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Changing Octane

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The DME adjusts ignition advance based on a map that expects 94 Octane. When fuel doesn't meet this, the knock sensors detect and the DME retards individual cylinders as required to stop the knock. The adjustment is effectively instantaneous; when you fill with the higher octane fuel, as the fuel reaching the injectors gets better, so does the advance and resulting power.

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As far as I've been told it can take upto 10 minutes or 15 miles for the ECU to fully adjust to the new fuel..... maybe because the fuel at the bottom of the tank is the old stuff.

Also is there a difference between US and European fuel ratings? My 987S says to use 98RON fuel as a minimum...... our standard grade fuel is 95RON (Octane) with Super being 97, 98 or 99....... I generally run on 99RON that contains 5% Bioethanol and its MUCH better than standard 95RON gas.

Edited by Wads
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Wads --Here in Canada Octane is RON + MON divided by 2 I think.

Where I am we can only get 91 octane "premium" but one chain has 94 octane ethanol blend. I've never tried it because of the vague warning in the manual about ethanol blend gases.

During the summer I go out to the coast where Chevron carries 94 octane- hence my original question.

With your experience I may have to try the ethanol blend.


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