Not getting full Spark Advance on WOT runs. Why?

[SiskMan]

I am commanding 29 degrees towards the top end of my high octane table. My low octane table is .85 of the high table. When I make a pull it sees a max of 26.5 degrees or so. I don't get what is happening.

Burst Knock is zeroed out on both tables. I get no KR during the run at all. But the timing is sort of a mix between the high and low tables. To test my theory, i copied the high octane table into the low. And now I get full advance at WOT. I don't think you want to leave the high and low tables the same do you? Im running a MAF tune and this is the last step to get the tune dialed in. LTFT are +-1 and the STFT are really close as well. PE table matches the wideband on WOT runs.

What tables do I need to adjust to have full advance?

[5FDP]

Post the tune, someone might see something you aren't.

[SiskMan]

tune1.hpt

Sorry, I should have posted that to begin with.

[Dweezil427]

It remembers that you have had knock and interpolates between the high and low octane tables. Eventually it works its way back to the high octane table after it sees a history of less knock. By making the tables the same you tricked it into not being able to do that. I think it is called knock learn or something like that. I believe you can reset that.

[jtrosky]

You can also log Knock Learn in order to see when it is in effect and how much it's going towards the Low Octane table. I think it's different on different platforms - some platforms it starts a 0 (full high ontane values) and works it's way up to 1 (full low octane values) as it sees knock and other platforms it starts at 1 (high octane table) and works it way down to 0 (low octance table) as it sees knock. Logging the Knock Learn PID will show you exactly what it's doing.

Everytime you write a new calibration, it resets the knock learn values. They will also go back towards the high octane table in teh absense of knock. It keeps track and applies knock on a per-RPM/cylinder-airmass basis. So it only applies the timing reductions in specific areas when it's seen knock previously.

[mowton]

For a quick check, set the Lo octane equal to the Hi Octane and see if the timing returns, if so then it is some sort of knock scaling as stated above.

Ed M

[Mep_q8]

Also check for any knock sensor DTCS. If you have any, the ECU automatically switches to the LO tables for safety.

[SiskMan]

Thanks for all the help everyone. Someone on another forum suggested that if it kept happening, i could copy over the WOT values in the high table to the low table. But keep the rest of the low table just in case I get some bad gas or the wife puts 87 in on accident.

[SiskMan]

You can also log Knock Learn in order to see when it is in effect and how much it's going towards the Low Octane table. I think it's different on different platforms - some platforms it starts a 0 (full high ontane values) and works it's way up to 1 (full low octane values) as it sees knock and other platforms it starts at 1 (high octane table) and works it way down to 0 (low octance table) as it sees knock. Logging the Knock Learn PID will show you exactly what it's doing.

Everytime you write a new calibration, it resets the knock learn values. They will also go back towards the high octane table in teh absense of knock. It keeps track and applies knock on a per-RPM/cylinder-airmass basis. So it only applies the timing reductions in specific areas when it's seen knock previously.

Yea I did log knock learn during one of the runs. And it started out at 0 and went to a higher value towards the end of the run, so what you say makes sense. I'll just have to keep messing with it

[jtrosky]

Personally, I would be very hesitant to make the high and low octane tables the same - especially in the WOT areas. You'd essentially be ignoring knock at the point since the timing will not lower if needed due to knock. I know that some poeple will make the tables the same during a run just for tuning purposes, but you don't want to leave them that way. Knock is especially harmful @ WOT - so you don't want to ignore it if it is occurring! I'd concentrate on identifying the cause of the knock instead of just trying to avoid the results of it.

[THE CHAD]

Also if not getting the Commanded timing check your ECT/I.A.T temps and then have a look under the Spark Advance TAB (spark correction- adders/reducers I.A.T/ECT) Cheers Bret

[Pekka_Perkeles]

Personally, I would be very hesitant to make the high and low octane tables the same - especially in the WOT areas. You'd essentially be ignoring knock at the point since the timing will not lower if needed due to knock.

Not true.

[67firebird455]

I had a problem last night with AFR Spark Correction adding timing to my setup, definitely check out ECT/IAT as previously mentioned.

Also - I have my High/Low octane tables set to the same, but am boosted, and am under the impression that once the MAF is failed for SD that it reverts to low anyway.

[jtrosky]

Not true.

Just to clairfy - while the computer will still retard when it sees knock, it won't atually "learn" about the knock that it sees if the high and low octane tables are the same. So it may repeatedly knock every time it sees that situation (airmass/RPM combination). Whereas if the low octane table was lower than the high octance table, the computer would automatically reduce timing in areas were it previously experienced knock, thereby avoiding the knock completely. So if you have the low and high octane tables set to the exact same values, you'll most likely experience more knock overall than if they were setup properly.

The low octane table is a "safety net" in case the timing is set too high or you get bad gas or situations like that. There is a reason why they have high and low octane tables. The correct way to fix the problem (in my opinion) is to fix the cause of the knock- not just set the high and low octane tables to the same values to avoid timing reductions. Besides, since the engine will still retard the timing if it sees knock (even if the high and low octane tables are the same), you'll still get the timing reductions anyway - it's just that you'll also experience more knock in the long run.

At least that is my understanding. Please, if that is incorrect, please correct me!

[Pekka_Perkeles]

Just to clairfy - while the computer will still retard when it sees knock, it won't atually "learn" about the knock that it sees if the high and low octane tables are the same. So it may repeatedly knock every time it sees that situation (airmass/RPM combination). Whereas if the low octane table was lower than the high octance table, the computer would automatically reduce timing in areas were it previously experienced knock, thereby avoiding the knock completely. So if you have the low and high octane tables set to the exact same values, you'll most likely experience more knock overall than if they were setup properly.

The low octane table is a "safety net" in case the timing is set too high or you get bad gas or situations like that. There is a reason why they have high and low octane tables. The correct way to fix the problem (in my opinion) is to fix the cause of the knock- not just set the high and low octane tables to the same values to avoid timing reductions. Besides, since the engine will still retard the timing if it sees knock (even if the high and low octane tables are the same), you'll still get the timing reductions anyway - it's just that you'll also experience more knock in the long run.

At least that is my understanding. Please, if that is incorrect, please correct me!

Much better now!

High octane and low octane tables are there for, well, for higher or lower octane gas.

If and if there would be a sensor detecting the octane itself (a bit like there is a sensor for detecting E85/alcohol ratio in some GM cars), there wouldn't be a need for octane-related functionality in a GM PCM.

Now, for car enthusiast, using low octane gas doesn't make sense. But at the same time, realtime detection of knock is very useful.

It is of course a tuner and car owner preference, but I personally don't see a point having a high and low octane tables using different values. Just tune the engine right, have a proper load test and let the GM algorithm for knock detection to do it's best.

- knock detection - and reaction to it - happens realtime

- usage of high/low octane tables and a percentage between those is a slow(er) PCM learning process

Two totally different things and for different purposes.

Never disable knock detection (except maybe some race engines with gear-driven cams or so)! But with high and low octane tables, well, use your own judgement.

[Pekka_Perkeles]

Also - I have my High/Low octane tables set to the same, but am boosted, and am under the impression that once the MAF is failed for SD that it reverts to low anyway.

Yes, unless you have 2 bar (etc.) Custom Operating System.

[jtrosky]

Like you said, I guess it all depends on the specific use of the vehicle. For a "daily driver" car, I just feel better having some "breathing room" between the high and low octane tables. Even if my car was tuned perfrectly and I used 93 octane fuel all of the time (which I do) you can always get a bad tank of gas or something like that. I'd rather the car have the ability to reduce timing and "learn" from the knock, just in case. I mean if the car in tuned perfectly, then it would rarely be knocking anyway, so the low octane table shouldn't come into into play much, right? :-)

Like you said, owner preference I guess, depending on their use of the car.

[Pekka_Perkeles]

I mean if the car in tuned perfectly, then it would rarely be knocking anyway, so the low octane table shouldn't come into into play much, right? :-)

So true.

But let me give you an example of an owner preference.

Let's say a certain engine combo is tuned with certain gasoline and let's say it does not knock even under proper load test at dyno.

Now, this engine at a road racing track and with active knock sensors may indeed have (and PCM detect) knock. Which is good for PCM to notice.

But for a road racing car/owner, it is not a good thing if the knock is based on a false mechanical noise.

However, me as a tuner, or a driver of the car, don't really know what is the case. Knock is real or false.

With this kind of combination, I would not want to use low octane table. Because me and the owner wants the car to perform. And at the same time, to react when the knock is detected. But not to stay with the low octane values (table) longer than needed. Thus having the high and low octane tables identical and just let the realtime knock detection to lower the ignition advance as long as necessary.

Like you said, owner preference I guess, depending on their use of the car.

Yep.

[mowton]

Personally, I would be very hesitant to make the high and low octane tables the same - especially in the WOT areas. You'd essentially be ignoring knock at the point since the timing will not lower if needed due to knock. I know that some poeple will make the tables the same during a run just for tuning purposes, but you don't want to leave them that way. Knock is especially harmful @ WOT - so you don't want to ignore it if it is occurring! I'd concentrate on identifying the cause of the knock instead of just trying to avoid the results of it.

It was only recommended as a test to see if that was where the timing was being removed.....not permanent

Ed M

[justhereforinfo]

While we're on the subject. I recently had some poor quality fuel, and noticed it right away. Did some logging and see the timing is being pulled, for a couple different reasons. One being IATs are too high (cause I'm in florida, and its freakin hot), and the other reason is still a mystery to me. I assume, that knock was detected, the knock learn feature reverted to the low table. Fine, since I am no longer seeing knock (never actually did catch it in a log), and have replaced fuel with a tank of 93 and octane booster, I copied the hi to low. But I still don't get everything I command. Of course that's due to the IAT pulling some, but I should still be seeing another degree or two.

How can I reset the knock learn timer? or whatever its called, so that it forgets there was knock...