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Old 09-22-2011, 11:35 PM   #1
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Octane Rating Relevancy

How important is using the appropriate octance for the engine in regards to MPG? I've decided to run a small experiment to find out, but I'm definitely looking for any input or experience that anyone else has had with this.

I've got a Toyota Matrix '03 XRS, which has a Celica engine in it. According to the manual, it's supposed to receive 91 octane. I didn't know at first, so I used regualar. Then after talking to my mechanic at the time, I switched to the midgrade. All of my past mileage results have been while using midgrade, and I'm right near the EPA estimated MPG. But as the car nears 100k miles, I've filled up a tank with 93 octane, and I'm thinking about making the switch permanent. I'll probably go with 2 or 3 full tanks of 93 octane gas to see if it raises my MPG at all. Any thoughts or experiences would be very welcome. Thanks!
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Old 09-23-2011, 12:44 AM   #2
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Octane is the resistance of the fuel to detonate. Higher compression engines require higher octane so that the fuel doesn't ignite too early in the cycle.

Using a higher octane in a car that doesn't require it will reduce efficiency of the engine. Using too low of an octane in a higher compression car will introduce knocking (pinging), which is the fuel igniting too early, which will actually make the engine twist against itself, as the piston is still moving upwards in the cylinder, not moving downwards as it should.

Use the octane recommended to you by the car manufacturer.
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Old 09-23-2011, 02:36 AM   #3
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Your XRS needs premium fuel, which is why the manufacturer recommended it. Your engine is a high-performance motor with very high compression (11.5:1 if I remember correctly) and thus performance and mileage will be degraded with a lower octane fuel. Twenty years ago driving a similar high-performance motor with regular fuel would have destroyed it, but today the OEMs use knock detection feedback to allow people who don't read their owner's manual to drive on regular or midgrade with just degraded performance instead of a blown-up motor. It may just be a grocery getter, but the motor in your XRS also powers the Lotus Elise.

For all cars it depends on how the car is tuned. Most newer cars use knock sensor which can sometimes increase mileage slightly when using higher octane than is called for. But in almost all cases using lower octane than recommended will lose a bit of mileage. (This doesn't apply in high altitudes.)

So to answer your question, it will probably increase mileage a little bit, and it will certainly increase performance! But I have an inkling that you may not be taking advantage of all of 8200 rpm available to you in the first place.
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Old 09-26-2011, 05:03 AM   #4
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yeah very true... i don't really push my car, and rarely do i get above 5000 rpm, but when I do, it sure is a treat... yeah i'm making the move to premium gas permanent... to drive this car for a while longer, so if it takes better care of the engine, i'm all for it.
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Old 10-04-2011, 01:58 AM   #5
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Agree with Camflan.
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Old 10-12-2011, 01:53 AM   #6
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I Agree with Camflan.

On another note, paying on average 20 cents more per gallon in the hopes of better MPG doesnt make much sense. At that rate, you would need to boost your current MPG from 28 to 30 just to break even at current prices.
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Old 10-13-2011, 05:30 AM   #7
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Camflan is close, but not totally accurate...

"Using a higher octane in a car that doesn't require it will reduce efficiency of the engine."

This is incorrect. Using higher octane rated fuel than required only drains you pocket quicker...the octane rating is a measurement of the "flash point" of the fuel...which has nothing to do with the actual ignition of the fuel...the spark from the plug is much hotter than the flash point rating, therefore you can use a higher octane fuel without any loss of performance or economy. The knocking isn't something that is caused by the spark plug, but the residual heat in the cylinder that "pre-ignites" the fuel before it is supposed to...this can be due to the tuning of the engine (in the OP's case) or due to a high mileage engine that has some carbon deposits on the under side of the valves or on the top of the piston that don't cool as fast as the other engine parts and cause the fuel to ignite prematurely...that's why some older cars start to develope a "knock"...go up a fuel grade in these old cars and the knock should go away...

BDC was much closer with his info on the knock sensor in engines...

"...today the OEMs use knock detection feedback to allow people who don't read their owner's manual to drive on regular or midgrade with just degraded performance..."

That feedback causes the engine controls to "retard" the timing (fire the plug a little later) to minimize the impact of the pre-ignition situation...so it basically "detunes" the engine to allow it to run on a lower grade of fuel without any real consequences.

But Camflan nailed this one...

"Use the octane recommended to you by the car manufacturer."

Later.
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