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Author Topic: 85 Octane  (Read 265 times)
dickreid1
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« on: August 20, 2014, 01:15:13 pm »

Last summer I commented to the group about encountering 85 Octane "regular gas" at Cody, WY.  My feeling then was that this was a marketing gimmick to increase profit.

This summer driving coast to coast I encountered both 85 octane in the mountains and 86 octane gas in high altitude areas.   Curious, I found the following in Wikipedia so I guess this gas is legitimate.

"United States: in the US octane rating is displayed in AKI. In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel.[38] The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage..."

Because my mpg was lower this summer and the engine lugged on hills and downshifted more than usual when using this gas, the following is of interest:

"Many modern engines are equipped with a knock sensor (a small piezoelectric microphone), which sends a signal to the engine control unit, which in turn retards the ignition timing when detonation is detected. Retarding the ignition timing reduces the tendency of the fuel-air mixture to detonate, but also reduces power output and fuel efficiency. Because of this, under conditions of high load and high temperature, a given engine may have a more consistent power output with a higher octane fuel, as such fuels are less prone to detonation."
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Carol
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« Reply #1 on: August 20, 2014, 01:28:25 pm »

I am far from an expert on anything, but just passing along that when I picked up my PC in early July, Earl said that even if I saw the 85 octane out there, that I must not use it.  He said to stick with 90 percent octane.  I didn't ask why, since I probably wouldn't understand the answer anyhow.
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ron.dittmer
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« Reply #2 on: August 20, 2014, 05:08:47 pm »

Maybe I've been wrong all these years.  I had always understood that the lower the octane, the faster the fuel burns, the more volatile the fuel is.  Lower octane gas is sold at high altitude to compensate for the thinner air.  Octane slows the burn to reduce pre-detonation we all know as knocking.  High performance engines have a pre-detonation condition due to higher compression, heat, and other factors which is why they require higher octane.  But like I said, maybe I am wrong.

It would be interesting to compare fuel economy versus octane.  We have always used the lowest octane due to cost per gallon, but wonder if spending more on premium would pay off with improved fuel efficiency.

Regarding power, we never had any issues such that I've wondered if we could have gotten by with the smaller Triton 5.4L-V8 engine.  Especially when not towing.
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