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Old 04-21-2005, 02:12 AM   #1
m4r10
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Default question about octane and mileage

I feel silly for asking this but what's the relationship between octane rating of gasoline and miles per gallon. For example, would you get better gas mileage from 93 octane or 87 octane assuming all the variables were the same?
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Old 04-21-2005, 03:52 AM   #2
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1. there is no direct relationship between mpg and octane level. you think that because wrx cars have lower mpg than rs. this is cuz the turbo sucks in more air therefore more air means more fuel...

2. use only the specific octane level that is instructed in your car's manual. if you use too high or too low, you will just wreck you engine. you dont want detonation nor carbon build-up do ya?
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Old 04-21-2005, 04:17 AM   #3
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It depends on the engine and its specific need for octane with your driving style.

There is no direct connection between a fuels octane and the miles per gallon it will give. The fuel energy contained in 87 octane gasoline is essentially identical to the fuel energy contained in an equal amount of 93 (or higher octane) fuel.

What is different is the ignition timing and boost level you can run with the two different fuels. You will get the best gas milage with the fuel that best matches the factory ignition timing curve. On the WRX we have a very flexible ECU that can give and take away a considerable amount of ignition timing based on detected knock. You want to run the fuel octane that will just allow the ECU to give you best timing.

The owner manual specifies a fuel of at least 91 octane (AKI), anything lower than that will give you less than ideal performance as the ECU will "detune" the engine to keep from hurting the engine due to detonation on lower octane fuels. The lowest octane you can use in "emergencies" is 87 and this is intended only as a stop gap to get you to a location where you can buy a proper fuel.

The WRX actually likes more than 91 octane and gives noticably better performance with fuels up to at least 94 octane.

Using a fuel of "too high" an octane cannot and will not hurt your engine provided your not using a leaded high octane fuel.

Buy the best pump premium fuel you can find the few pennies a gallon you save by using cheaper fuels will cost you money in the long run through lower gas milage, and possible damage due to too low an octane of a cut rate fuel that does not have proper additives to keep your injectors clean.

The driver and how he drives the car has much much more effect on gas milage than the fuel does. Proper tire pressures ( or a few PSI above recommended pressure) and not flogging the engine hard all the time will make a huge difference. Learn to cruise in 5th gear and down shift to 4 th when you need to pass traffic is also important.

Larry
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Old 04-21-2005, 06:52 AM   #4
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^awesome explanation

and in addition: the octane number is a measure of the fuel's ability to resist self-ignition when burnt in an engine (knock), so the higher the octane rating, the less knock prone your car will be. Apparently higher octane fuel is supposed to be denser then regular fuel... which in turn means you'll get a bit better mileage (not significantly more...)

and ya.. the easiest way to get significantly better milage is by changing your driving style... I can get from less then 16mpg to more than 23mpg on average for the tank depending on the way I drive.. proper tire pressure makes a noticeable difference too..
If you want to be anal about it, keep your windows rolled up when on the highways, use vehicle electronics only when needed, don't use the a/c, try losing some weight, don't carry passengers, scrape off that sound deadening stuff from your car, etc etc.

and you shouldn't even think about using 87 octane if you have the option of getting better, the wrx doesn't like it

Quote:
there is no direct relationship between mpg and octane level.
there is somewhat actually..
Quote:
you think that because wrx cars have lower mpg than rs. this is cuz the turbo sucks in more air therefore more air means more fuel...
true
Quote:
use only the specific octane level that is instructed in your car's manual.
owner's manual lists the recommended octane level, so use that or better
Quote:
if you use too high or too low, you will just wreck you engine.
high = no prob, too low = *knock knock*

Last edited by GlobalWaste; 04-21-2005 at 07:07 AM.
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Old 04-21-2005, 07:26 AM   #5
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A couple of years ago when fuel prices started creeping up, I stopped buying 93 octane and started filling up my old Eagle Talon turbo with regular 87 octane.

I ran that for several months with no issues or problems. At some point, I calculated out the additional cost to purchase 93 octane versus 87 and decided it was worth it. So I filled up with 93 octane.

I was amazed at the difference. Had you asked me the previous week if the 87 octane yielded lower performance, I would have said, yes, but it's hardly noticeable. But after that first tank of 93 octane....wow...it was a huge difference!

To your original question, I noticed absolutely no difference in MPG using either octane...it was exactly the same as it was every week over the 9 years I owned the car.

I can only assume the same is true for the WRX. I know I put only 93 octane in my STi!

-Michael
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Old 04-21-2005, 09:14 AM   #6
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+1 for amarpauld's post.
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Old 04-21-2005, 09:42 AM   #7
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A little more about Octane. Octane rating was created to explain the ratio of octane (an 8 carbon chain) compared to heptane (a 7 carbon chain). Octane is extremely stable under high pressures, heptane is not. The higher the number the more stable the product is under compression. A lower octane fuel will self detonate (no spark required) at a lower pressure or compression cycle than a higher octane fuel. The higher the compression ratio an engine has the higher the octane required OR you will get premature detonation aka knock which you do NOT want. Basically the fuel will explode early in the compression cycle instead of at the properly timed point when the spark is applied by your spark plug. The compression ratio on the WRX is only 8:1 BUT it is a pressurized charge before the compression cycle so it would be more like a 12:1 naturally aspirated engine. Gallon for gallon the actual amount of chemical energy is almost identical but the chemical STABILITY is slightly different.
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Old 04-21-2005, 09:46 AM   #8
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Quote:
Originally Posted by Dueysan
A little more about Octane. Octane rating was created to explain the ratio of octane (an 8 carbon chain) compared to heptane (a 7 carbon chain). Octane is extremely stable under high pressures, heptane is not. The higher the number the more stable the product is under compression. A lower octane fuel will self detonate (no spark required) at a lower pressure or compression cycle than a higher octane fuel. The higher the compression ratio an engine has the higher the octane required OR you will get premature detonation aka knock which you do NOT want. Basically the fuel will explode early in the compression cycle instead of at the properly timed point when the spark is applied by your spark plug. The compression ratio on the WRX is only 8:1 BUT it is a pressurized charge before the compression cycle so it would be more like a 12:1 naturally aspirated engine. Gallon for gallon the actual amount of chemical energy is almost identical but the chemical STABILITY is slightly different.
Thanks!! I was about to post a question about this...but you already answered it and then some. A friend at work claim that there is no difference in these fuels based on octane and the cars that "require" 93 are full of it. I have never had any real knowledge to argue with...until know.
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Old 04-21-2005, 10:08 AM   #9
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Great explanations, I've learned a lot, thanks.
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Old 04-21-2005, 10:30 AM   #10
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Quote:
Originally Posted by Dueysan
A little more about Octane. Octane rating was created to explain the ratio of octane (an 8 carbon chain) compared to heptane (a 7 carbon chain). Octane is extremely stable under high pressures, heptane is not. The higher the number the more stable the product is under compression. A lower octane fuel will self detonate (no spark required) at a lower pressure or compression cycle than a higher octane fuel. The higher the compression ratio an engine has the higher the octane required OR you will get premature detonation aka knock which you do NOT want. Basically the fuel will explode early in the compression cycle instead of at the properly timed point when the spark is applied by your spark plug. The compression ratio on the WRX is only 8:1 BUT it is a pressurized charge before the compression cycle so it would be more like a 12:1 naturally aspirated engine. Gallon for gallon the actual amount of chemical energy is almost identical but the chemical STABILITY is slightly different.
used to work at a refinery ... it's actually pretty neat to see how they seperate all the junk in crude ;p
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Old 04-21-2005, 11:07 AM   #11
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Yeah....I watched some show on the History Channel about the history of gasoline (Modern Marvels was the show, I think) and it was pretty cool.

They went through how the refining processes kept improving, and how today, through various processes nearly 50% of a barrel of oil now becomes gasonline....up from about 35% 20 years ago.

They also went through how octane ratings came to existance and why, as well as how much it has improved.

It also had some very good animations of how the raw crude is broken down into the various fuels in the refinery stacks.

Very informative....catch it if you can.

-Michael
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Old 04-21-2005, 11:35 AM   #12
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Quote:
Originally Posted by amarpauld
If you want to be anal about it, keep your windows rolled up when on the highways, use vehicle electronics only when needed, don't use the a/c, try losing some weight, don't carry passengers, scrape off that sound deadening stuff from your car, etc etc.

and you shouldn't even think about using 87 octane if you have the option of getting better, the wrx doesn't like it
Yea I'm not anal about it, just curious cuz someone asked me the same question and I drew a blank. I knew about the knocking with the lower octane and btw, I always use 93 octane.
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Old 04-21-2005, 12:12 PM   #13
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Quote:
Originally Posted by m4r10
Yea I'm not anal about it, just curious cuz someone asked me the same question and I drew a blank. I knew about the knocking with the lower octane and btw, I always use 93 octane.
if you missed it, that whole "if you're anal about it.." paragraph wasn't me being serious for the average guy, that stuff is going too far
and no worries man, I asked my mechanics teacher this same question a few years ago
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Old 04-21-2005, 12:49 PM   #14
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I was considering using a tank of 89 and see what happens. I would never touch 87 though for my WRX.
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Old 04-22-2005, 10:22 AM   #15
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Would someone care to comment about the difference in speed of the flame front and potential leftover unburned fuel at the end of the ignition cycle? We've all seen pix of racecars with flames coming out the exhaust....which is unburned fuel making their way to the exhaust. Is there a similar, to less degree effect in a street car?

jack (uses lowest available octane except in racecar)
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Old 04-22-2005, 10:58 AM   #16
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.....and the moral of the story is...???

If you really don't understand, exactly, what you are doing-----you should do what the 'book' tells you or common sense(going out on a limb here sometimes, with this one) dictates.

Stupid people do stupid things for stupid reasons......and the rest of us(sometimes secretly ) laugh our asses off at them.
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Old 04-22-2005, 11:05 AM   #17
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Well I'm not an expert by any means (I probably know enough to make myself dangerous to myself) but my guess is they are just throwing so much juice into their cylinders for maximum output that the fuel efficiency from a complete burn isn't really a concern. Maximum output at all cost is. It's kind of like a magnum rifle cartridge versus a medium power cartridge in the same caliber with the same barrel length...you are going to get a much bigger muzzle flash (gunpowder burning but now outside of the pressurized system of the barrel) in the magnum which is basically wasted energy but you are getting an increase in ballistic performance out of roughly the same barrel at the cost of efficiency (which is rarely a concern in such a case). Exhaust temperatures are going to be much higher on race cars and there is a lack of a catylictic converters because of this. This is why when you pump up a car like a WRX far beyond it's stock output you must do something with the CATs otherwise they will simply melt from the high EGTs (exhaust gas temperatures). In street cars to meet EPA mandates a complete combustion is desireable for fuel mileage ratings and emissions control....however there are so many factors now in pollution control regulations that I believe that combustion is no longer optimized for max output/efficiency but more to comply with pollution laws which although not being completely arbitrary are often in conflict with engine output/efficiency. Why the 04'-05' WRX's have a different fuel management program than the 02'-03' with the 04-05 version running far too lean in certain high RPM situations? Well after reading a lengthy thread on the subject by people waaaay more knowledgeable than I, there conclusion was it's probably due to new EPA laws and changes in car classifications more than anything else.
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Old 04-22-2005, 06:51 PM   #18
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From several real-life measurements that I have done on cars factory designed for 87 octane:

1) MPG with Amoco 93 Octane > MPG with Amoco 87 Octane

2) MPG with Shell 93 Octane =~ MPG with Shell 87 Octane

3) MPG with Shell 87 Octane > MPG with Amoco 87 Octane

4) Any brand of 93 Octane = weaker low end torque, easier to get engine to rev higher (when it finally gets there), and less vibrations overall

5) Hwy cruising with the Impreza RS wagon on 89 octane = sweet

I haven't done enough measurements to compare Amoco 93 to Shell 93.

I few days ago I was reading on the Chevron web site that their premium gas has a greater heat content (ie more energy), than their regular grade, but the difference was rather small. Shell claims that all of their grades of gasoline have added lubricants which reduce friction and increase mileage. Other brands may only add extra lubricants to their higher grades.

When comparing gasoline brands, give it at least three tanks of gas to come reasonably close to completely flushing out the previous brand/grade from the tank.
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