How does the FJ system detect the grade of fuel and adjust engine timing parameters (stated in an earlier post)? That seems implausible to me. Buying lower octane fuel (lower octane=faster burning as I recall) from a quality fuel provider may not cause a reduction in fuel mileage unless the engine was working at a level at which the higher octane would provide a benefit, such as higher cylinder pressures under open throttle conditions during towing, heavy acceleration or higher altitude driving. My 2WD has never gotten less than 20MPG mixed driving on shell 87 regular, but I rarely run the engine at higher rpms or full throttle. For flatland driving I have not detected any pinging. I could understand a mileage penalty for a faster burning fuel if the fuel causes incomplete burning due to detonation - but that is rarely the case for most common driving conditions.
Post edited by: kawagumby, at: 2008/03/30 03:09
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Re:High octane fuel?
Date: 2008/03/30 09:01
By: fjd08se
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It doesn't detect anything about the fuel, but it monitors the performance of the engine. I recommend you read this article, especially the part about "Effects of Octane Rating"
Interestingly, the octane rating itself has no bearing on the BTU content of the gas. The direct quote applicable here is "Modern engines have anti-knock provisions built into the control systems and this is usually achieved by dynamically de-tuning the engine while under load by increasing the fuel-air mixture and retarding the spark".
Does your FJ have a roof rack? Standard tires? 20mpg is great.
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Re:High octane fuel?
Date: 2008/03/30 16:32
By: adambomb510
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I completely agree with you and the fact that the engine cannot detect octane levels, but there is something different here. I towed a large load of furniture this weekend and tried both low and high octane - the low octane resulted in nauseatingly lower mileage than the high. In mixed driving conditions, (I have the manual) I get 30-40 miles more out of a tank simply by using high octane and driving normally (not flooring it or running excessively high rpms). Its enough to convince me to use high octane all the time. I also live near sea level, I would imagine there would be a difference at higher altitudes too. All that being said, I am continually impressed with 20mpg numbers in the FJ. I have friends with those mini-SUVs (Escape, Santa Fe, etc) and they are all seeing the same numbers! Plus theyre SUVs are ugly.
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Re:High octane fuel?
Date: 2008/03/30 18:47
By: mwhunter
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Hey fjd08se;
I'd really like to know how your MPG came out after running a full tank of 89 vs the 93 octane? Please post your results.
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Re:High octane fuel?
Date: 2008/03/30 19:08
By: mwhunter
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Hey fjd08se,
That wiki article you provided is a Great read for everyone who 'thinks' they know about gas and octane ratings, etc. Dealerships should post that for their Service Center guys to read.
