I forgot to explain something: When gas goes stale, the octane goes down dramatically, so (if the gas is bad) you may have knock problems. That's why the ECU would cut timing, based on the knock sensor readings. This is the same reason that you can get more power out of higher octane gas (with no additional tuning) with a knock sensor equipped engine. The ECU will advance timing, and the higher octane can support it, and the ECU will adjust timing until it detects a problem. When you go back to lower octane, you lose power because the ECU will detect that timing can't be advanced as far before problems are detected.