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What octane are you running in your mk3?

Believe it or not, higher octane has no positive effects on performance or fuel economy.

Fuel requires two things for combustion; heat and compression (keep in mind this is after the fuel has been atomized and mixed with air). You dont NEED spark, spark is added to compensate for a lack of the proper amount of heat. This is done on purpose so timing can be tuned.

I used to believe (from the time I began driving up until about a year after I got out of the military) that running higher octane in a car that was designed to run 87 octane would improve performance. In one of my Mechanical Engineering courses last year my professor explained several reasons NOT to do this.

The octane listed in the owners manual is what it is FOR A REASON. The car companies are not trying to dupe you out of performance. They list the octane required for optimum performance. This means that the motor was designed to run at its best while using THAT octane.

Running the wrong octane fuel only has an effect on vehicles designed to run a higher octane. This is because lower octane has a lower flash point and requires less compression for combustion. Hence why vehicles that have higher compression motors typically require 93 octane.

Running 93 in an engine that is supposed to run 87 will not have immediate negative consequences, other than putting a larger dent in your wallet, but in the long run it can have an effect on timing and eventually cause misfires.

High octane + low compression = unused fuel, if the compression doesn't get up high enough your engine cant burn all the fuel upon combustion. This fuel burns off into your exhaust and your 02 sensor will assume you are running too rich and the mixture will lean out. When the mixture leans out there is a possibility that it wont be enough to run the engine smoothly and so the ecu will again attempt to compensate. While you may not notice any issues at first, over time it will become more apparent.

If you want to upgrade for performance and run 93 more power to you, just make sure you tune it for it unless you want to deal with the consequences later on down the road.

Mind you this is what I can remember from the discussion so some details may be off or missing but the main point is still fairly clear.
Feel free to add, retract, and debate (I love a good debate).
 

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It would be very interesting to see 87 vs. 89 vs. 91 vs. 93 AKI dynamometer readings on the 2.0 GDI naturally aspirated.
 

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Meh, I heared that our direct injection gets more clogged with lower octane fuels, and the higher helps prevent this. I have found that with the higher oct the car down-shifts much less often and pushes it self. These cars are tuned down a lot to what they can handle even safely.
 

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87 octane E-10 from a high volume geriatric station works fine for me & the price is right :)
 

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Jesus H. Tap Dancing Christ, how much does it cost to run a car on a dyno twice? I'll solve this mystery myself if I have to.

EDIT: This is kind of a lie, since I'm saving for a house and still have thousands upon thousands of dollars in student loans.

Double EDIT: If I did this, I would use ethanol-free 87 and 91, since I kind of have access to both - I just need to drive 40 minutes to get to it.
 

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Yeah - send it in to Mythbusters, getting a little old here.

Mostly factual, WHEN CONSIDERING ENGINES WITHOUT ANY ADJUSTMENT CAPABILITY.

Even then, the "best" choice was the one proven to work well for that vehicle - at that time - in it's normal conditions of use.

YES - for older vehicles it is ABSOLUTELY a "Myth" that you'd get a lot of performance JUST by changing to higher octane when not required - in fact you loose performance.

Your professor would explain that it's a LOT more debatable, with results that vary in degree according to circumstances, when dealing with a high compression, GDI, Variable valve timing, variable spark timing, variable injection timing, knock sensor equipped engine such as the one in the MkIII.

Not all those capabilities are even used to their fullest in this model AFAIK, leaving room for more debate as future improvements are made.

You'll note that "factory spec." is now a minimum? There's some reason behind that, as has been commented on at length in prev. discussions.
 

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Yeah the op seems to be running for 'troll of the year'. But I will give the person a pass. maybe they really believe in the tooth fairy?

When I read these assertions, it is always some other guru/expert sousing about the topic, and the poor 'expounder' is just passing along the 'truth' for all to see.
Right.
Anyway I go with what my OWN EXPERIENCE has shown me.

In my previous ride Premium was a little better. The main thing which was a LOT better was 100% gasoline gas. 10% ethanol had less power, and poorer gas mileage.

In my current Focus SE, I find using gas which is a little higher than 87 is the best. 89 works better for smooth idle and take off from idle.
I now use 93 all the time, though I do buy partial tanks of 87 if the gas price is rediculously low for regular compared to premium that day.

For folks who think it is all hogwash. sure. no problem. I do not worry about what YOU use. Why do you worry about what I use?
that is the mystery... [poke]
 

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Believe it or not, higher octane has no positive effects on performance or fuel economy.....

....blah blah blah
Yawn!



He must be talking a completely cast iron, carborated, points ignition, push rod engine!
 

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Your professor's views are dated. They may have applied to simple knock sensor systems but not to the sophisticated system in the MK3.

Wrong about hp gain 87 vs. 93 octane as well. Mustang in this example:
 

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For years, Nissan touted better performance from their 3.5 V6 when run on premium fuel, simply because the ECU was able to detect it and advance timing. That was 2002. 12 years later, I'm betting most cars can make these adjustments.

As for the long-term side effects of higher-octane in engines that don't call for it, my '93 Sundance V6 (pretty simple engine in today's standards) was spec'd to run '87, and after 73,000 miles run on nothing but Amoco Ultimate, it was still purring on the original spark plugs the day I traded it in. So, how many miles should we expect to travel problem-free before the evil octane grabs ahold and ruins our engines?

To assert that any modern engine will start mis-firing due to running 4-6 extra grades of octane is ridiculous, and untrue according to my '11 Camaro (22,000 miles) and '05 Sport Trac (110,000 miles). True enough that a butt-dyno and MPG minder aren't likely to give a realistic picture of any hp or MPG gains that are worth mentioning, but the complete lack of spark knock help me feel a little better about driving my vehicle hard in the Texas heat. The few times I attempted to run 87 in the Altima, I did hear knock at lower rpm under load. It was absent on 93.
 

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Running the wrong octane fuel only has an effect on vehicles designed to run a higher octane. This is because lower octane has a lower flash point and requires less compression for combustion. Hence why vehicles that have higher compression motors typically require 93 octane.
Everyone else seems to have beat you up well enough. Figured Id throw my 2 cents in. Im not a mechanical engineer nor did I sleep at a Holiday Inn Express last nite....

But you are aware the MKIII engine runs at 12:1, right? Sure it can adjust valve duration, timing etc, but all these concessions are made to safely use a lower octane fuel. The MKIII engine will run better, and more efficiently, with a higher octane fuel.
 

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The GT is recommended for 93 and requires 87. The 2.0GDI is recommended for 87. So far I have SEEN absolutely nothing to indicate that FoMoCo engineers tuned the Foci to take advantage of higher octane fuels, MPG or HP.

A recent Motor Trend article on the Kia k900 with GDI, a vehicle that is recommended for premium but can also run lower grade fuels, showed the same fuel efficiency with regular and premium.

Every article I have seen seems to say there are no gains.
 

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Something I noticed, those that agree with the OP are focus rookies. The ones that disagree and show valid points are enthusiasts and above. Hmmmmm coincidence?
 

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What octane are you running in your mk3?

Believe it or not, higher octane has no positive effects on performance or fuel economy.
Whoever told you that is obviously not familiar with engine management of the mk3 Focus.
 
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