I'll try that. I really don't want to have to switch to 91 for the summer though. . .
So it is common then? I googled and found this:
Quote:
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In the timing maps the closed loop region, also called the ‘knock zone’, is only in a narrow stretch of RPM range in the high load area. This is because the knock sensor cannot distinguish between detonation and normal engine vibration. But in the event poor gas is put into the car or you are simply pushing the limits too far, the ECU will attempt to save the engine by retarding the timing when it detects detonation.
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It wasn't specific to the Impreza, but seems to apply.