level. (Tests indicate that this elevation effect varies between 0.8 and 1.2 octane and averages about 1.0 octane). Engine makers normally design their engines to operate at or near sea level, and some (including Nissan) acknowledge that at higher elevations lower octane gasoline is acceptable. Almost all regular gasoline in the Rocky Mountain West (generally sold at 4,000 feet elevation or above) is produced at 85 octane (rather than 87 octane in the remainder of the USA)"

From:http://www.wisegeek.com/what-is-high-octane-fuel.htm

[R-Audi]

Re: K04/RS6/What Octane is best in the city Q&A

Alsofrom the same link:

Your discussion on gasoline octane is correct as far as it goes. But some of your information is incomplete, particularly the paragraph about what determines an engine's need for higher octane gasoline. You state that "The octane level required by an engine is determined by its compression ratio." -- which is partly correct. In fact an engine's need for octane is determined by these three basic factors:
- Pressure (the pressure inside the engine, which is determined by the ambient outside air pressure, the engine's compression ratio, and -- if installed -- the increased pressure produced by a turbo-charger). Higher pressures require higher octanes.

- Temperature (the temperature inside the engine, which is determined by the ambient outside air temperature, the temperature of the inside of the engine and -- potentially -- the presence of engine deposits which tend to get hotter than the inside of the engine). Higher temperatures require higher octanes.

- Engine Spark Advance (the advance in the firing of the spark in a gasoline engine before top-dead-center). More spark advance requires higher octanes.


All 3 of these factors work together to determine an engine's need for higher octane gasolines. For example, the new Honda Accord engine has a 10.0 to 1 compression ratio and Honda recommends regular gasoline. The new Nissan Maxima engine has a 10.3 to 1 compression ratio (not that much higher than the Honda) and Nissan recommends premium gasoline -- but acknowledges that regular gasoline will work, but will potentially reduce performance -- which may be caused by reducing the spark advance in the Nissan engine if the knock sensor detects knocking with regular gasoline

[R-Audi]

Re: K04/RS6/What Octane is best in the city Q&A

And also a newspaper article from Colorado, quoting from someone in the Department of Energy in the US


http://www.summitdaily.com/article/2...NOTES/50812008

[Smartiepants]

Re: K04/RS6/What Octane is best in the city Q&A

In the Rocky Mountain (high altitude) states, 85 AKI is the minimum octane, and 91 AKI is the maximum octane available in fuel. The reason for this is that in higher-altitude areas, a typical combustion engine draws in less air per cycle due to the reduced density of the atmosphere. This directly translates to reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a car with a carburetor that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains

From wikipedia

[The_Jerbel]

Re: K04/RS6/What Octane is best in the city Q&A

Ok, so everything that Mark posted, as well as Nick, indicates that nothing about the fuel changes chemically at higher altitudes, but that with equal engine compression ratios the absolute pressure decreases slightly with lower density air.

That I can absolutely agree with, but just how much difference does it make? The Ford "shop foreman" from Colorado claims that it's 1 octane rating for every 1000 feet above sea level. I have a HELL of a hard time believing that. There is no way that our 91 is equal to 95oct down at sea level.

Guys, I'm not sure if anyone here has any experience logging cars with fuel from the east coast, but based on what Mike as VAST tells me when looking at our logs (from a few different vehicles), our fuel of equal octane rating gives a lot more knock than their fuel over there does...

Maybe it's because we use anti-freeze additives after the fuel has already been assigned an octane rating, I don't know for sure, but I can't deny the actual results that I have seen.

[The_Jerbel]

Re: K04/RS6/What Octane is best in the city Q&A

I also posted this in the Tial650 thread...

Ok, the results are in for Husky 94oct vs. Shell 91oct. The logs of the two fuels are almost identical, with very small deviations between them, but here is how they stack up in this Tial650R S4...

94oct fuel gave knock values equal to 91oct.

94oct fuel gave timing CF's as high or higher than 91oct in every log.

94oct fuel gave LESS peak and overall acceleration than 91oct as measured by the accelerometer.

94oct fuel produced lower overall timing advance numbers than 91oct.

The temperature when running 94oct fuel was 20degrees F lower than when running 91oct, which should help the case of 94....

The results from my logs taken in the last couple of days pretty much mirror the results of my testing done in the winter.

[NeonGTI]

Re: K04/RS6/What Octane is best in the city Q&A

The other thing to remember about the Mohawk ethanol blended gas is that ethanol has a MUCH lower energy density than regular old gas gas so unless you are tuning it aggressively for 94 octane you will make less HP than you would on 91 octane, all othre factors remaining equal.