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It’s worth discussing how the engine is designed. I’m not familiar with the exact design of the LT1 and LT4, but generally how it works for modern high compression engines is the engine has a knock sensor and when it detects detonation caused by too low octane for the compression ratio, the ECU retards timing to correct for that situation which will be something less than optimal. The net result is lower horsepower. No question about it. The amount might be debatable, but the effect is not. So when someone says, I ran lower octane without knocking, this should be obvious because the engine is designed to compensate. What will be much less obvious is the loss in horsepower. Published horsepower ratings are determined under standard atmospheric conditions at a given RPM. So if you aren’t running your engine at red line, you probably won’t notice any difference, and even in that situation it will be hard to objectively tell the difference without timing something like 0-60, quarter mile, etc. Another objective method would be to measure manifold pressure and RPM which directly relates to HP for a given engine. Putting the car on a dyno, would be another method.
Another aspect is the ECU learns how to adjust timing to prevent knocking. It makes sense the effect will be near immediate if knocking is detected and much less immediate going the other way. I have no idea how much higher octane fuel it takes to return to optimum, but my guess is it’s going to be more than one or two tanks full.
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