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Originally Posted by USNA94
It's my understanding that it "tunes" itself using the knock sensor. There is no octane sensor.
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We agree on this, although I think the new knock sensors are supposedly picking it up much earlier, when it's a tiny hint of a knock. You don't actually have to get to where you're risking damage to have the sensor "see" it and adjust.
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When it detects the onset of knock using the knock sensor the ECU throttles back thereby reducing performance. If there is no knock when running 91 octane and definitely no knock when running 101 octane, the ECU would never know the difference and run the same. It doesn't open up the throttle any more or provide more power. It just knows there is no knock happening. And the energy content of the gasoline does not go up with octane. 91 octane fuel has ~32MJ/liter of energy, 101 octane fuel has ~32MJ/liter of energy, 121 octane fuel has ~32MJ/liter of energy so the power of the engine is the same. The higher octane will allow you to run a modified engine with a higher compression ratio to get more power though.
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Here's where we potentially differ (not that I'm solidly in the other camp either, I'm just playing out their side of the argument as best I can, and it's reasonably sound, but has question marks):
The way that the ECU responds to knock is not to "throttle back" literally with the throttle, but to retard the timing, which eliminates the knock at the cost of performance and efficiency. You mentioned this yourself in an earlier post, but I just wanted to be clear on that issue:
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However, burning fuel with a lower octane rating than required by the engine often reduces power output and efficiency one way or another. If the engine begins to detonate (knock), that reduces power and efficiency for the reasons stated above. Many modern car engines feature a knock sensor which detects knock, and then sends a signal to the engine control unit to retard the ignition timing. Retarding the ignition timing reduces the tendency to detonate, but also reduces power output and fuel efficiency.
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Similarly, if you put 110 race gas in a car that was designed and tuned for 93, there's lots of "knock headroom", so to speak. With the higher octane fuel in the tank, a tuner can go into the ECU parameters and advance the ignition timing more aggressively than stock to create more power than was possible on 93 octane due to the higher resistance to knocking the 110 fuel has (the inverse of what we described above, where retarding it reduces power and helps avoid a knock). There's still only X MJ/liter of energy in the higher-octane gas, but by being more knock-resistant, it allows a more aggressive spark timing advance, which results in increased horsepower.
I *think* we both agree on the above - that by tuning the ignition timing manually with an ECU programmer up as far as you can without knocking, on any modern-ish car where that is possible, you can get more horsepower out of 110 gas than 93 gas, even though they both have the same MJ/liter of potential energy within them.
The question mark is whether the ECU is smart enough to do this by itself. Some have claimed that when it detects absolutely no hint of knocking, it slowly advances the timing more aggressively until it finds the first hint of it, and thus self-adjusts upwards (more aggressive spark timing, more power) for higher-octane fuel in much the same manner that we know it self-adjusts downwards for crappy fuel.
Nobody has put out any hard evidence that this is true, but people have mentioned it, and the 110 race gas dyno results seem to indicate that *something* is going on, and this explanation kinda fits the bill.