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Mike Wallace

Octane: 90 vs 87. Worth $.10 per gallon?

15 posts in this topic

I'm not the greatest, or even second greatest, with math so is there a significant performance/mileage benefit of running 90 octane vs. 87 octane?

I have a '06 Kia Sedona (yes, it's a minivan, please be kind) and see that these two octanes are always $.10/gallon apart. What gives? would I be better off using the higher octane?

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There is such a big misconception out there about what the octane rating means.

Simply put, octane is a measure of resistance to detonation. If the owners manual of your vehicle states that 87 octane is the suggested fuel, that is what you should run. Many folks equate higher octane with better milage/performance. Not necessarily true. If you have an engine that is built up for high performance, then a higher octane fuel is in order.

Sure, your vehicle may run on 90 octane just fine, but you are not really saving any money.

Most vehicles in the last 10-15 years are designed to run on 87 octane.

I had a pal put 100LL into stock snomobile about 10 years back. It fouled plugs and ran terrible until he got some regular pump gas back into the system. The engine just did not have enough compression to get the fuel to burn efficiently.

I think you'll be just fine on 87 octane.

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You are correct that higher octane does not mean better performance or mileage. In fact, my 2001 Triton will lose both if I run on anything higher than 87 octane.

Using octane that is not what you engine is tuned for has an effect on engine timing. Simply put, different octanes react differently combusting at different rates and the engine timing needs to adjust accordingly.

Bob

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The super unleaded higher octane is usually achieved by adding ethanol. Ethanol does not have as much energy as gasoline. That's why people who run e-85 get 2/3 to 3/4 the fuel mileage as if they ran straight gas. So it stands to reason that you won't get better gas mileage, you will probably get a little worse.

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Thanks, everyone for the info. Much appreciated.

Mike

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Good answers everyone!!

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Quote:

The super unleaded higher octane is usually achieved by adding ethanol. Ethanol does not have as much energy as gasoline. That's why people who run e-85 get 2/3 to 3/4 the fuel mileage as if they ran straight gas. So it stands to reason that you won't get better gas mileage, you will probably get a little worse.


All grades of gas have E-10 added. There are some 93 octane pumps that are non oxygenated but they are for collector cars, snowmobiles, boats...not standard passenger vehicles.

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Good answers everyone!!


Agreed, great answers by all.

I would like to add that if your vehicle does recommend a higher octane than 87, you are best off running the higher octane gasoline.

For example, Nissan recommends that I use 91 octane in my 98 Maxima. Whenever I have tried running the lower octane, there is power loss and the milage goes down. So much so, that my girlfriend once filled my tank for me. and I could tell she didn't use 91 octane! It felt so sluggish!

The Maxima has high compression. So, when I run 87 octane, it senses pre-ignition, and backs off the timing.

By the way, I would realize no benifit from running 92 or 93 octane.

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This topic always interests me now since I have a newer vehicle that states in the owner's manual to run no lower than 87 octane, and up to 93 octane for optimum performance. "Optimum" performance is simply allowing the knock sensor to time the ignition spark a bit earlier (more degrees before TDC than with 87 octane) which gives the engine a bit more power, I suppose. I get plenty of power and mileage with regular gas so thats what I use. Higher octane will resist detonation, but also it burns hotter. If heat is the enemy of all things engine and transmission related, why use the higher octane? Because older cars that don't use knock sensors and have high compression ratios (about 10.5:1 or higher) require the resistance to detonation high octane fuel provides, especially under high heat conditions like racing. OK, no more rambling, just my $0.02.

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High octane gas does not burn hotter.

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Quote:

High octane gas does not burn hotter.


Right. In some respects it burns cooler - well, actually slower, I believe.

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All grades of gas do not have e-10 added to them. It states right on the pump if ethanol is added or not.

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Actually, for Minnesota, I believe all grades DO have ethanol unless it's specifically stated they don't. Those that don't must have the non-oxygenated decal affixed to the pump.

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Quote:

In some respects it burns cooler - well, actually slower, I believe.


Your correct- The higher the octane, the slower the burn. There is a relation the how much compression you have and how good the airflow( heads) are. In my stock car motors we use 110 octane fuel with 13-1 static compression. Any lower in the octane and we burn up the pistons because the explosion is so much quicker and detonation occurs. Yet on the 9-1 motors that I work on, which are similar in compression to today's motors, regular 87 octane fuel produces more power and cooler temps than the higher octane. Found this all on the dyno and on the track and supported the theory when I tried 93 octane in my truck pulling the River Pro up to the Sturgeon Exersion last year and averged 10 mpg. Tried the 91 octane on the next tank and averged 11.5. Put 87 octane in on the next tank and averaged 14 mpg.

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That slower burning characteristic of higher octane is precisely why it was advertised for older engines that had developed a knock. The knock was caused by ignition timing that was too early and using higher octane delayed ignition just enough to compensate. It was a bandaid for a timing problem but the fuel industry didn't say that. Not only did it solve the knock the engine performance improved because the timing was more accurate for the fuel being burned. It was this advertising that burned it into our brains that higher octane was better and it stuck. Take the vehicle to a mechanic and have it properly tuned and return to using 87 octane and everything gets rosy again.

Bob

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