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Old 01-14-2006, 02:18 PM   #1
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Default High Octane Vs. Low Octane Fuel

One question I have always wanted answers to is: what are the differences between high octane and low octane fuels in terms of wear n' tear on the engine and fuel economy? I have searched high and low and couldn't find an answer anywhere, so I did my own little study and thought I'd post the results here...

Control Group:
  • 1997 Civic EX Sedan, 80k miles and in excellent condition
  • Short Ram Intake (I thought this may have changed performance a little when compared to stock so I'm including it in the list)
  • 11.9 gallon fuel tank
  • Dead weight includes the sub-woofer box, amplifier, and myself
  • Transporting people other than myself was avoided as much as possible to
    keep variances in the weight of the vehicle to a minimum
Octane ratings compared: 86 and 91

During the testing, I ran the gas tank as close to empty as I could, so the fuel weights were the same throughout both tests. I also limited my RPMs to no more than 3000 which is what Honda recommends for maximum fuel economy.

My hypothesis was that the higher octane fuel would yield lower MPG ratings. My hypothesis was confirmed. When running on 86 octane fuel, I averaged about 32 MPG in the city. When running on 91 octane fuel, I averaged about 29.5 MPG in the city. I do live in Albuquerque (about a mile above sea level), so perhaps altitude played a factor in all of this but I'll never know for sure. Maybe one of you guys who live in a lower altitude area could duplicate my experiment - it would be interesting to see results for sea level or even higher altitudes.

I did not get a chance to do any highway testing because I don't typically use the freeway but if I go on a long road trip, I'll test that later.

Some reasons why higher octane fuels give lower MPG rating may be because they burn slower. Lower octane fuels are more explosive and probably yield a higher power output per gallon than their higher octane counterparts.

So why the heck would you want to buy high octane fuel if it costs more and gives worse gas mileage? Well, because it burns more slowly, it can lengthen the life of your engine by subjecting it to less stress. I suspect though, that if your engine was designed for regular unleaded, you probably won't see any benefits in using higher octane fuel. However, if you run a high-compression or boosted engine, high octane fuel will help your engine run more smoothly. Whether it is worth the extra 20-30 cents per gallon is debatable.

Last edited by 94accordex; 12-04-2007 at 11:56 PM. Reason: Added additional formatting, fixed spelling errors
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Old 01-16-2006, 07:05 AM   #2
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Default High Octane Vs. Low Octane

Good post! I'm gonna put this in the FAQ section.
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Old 05-14-2006, 08:26 AM   #3
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Default Re: High Octane Vs. Low Octane

No offense to you, but this experiment is pretty much bogus. Unless you do the test in a wind tunnel, or at least on a track in consistent weather on cruise control, there are just way too many variables.

With that in mind, it should also be noted that it's mostly just a myth that higher octane results in better performance and changes in fuel consumption. The only reason for using high octane gas is to resist knocking at higher compression. And with a D16Y8, you definitely don't have a compression value high enough to need high octane gas. Either your engine knocks or it doesn't. If you have a lower compression engine, you won't reap the benefits of higher octane gas and you're basically throwing away money.
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Last edited by 94accordex; 12-04-2007 at 11:57 PM. Reason: Fixed spelling errors
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Old 05-16-2006, 07:52 PM   #4
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Default Re: High Octane Vs. Low Octane

I wouldn't be so quick to say it's "bogus". This has been an ongoing experiment for the last few months and my results have consistently shown that higher octane fuels don't exhibit good gas mileage when compared to lower octane fuels (in a 1997 Civic at the VERY least). I live in New Mexico so the weather almost never changes; it's sunny and calm for the vast majority of the time. If there is ANY margin of error, it would be very small due to the amount of time I have put into this experiment. I never said that anyone needed high octane gas, I did this simply because I was curious in terms of fuel economy.

As far as having a D16Y8 and compression values and whatnot, you're absolutely right. You don't need high octane gas because low octane gas burns slowly enough that it doesn't cause a D16Y8 to knock. BUT, when you burn high octane gas in a low-compression engine, it is more likely that your gas will not burn as completely because of its slower burn time and the fact that smaller engines often run at much higher RPMs than larger engines with higher compression ratios (like a V8 HEMI for example). It is entirely possible that the higher octane gas could still be burning at the time the piston is at or near the end of the down-cycle. Ideally, the explosion should occur when the piston is moving fastest (between the top and middle of the down-cycle). At any rate, I appreciate the input but I stand by my results. I would be more apt to say that the test was bogus if I didn't see the results I've gotten for so long.

Last edited by 94accordex; 12-05-2007 at 12:00 AM. Reason: Fixed spelling errors
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