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Discussion Starter · #1 ·
In Colorado you can go down 2 points on your Octane rating because there is not enough oxygen to burn the gas anyway. You won't have ping problems because the gas won't predetonate.

So I have run 85 Octane in my cars rated at 87 with no problem for years.

I had been running 85 Octane in my 2 door JK just out of habit. I also track fuel mileage out of habit. I also make sure I find gas that is not watered down with alcohol. It definitely reduces the energy potential of the fuel.

I was getting only about 17+ to 18 on the highway with 85 Octane.

So we go up to the mountains yesterday and I had filled the tank with Shell 87 Octane gas. Between Denver and Buena Vista, traveling up some fairly serious hills going around 65 to 70 and all at about 8000 feet altitude, turning the overdrive off for 80% of the ride and shifting into 2nd frequently I got 20 MPG!

OK so maybe the fuel management system is using the gas better, I don't know, but I will definitely use 87 Octane from now on!

I was pleasantly surprised!
 

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I have over a milliong miles in Jeeps. (owned 10 of them and drive a lot). I have always gotten better mileage with 91 here in Colorado. In my YJs and TJ it was always about 40 more miles to a tank. So the extra $1.50 if cost to fill up usually saved me 2-3 gallons of fuel (at $4 a gallon thats $8-12 bucks a tank).
 

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In Colorado you can go down 2 points on your Octane rating because there is not enough oxygen to burn the gas anyway.


You do realize that our fuel here in Colorado is already 2 points lower than everywhere at a lower elevation. We have 85/87/91 here while most places have 87/89/93. So the two points is alread accounted for in the fuel we buy.


I have read every thing that says octane will not increase mileage. But my real world experience on the Jeeps has always proved otherwise. Jeep might just run enough better with the higher octane that is does not work quite as hard and gets better mileage.
 

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The octane does not get you better gas milage in itself but, it does allow you to tune leaner and that will make a difference in gas milage.

The engine does have a knock sensor that will pull timing. It does so even slightly with low knock levels creating richer AFRs. Less air could also contribute to a richer mixture.

I would assume that with the higher octane the engine sees less knock events and the ECU allows leaner AFR's in the end netting better MPG's


just another theory to throw out there....
 

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So how do you explain the results on my old carborated YJ and CJ? (just curious)

I think your post before may be a big factor in what is helping on the carb engine.To be honest I have no idea, I really don't know enough about carborated systems to say.

Another factor may be the higher quality fuel. Shell is my personal favorite. On turbo engines you can see a pretty big difference in performance and MPG when you use higher quality fuel even if it is the same octane rating. I just moved to Colorado so I have not looked into this but I assume the places that do sell the higher octane fuel may also be a higher quality fuel as well???

If that is the case, it could have to do with the additives. Most stations actually use the same gas in the begininig, just different additives. Some put "cleaners" in the higher octane fuels that they do not put in the lower octane. I imagine on a carberated engine (especially higher mile engine) that could have a huge impact on effincency with fuel delivery and spark. Only other thing I can really think of would be if the higher octane is helping to partially cure an existing preignition situation that wasn't obvious. Preinition and the temps it creats could lower gas milage so that could be what would be helping. Especially in certain cylinders in the firing sequence.

I don't think a singal one of those issues would result in that large of a gain in MPG on the Carb engine so I would assume it is a litttle of each of those in combination with something I don't know about. Like I said, I really don't know much on those systems so I am only guessing....:beer:
 

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The octane does not get you better gas milage in itself but, it does allow you to tune leaner and that will make a difference in gas milage.
X2

There is no benefit to running a higher octane gas than is required by the engine. Higher octane does not mean higher quality, nor does it mean higher energy.

Octane is an arbitrary number which is calculated as the average of the Research Octane Number (RON) and the Motor Octane Number (MON), and is simply an indication of sensitivity to knock, nothing else!

In simple terms a chemical reaction occurs in unburned fuel in the combustion chamber before the spark plug ignites it. This reaction can create hydrocarbon molecules that have a nasty habit of igniting before the spark plug ignites them. When this pre-ignition occurs, a rapid increase in air pressure inside the chamber results. The increase in pressure yields a distinctive knocking sound.

The higher the octane rating the more resistant the gas is to developing these hydrocarbon molecules that cause knocking. That's it, no extra power, no more efficient, simply a higher tolerance to knock.

Some high performance vehicles with high compression engines do need high octane gas to run at peak efficiency, although you can often run 87 octane and never noticed it unless you drive hard or during colder months.

The octane value is no indication of how much energy is in the gas, nothing to do with it. Most race gas has no more energy than regular gas! Even methanol contains only 10% more energy than regular gas. The real benefit of race gas and methanol is that they allow you to run a much higher compression ratio. It is that higher compression ratio which is allowing more power to be produced, not anything in the gas.

So, unless you are going to change the compression ratio on your vehicle or change the timing, etc, it will produce no more power and be no more efficient running higher octane gas than running regular gas.

Gas companies have made a fortune in the past by misleading people into believing that octane improves performance and is higher quality. Over the years the FTC has sued a number of oil companies for making misleading claims about higher octane gas.

That said, as previously mentioned, if there are differences in the way the gas is blended for various octane levels, then you may see different fuel efficiencies. However, this would not be a direct result of the octane rating, but as a result of the blending.
 

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Even methanol contains only 10% more energy than regular gas. The real benefit of race gas and methanol is that they allow you to run a much higher compression ratio.
Methanol only has half the energy of gasoline. An engine setup to run methanol can produce 10% more HP than the same engine setup to run gasoline. You just have to run twice the fuel flow to do it.
 

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Some high performance vehicles with high compression engines do need high octane gas to run at peak efficiency, although you can often run 87 octane and never noticed it unless you drive hard or during colder months.
I once read or heard somewhere that an older Carburated engine, the gunk can in some way raise compression. I don't know if this is true and have not seen it myself but, I wonder if in Xtremjeepn's example some (maybe a little) of the increased MPG's could be A partial result of increased compression and therfore a little better efficincy with the higher octane???


Sorry, lol, still stumped on Xtremjeepn's because all my experiences have shown lower performance and MPG's running regular in a High octane engine or high octane in a regular engine......
 

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I once read or heard somewhere that an older Carburated engine, the gunk can in some way raise compression. I don't know if this is true and have not seen it myself but, I wonder if in Xtremjeepn's example some (maybe a little) of the increased MPG's could be A partial result of increased compression and therfore a little better efficincy with the higher octane???
In which case I would expect to see knocking with lower octane fuel, but I'm not convinced that you'd see better efficiency as low and high octane would be working on the same compression ratio.

I've known people that swear they get better mpg with higher octane gas. Even though my own experince over many vehicles does not reflect this, and the theory certainly doesn't, but I wonder if there are other factors, like the amount of ethanol blended in different octanes, that may make a difference.
 

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I've known people that swear they get better mpg with higher octane gas. Even though my own experince over many vehicles does not reflect this, and the theory certainly doesn't, but I wonder if there are other factors, like the amount of ethanol blended in different octanes, that may make a difference.
The only engines that I know that get better mpg with higher octane gasoline are the ones with a knock sensors and an ecm program that keeps pushing the timing and fuel flow until the engine knocks. Our 3.8l engines don't have knock sensors and high octane gasoline will not help.

Phil has the best explanation, varying ethanol blends.
 

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I always run 89 and get between 17 and 20 consistantly. I've gotten as much as 24.
 

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There once was a theory that Higher octane fuel burnered slower then lower octane fuels. This slower burn would increase the engines torque due to a slightly (very slightly) longer push on the piston during the power stroke. Somewhat like the comparison between modern gunpowder (burns fast like low octane) and blackpowder (just as powerful but slower burn hence slower push) the slower push creates a more sustained power to the bottom of the stroke. So, for a working engine pulling loads up hills the increased torque would increase power hence better mileage.

With that said my 98 RAM 5.2L always got better mileage on 91 Octane. No anti knock sensor on the 5.2L either and I knew as soon as I did use a poor grade of regular, as it would knock unbareably. I too live in the hills not like Colarado mind you. On the other hand my 2000 Honda ATV was set up to run regular gas and runs like crap on premium fuel.

As for carbureted engines, I believe they were pretty much all designed to run on 91 octane to start with. Can't remember what year it was when they dropped the Octane level for regular from 90-91 down to 87 or in some cases 85. Early 70's and older also had lead in the fuel which also increased the octane.
 

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So we go up to the mountains yesterday and I had filled the tank with Shell 87 Octane gas. Between Denver and Buena Vista, traveling up some fairly serious hills going around 65 to 70 and all at about 8000 feet altitude, turning the overdrive off for 80% of the ride and shifting into 2nd frequently I got 20 MPG!
So does it feel like it has more torque for climbing hill as well as getting better mileage?
 

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better fuel use

I have found that the v6 in my Jk just likes to spin a little higher than any of my previous vehicles. 6th gear (I have a man trans) is just used for interstate travel, I generaly keep the engine spinning like I do on my motorcycles. I average around 20 mpg measured (also what the dash info says). Don't be afraid to spin the engine up on the rpm scale,lugging the engine is no good any way and the engine appears to like to spin.
 

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Discussion Starter · #20 ·
I can't really tell an outright power difference because about the time I tried this I also had the dealership reset my speedo for 32 inch tires which also adjusts the shiftpoints a bit. That was noticable.

But we did go up in the mountains again last week end. Went south of Fairplay and west over Weston Pass then up to Leadville and back east across Mosquito pass.

I am from Colorado so I have no problem turning the od off or putting it in second (mines an automatic). I nearly never touch my brakes on the downhill side of any pass because I use the engine to control my speed. It has no lack of power if you are willing to rev it up a bit. Never bogs.

With about 200 miles of highway up and back and about 50 miles of under 20mph trails we got just under 18. Most of that driving was well over 8000 feet above sea level. That is pretty close to phenomenal to me.
 
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