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Old 10-24-2000, 09:12 AM   #1
Jonathan Kent
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Question changing octane and ECU reset

Hey folks. Can someone please clarify as to whether or not an ECU reset is necessary after changing gas octanes? I've always used 93 in my 99RS. However, this past weekend, I filled my empty tank with 87 for the first time. I didn't notice any changes (power, mileage, etc.)

So, to ECU, or not to ECU?

Thanks,

J. Kent
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Old 10-24-2000, 02:00 PM   #2
cakagan
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I'm not sure how long it takes the engine to notice the lower octane, but I've heard that it takes 3 tankfuls of a higher octane for the ECU to adjust.

I would only reset it when you go to a higher octane than 87. Why the change?

-Chris
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Old 10-24-2000, 02:47 PM   #3
sasim
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Smile

The ECU should adjust to the lower octane quite quickly on a MY99, a reset should not be required.

The whole ECU reset is a subject of great debate here in the UK, but my simple understanding is that MY98 models or later should never need a reset, not sure about pre MY98 ones......hmmmmmmmm

SS
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Old 10-24-2000, 03:10 PM   #4
silverSurfer
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$$$ is why'd i'd change too...
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Old 10-24-2000, 04:56 PM   #5
Catfish
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In case you don't already know....

You don't need high octane gas unless you are experiencing detonation. Just fill it up with the minimum octane required by the manufacturer and live happy. As you do mods to your car, upgrade to the next octane if you detect any knocking. Most mods outside of cams or forced induction will not require an increase in octane. To use the highest octane available just because that's what all the "sports cars" use is a waste of $$$ and gas mileage. You'll actually get better gas mileage with the least octane. Higher octane fuels are less prone to detonation because they are more difficult to burn. Being more difficult to burn means they burn slower and also are more difficult to ignite by your ignition system. So, unless your car was designed or has been modified in such a way to warrant the higher octane, stick with what works and enjoy the savings.

Dave
'00 Subaru 2.5RS Sedan
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Old 10-24-2000, 05:01 PM   #6
ColinL
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Flame speed is not related to octane.

You can trust me on that or you can read a very informative post here: http://www.i-club.com/ubb-files/Foru...ML/003898.html

In it you will also learn that engines can require considerably different octanes depending on wear, atmospheric and local conditions, and manufacturing tolerances. It is by no means unreasonable to think that some 2.5RS will require 91 octane or above to attain nominal performance, and others will require less than 87.

[This message has been edited by ColinL (edited October 24, 2000).]
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Old 10-24-2000, 10:05 PM   #7
ImprezaRS dot com
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ColinL,

Don't you know that nobody ever reads that post? Except me...

Larry
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Old 10-25-2000, 07:19 AM   #8
Liquid Calm
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WOW that was a long post.....but some stuff I did not know in there
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Old 10-25-2000, 10:12 AM   #9
Gambit
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Bah that post is like Colin's Tne Commandments tablets.
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Old 10-25-2000, 11:58 AM   #10
BongMan
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Thumbs up

That post was crazy long.
Lawrence
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Old 10-25-2000, 09:09 PM   #11
Ken Levin
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I get improved mpg with 92 octane vs. 87 or 89 with my EJ22. Enough to make the added $ worth it.
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