Highest Octane For Stock ECU??

wstar

I agree that with an older engine/ECU, given a fixed set of engine parameters from the factory, running a higher-than-necessary octane rating is total waste in every respect. However, we know that even on a traditional engine, higher octane fuels allow tuning headroom. When tuning the A:F ratios and the spark timing, you can account for higher octane fuel and get more power that way. The flipside is by tuning for say 110 octane race fuel, you've now made a car that will knock on regular pump gas.

The big question mark with our cars is: given a stock ECU, is the stock ECU smart enough to tune itself upwards for a higher octane fuel automatically? We already know that the stock ECU knows how to de-tune itself when presented with lower-octane fuel by using sensors to detect the onset of knocking and making the appropriate adjustments.

There seems to be some evidence that, when given a much higher octane fuel, our ECU will slow retune itself for higher power using those same sensors as a guide. I find it a bit hard to believe myself, but someone did do a dyno with some 110 race fuel and showed some statistically significant gains a while back.

Edited to add: Even without "oxygenated" fuel, just plain higher octane does allow a higher horsepower tune. By making the fuel more knock resistant, the timing can be advanced further without knocking, which results in more power.

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USNA94

It's my understanding that it "tunes" itself using the knock sensor. There is no octane sensor. 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.

wstar

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.

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:

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.

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USNA94

I agree with all of your points.

As far as the ECU being smart enough to adjust up on it's own I don't know enough about how it works to say. Would be cool if this were true though I wonder what the limit is as you can only adjust timing so much I would think.