"We always want to tune on the best octane fuel that's available to us". But, do we really?

Hello.

The quote in the title has been taken from the "Understanding Knock" section of the "EFI Tuning Fundamentals" course. The reason I'm writing this post is because I'm not understanding the rationale behind that statement.

In Europe, the commonly available pump gas can be found in 95 RON or 98 RON. 100+ RON exists in select fuel pumps.

Consider the engine was tuned on 100+ RON, and the ignition table was adjusted according to that, and the engine did not present itself to be knock limited.

What happens when one fills up on 95 RON? What if the engine then presents itself as knock limited, and the ignition timing is no longer adequate, and too far advanced?

What my intuition tells me, is that the car should be tunned with the lowest octane fuel the car is expected to be used with. That way, if a higher octane fuel is used, no problems happen (beyond maybe some lost performance).

Am I thinking wrongly?

Thank you.

What will happen if you fill it with 95RON and it becomes knock-prone (limited)? You will have a very sorry-looking set of pistons pretty quickly and a rebuild very shortly after, if driven in the knock areas, unless your knock control is set up properly and aggressive to remove enough timing to save the engine.

What I always tell my customers is to sacrifice dyno numbers for safety, I will tune their car on the lowest octane that they will ever run. If they intend to run octane boosters I will tune without them. As you've said, it means when they're running their normal fuels, they have an extra bit of headroom for safety.

If they are after dyno numbers, then the fuel will be dosed with as much as we can to hit a figure, then have the tune pulled back to a safer level

Ask the customer what fuel they intend to run the car on, and then tune it accordingly. If the customer says they intend to always run it on 98RON, and I tune it for 98RON, then the customer fills 93RON and destroys the engine, then that's on them. The tuner cannot be blamed for that whatsoever.

Explain to the customer the importance of always running the car on 98RON or higher (if available) as this is what the engine was tuned on, make sure they understood that very very well, and just leave it with them. 9 out of 10 times, knock detection and timing corrections will save the engine if well setup.