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Re: Better performance with lower octane fuel?
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Posted by jsteele [Email] (more from jsteele) on Tue, 22 Feb 2005 11:58:31 Share Post by Email
In Reply to: Better performance with lower octane fuel?, Finch, Mon, 21 Feb 2005 22:27:27
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Is your car turbo or NA ?

I'm no expert, just wondering out loud here. I believe the main reason for higher octane is to prevent detonation which occurs at too lean a A/F ratio or too high a compression. The A/F ratio is optimized by the ECU, and for a turbo the compression is as well. For a tubo, high compression is the whole point, so the need for higher octane is pretty obvious. But is the compression of the NA engines high enough to make detonation an issue ?

Another factor is the temperature of the air. If it's quite cold out, then either a higher compression or a leaner A/F ratio is required for detonation, so there's a larger range of parameters that the ECU can safely explore.

One scenario for the improvement (and I'm really just guessing here) is that the change in fuel somehow jogs the ECU out of a fairly stable set of operating parameters, causing it to "explore" a little more aggressively to find some improvement (a subtle version of what happens after ECU loses its settings altogether). Since the air is (I'm assuming) cold, it doesn't run into detonation as soon and might settle in on a more efficient operating point. Since detonation is such a serious issue, I would expect that the ECU's programming is extra paranoid about it, and under normal conditions wouldn't keep re-checking the boundaries of a known detonation area for slight improvements.

Sound reasonable ?


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