Just filled up the tank with 91 octane premium fuel

[jkuchta]

Originally Posted by 07WYarisRS

yeah we no it will allow it to advance the timing more
BUT why does it advance the timing? What is it that tells the ECU it needs or can go with more timing?

The engine MUST advance the timing more because the 02 sensors are picking up incomplete combustion from too high of octane and a rich air/fuel ratio...
by advancing the timing it creates more heat to help with more efficient combustion...
this is what I have been sayinall along....
the overall power remains the same because the advanced timing is just countering the effects of the overly rich mixture that would be reducing the power output.

Thats why I said if you ran them both in an engine with fixed timing the lower octane will always make more HP, in order for the engine thats running high octane to make the same power you must change the valve timing and ignition timing for more compression/different overlap and earlier spark for a more efficient combustion...
This is why when i'm setting up race engine we start out rich, watch the EGT and CHTs as well as power output. Then start leaning things out. if the CHT's spike but EGTS drop the engine is starting to detonate. if a higher octane is needed we usually have to adjust the timing as well to bring the power back that we lost.

But thanks for proving my point

That's an interesting idea...but a little out in left field..the ECU is programed to run the most advanced timing it can get away with without detonating...that's why the timing is more advanced with the higher octane.
The reason you would want to run the timing as advanced as possible is that it will lead to higher cylinder pressures, which equal more torque.

The octane rating of the fuel has no bearing on the mixture setting, other than the fact that with a more knock-resistant fuel you may be able to run a little leaner without problems.