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Geek help, Why does cheap gas get better mileage?

Old 03-21-2006, 04:23 PM
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Geek help, Why does cheap gas get better mileage?

at $2.59 a gallon I broke down and bought 89 octane gas. Forgive me, I won't do it again. BUT!

I'm half way through the take and have 125 miles? with 93 octane I'd be at 100 miles. I don't get it 25% better mileage? That gets me to almost 20 mpg vs. the 15 or 16 I normally get.

Oh well, long trips to the shore I'll load up on the cheaper stuff.



I've never had a car where I could watch the gas gauge go down.
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Old 03-21-2006, 04:43 PM
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Compare it to eating a Filet Mignon as compared to chewing on a shoulder steak. The filet practically melts in your mouth and slides down your throat... it's wonderful to taste and easy to consume. The shoulder steak is tough and hard to chew. It takes longer to eat and is less enjoyable. Get it?
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Old 03-21-2006, 04:48 PM
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<forehead slap>
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Old 03-21-2006, 04:50 PM
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less advanced ignition timing...(read...less power)
equals better mileage

be careful with the crap gas bro, anything under 87 and u run a serious risk...but as for me.....94 octane until the day i (or she) dies
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Old 03-21-2006, 04:56 PM
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I think it is slower burning as well.
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Old 03-21-2006, 06:17 PM
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Don't shift at high rpms... you'll get a CEL :]
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Old 03-21-2006, 11:15 PM
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Originally Posted by Nero
I think it is slower burning as well.
No offense, but it seems like this is covered everytime a gas related thread starts.

HIGHER octane burns SLOWER.

LOWER octane burns FASTER, which is why you run the risk of detonation with a low-octane fuel. It is more volitile, so it takes less to get it to prematurely ignite.
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Old 03-22-2006, 10:10 AM
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"The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting."

Ah so you can fit more 93 octane in the cylinder. Hence a bigger detonation and more power. So crappy gas will always have better mileage. Dooh. That sux.
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Old 03-22-2006, 12:10 PM
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Originally Posted by Smileynh
"The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting."

Ah so you can fit more 93 octane in the cylinder. Hence a bigger detonation and more power. So crappy gas will always have better mileage. Dooh. That sux.

No you can't fit more "octane." Octane rating refers to the effect additives have which determine the temperature point at which the fuel will ignite; it has nothing to do with quantity of fuel. The more you compress gas before it ignites, the more energy is released; however the engine system has to be designed and timed for that. Racing engines are "high compression" and have to run high octane fuel as a result. High compression/high energy release engines also produce high amounts of heat. The more the gas is compressed, the more volatile it becomes which makes it more sensitive to ignition. A spark plug merely creates a hot-point to trigger that ignition. However any hot-point can do that; so if the temperature inside the chamber is high enough, or there's a piece of carbon deposit that's glowing hot enough, it can ignite the fuel before it's intended to. This is a disaster for an engine and puts tremendous strain on it which can lead to failure.

Lower octane fuel is not "crappy" gas; it's the same gas as the higher octane fuel but with less additives. Because of the way a rotary engine burns fuel, the lower octane gas may actually benefit it. Many rotary racers use 87 regular because it burns quicker, longer and more completely/efficiently. There are many many threads in the forum archives on this very subject.

Under continuous high-rpm usage premium octane will be beneficial because it allows safer ignition under high-compression. Under daily average driving around town, it won't burn as efficiently and will leave more carbon deposits in your engine which is why it's VERY important to not use a cheap BRAND of gas. Shell, Chevron, Texaco use good detergent additives in their high octane fuel to help cleanse those deposits. Lower octane fuel doesn't need them since it burns quicker and more efficiently.

You are one of the lucky ones (not everybody seems to be able to run 87 successfully) who can run 87 with the benefits of better mileage. Try a few tanks of Shell or Chevron 87 regular and measure your mileage. At the same time see if you can feel any appreciable difference in performance. With my car it idled better and seemed a tad more responsive to throttle input as well as better mileage compared to 91 octane.
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Old 03-22-2006, 01:02 PM
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Naturally aspirated rotaries hae always loved low octane. I won't run over 87 in an RX-7 unless it is turbocharged. The RX-8 has mixed results with it. Some run good with low octane and others run better with higher octane. It all has to do with the tuning and nothing with the engine. Higher octane can get you more power because you can run more advance but even this is only a blanket statement and not necessarily true all of the time. If your car runs good on 89, keep using it. In general, run the lowest octane that works well in the car. Anythig above that is a waste. The extra horsepower or 2 you'll get out of the next grade up won't do a thing for you. Extra horsepower is only going to happen if the car is tuned around higher octane. In a nonturbo RX-7 you'll lose power with higher octane. As was stated above, cheaper gas is not worse gas. We aren't talking about oil here. A higher grade of oil is better. A higher grade of fuel is more or less just a different octane rating which has nothing to do with it's quality. You'll get more variation going from gas company to gas company than you will between octane ratings when it comes to quality.
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Old 03-23-2006, 05:33 PM
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When running 87 octane, I get pings at high rpms. So, I really thought hard about what gas to buy. Once I did the math, I have always used 93 octane.

I travel 234 miles per week. Lets say I can average 18 mpg with 93 octane and 20 (not likely) mpg with 87 octane. My travel is all highway. That means for 93 octane, I need 13 gallons. For 87 octane I need 11.7.

Where I buy my gas, 93 is only .14 more than 87. Its mostly always <= .20 more. At .20, it costs an extra $3.38 per week. I spend that on a latte.

So, if its just money, its not worth it to me to use the fuel that makes my engine ping.

Now, there are arguments for midgrade I guess. I have run it with some success. But, then you are just talking like $1.10 per week.

Last edited by brianlmoon; 03-23-2006 at 06:25 PM.
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