Octane and timing

[Kyp J]

Does anyone know how long it takes or what conditions it takes to get the ECU to advance the idle if you put in enough high octane gas to significantly boost it by several points (when you get to the track for instance)? I am under the impression that the ECU pulls timing if you get knock, but how does it know you jacked up it's octane?

[Tom @ Fast Forward]

Once you get sufficient octane for ping/knock to disappear, the ECU has no idea what the octane is. The ECU will try and mack the most advance it can without knock/ping. Under cruise conditions that is up to 30-35 degrees. Under WOT, that is typically ~24 degrees. Octane should have zero effect on idle as it is impossible (?) to have ping at idle. At idle, the ECU looks at the AC compressor on/off, TPS, Front O2 Sensor and MAF to make it's settings.

[Kyp J]

So is it constantly trying to advance till knock thus giving almost constant knock conditions or does it wait for: one or several cold starts, some delay period, every third stoplight, putting it in reverse and rewinding the ECU spring?

[Kyp J]

I just read my original post. I didn't mean to say "advance the idle". I meant the spark advance. Damn, maybe I am an idiot. I didn't believe it when they told me.

[Tom @ Fast Forward]

So is it constantly trying to advance till knock thus giving almost constant knock conditions or does it wait for: one or several cold starts, some delay period, every third stoplight, putting it in reverse and rewinding the ECU spring?

Actually, it isn't quite that way. The ECU has different limits under different circumstances. Under WOT, for example, it is 24 degrees (as far as I can tell). If it makes 24 with no ping, it stops there. If it detects ping it drops back off 24 until it stops the ping. It doesn't just keep increasing until it finds ping. It is an instantaneous change and is under constant correction. The key is to avoid ping and let it make the max timing.

when it pulls timing to avoid knock, it is always on the verge as it only pulls enough to stop the knock.

[Kyp J]

Its starting to make sense to me now. If I had a brain I might have been able to figure it out. If I could restate it in my version, tell me if I have the right idea.

The old mechanical advance always advanced the same way for various rpms as the engine changed speed the advance changed.

With the computer, it has to change advance with rpms but does it with and ear for knock so every rpm increase, it tries for max (unless other considerations limit it) so from my earlier question, "when does it react to higher octane?" it reacts the next time you push on the gas pedal.

My understanding of a car's computer program is very limited.

[Tom @ Fast Forward]

It reacts instantly to higher octane. Well, as soon as the higher octane gets through the fuel line and into the engine. The computer doesn't know octane, only knock and as soon as the higher octane gets there, the knock goes away. Think of the knock detector only as a safety valve.

Now, if you are running 91 octane and there is no knock and the timimg is maxed out and you switch to 100 octane, what happens? Nothing. Absolutely nothing. Can you go faster on 100 over 91 if there is no knock on the 91 and timing is maxed out? Nope. Well, maybe. :) If you have higher octane fuel, and a method to advance the timing, you might get a few more ponies out of it. Is it worth the trouble and expense? Probably too few ponies to make it worthwhile. Easier to up the pulley size and make more ponies that way. Increase boost until you can't stop the ping and then back off 5mm. :)

[Kyp J]

You need to invent a feature for the blowers to make ping smell for deaf people. I can't hear ping, especially over the noise of everything else. That way if I have a passenger and I have body malfunction I can blame it on the car since I don't have a dog.

[Tom @ Fast Forward]

Just tell them it was the dog. That's what i do and we don't have a dog.