Cruze 1.4 spark table question

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Has anyone ever tuned their high octane table for 91 octane then tuned their low octane table for 87? Can it be done working correctly? The car has a factory turbo so I know I would lose a bit of power running 87 but want the option due to fuel prices or in a pinch. 87 octane is a good $12 to 13 dollars cheaper when filling.

[07GTS]

it always uses the high octane table, its only because of sustained knock that it uses the low octane table, best to tune for the lowest grade fuel u are going to use and just be safer on the higher octane fuel

[sbarisic]

Factory numbers at least on the european models for the high octane table target RON 100 and there's usually a few spots where knock happens anyway. The engine is still advertised as being able to run on RON 95 and it does so by constantly detecting knock and relying on the knock retard to fix things.
Here's a log for a stock map with 95 RON. These are considered normal levels by the factory.

corsa_e_2016.hpl


Now, if you double or tripple the knock recovery rate in the 12744 table, and constant knock happens in a certain RPM range, the computer might learn that number and always fall back to a certain % spark between low and high octane tables and even lower boost.
For example, constant 3 degree retard from 3500 to 4500 can trigger the computer to always request the cylinder airmass at the "-3 degree Spark Retard" column in the knock airmass table, instead the default 0 degree one.
This is basically the "max boost" the ECU is allowed to achieve in relation to the knock level. It requests that much airmass and the final spark is a blend between hi and lo octane depending on the "Knock Learn Factor" number, 0 meaning take hi octane always, 1 lo octane always, and 0.5 is a halfway number between the two.

At least on my car, this resets as soon as you shut the engine off and turn it on again.


You could put small amount of low octane fuel in the car and watch the "Knock Learn Factor" in the log. Increase the sensitivity until the car "learns" the knock factor across the RPM range fast enough and then stays there. That number should stay somewhere above 0 on partial throttle in the critical RPM range (3000-4000 RPM where the max torque by default is) and the least amount of knock happens.

The downside of high enough sensitivity is that it might decrease spark or boost even with good fuel from time to time, it's a fine act of balance.

Personally, i would not bother with trying to run low octane, it's not that big of a price difference. But it can be achieved.
Don't blow your engine up.


Edit:

On that note, a lower heat range spark plug might help with knock prevention in the first place on tuned cars.
This is another experiment on the to-do list for next year! At least for this engine in particular, as it's quite knock limited by default.