When does it switch from high to low octane timing table?

[n0dih]

How does one know when the PCM switches from one table to the other?

Do I have to copy one to the other to ensure I can tune that particular octane table to nail it down to keep it from switching?

Thanks!

[g5 gxp]

it switches when it detects knock.

[n0dih]

Any tell tales? Status or anything?

[g5 gxp]

yea the scanner shows that your engine is getting so many degrees of knock and it corrects itself by looking at the low octane table and picking a number that's in between the high oct and low oct table to use. at worse case it will just use the low oct table if a lot of knock is detected.

[MMGT1]

When tuning I would copy the high to the low, then subtract 4* from the low table when it's done. If the MAF fails the car will look to the low table as well.
This is important if you are tuning SD.

[BoostedGTA]

Does it switch as soon as it detects knock? How soon will it switch back? The reason for my question is that I'm tuning an 07 Silverado 4.8L and I want it to run from the high table when he's got 93 in there, and the low table when he runs 87 octane. I just wasn't sure if he could expect it to switch that quickly (like as soon as he fills up with 93 after running a tank of 87).

[Texsrt4]

the E38/40 PCMs have what is called a Knock Learn Factor. it is a 0-1 scale, 0 being low octane map 1 being the high octane map. the car will scale itself depending on the knock sensor activity. typically it will take 30-50 miles to swing itself up from the low octane map. you can watch this in the scanner PID section. you will have to right click and select insert, then find the Knock Learn Factor parameter (it will be in the engine - timing retard section)

[n0dih]

You could zero out the low octane table and drive it, it would definately give you a clue when it goes to it and how long, and your datalogger would show it.

[JerryH]

I am not sure on other cars, but on the 98-02 GTPs you can have a consistent 15 degrees of KR for over 2 seconds before it kicks in to the low octane table. A scan will easily show when this event happens, as should your butt dyno (or the chunked pistons rattling should be a dead give-away that your tune was not quite on the ball).

A good tuner will obviously have their car in such a tuned state that this will never happen. So, that said, in the case of a well tuned GTP, your car should NEVER see the low octane table.

[n0dih]

What was wondering if the combination of the high and low octane table was actually a 3D map for the KR so it the PCM always had a defined plot of where to go at any cyl air/rpm relationship.
X = cyl air
Y = rpm
Z = timing between high / low map

i.e. You ping at 520 mg/cyl and 2800 rpm, it would, based on the attack gain, would start to go towards the low octane table, with the low octane table as the min timing value it is allowed to attain. Basically gives it a column of data to draw from, if your timing is 25 degrees on the high and 19 on the low, then it will have 6 degrees of range to work from with a floor of 19 degrees and needing to go lower due to excessive knock would result in audible ping or broken parts.

Maybe I am off, but I did find that when my low octane table was not sufficiently far enough away from the high, I could not control KR at all by tuning only the high octane table the moment I lowered the low octane table, I had control again.

I am not sure on other cars, but on the 98-02 GTPs you can have a consistent 15 degrees of KR for over 2 seconds before it kicks in to the low octane table. A scan will easily show when this event happens, as should your butt dyno (or the chunked pistons rattling should be a dead give-away that your tune was not quite on the ball).

A good tuner will obviously have their car in such a tuned state that this will never happen. So, that said, in the case of a well tuned GTP, your car should NEVER see the low octane table.