Gen IV Octane Spark Table question

[Protec]

On the GEN IV E38 I've seen some people talking about having up to a 10* difference between the high and low octane spark tables (B5101 & B5102). I've also seen where people have these tables exactly the same.

What is the preference on the GEN IV systems? I understand there is supposed to be a blending of the two, but if they are the same, how does that happen?


Respectfully,

John~

[gmh308]

On the GEN IV E38 I've seen some people talking about having up to a 10* difference between the high and low octane spark tables (B5101 & B5102). I've also seen where people have these tables exactly the same.

What is the preference on the GEN IV systems? I understand there is supposed to be a blending of the two, but if they are the same, how does that happen?


Respectfully,

John~

If they are the same that is to stop any chance of using anything else but the intended hi table spark numbers. :)

[swingtan]

To be totally accurate, keeping the tables the same will not force the ECM to only use the "hi" table numbers, it will still blend between the hi and low tables if the Octane scaler is active and greater than "0". However, if the "hi" and "low" table have the same values, the resultant spark value will alway be the same. I know I'm getting really picky though, the the end effect is the same.

Having the hi and low tables the same is fine, as long as the tune is perfect, the conditions the engine runs in are always the same and the fuel quality never changes. If you were full on racing, you could probably use this technique to ensure the engine runs the same all the time. It's not the same as running a daily driver though. The other place I've seen this is for tuning, where you don't want the Octane scalar altering the spark timing. Sometimes shops "forget" to then change the low tables again...

With a daily driver, conditions change and you can't always get the same quality of fuel all the time. This is where running lower commanded spark helps a great deal. Personally in my daily driver, I run about 8' between the hi and low table, but I'll run the same tune when going for a cruise on the open road or towing a heavy trailer 550 KM where I can't get Premium ULP. If you get the "hi" table correct though, you rarely see any knock and rarely have the octane scalar higher than "0"

Simon

[gmh308]

To be totally accurate, keeping the tables the same will not force the ECM to only use the "hi" table numbers, it will still blend between the hi and low tables if the Octane scaler is active and greater than "0". However, if the "hi" and "low" table have the same values, the resultant spark value will alway be the same. I know I'm getting really picky though, the the end effect is the same.

Simon

:) Apologies for poor expression/articulation my part :). Yes the blending still takes place but as x = y & x - y = 0... hi and lo appear to occupy the same quantum space :)

[swingtan]

Yep, that's exactly the way I read it, but just wanted to be clarify ;).

[gmh308]

Yep, that's exactly the way I read it, but just wanted to be clarify ;).

And your description is correct and accurate so clarification worthwhile and appreciated :).

[Protec]

Gents,

Great information thank you both...

Simon - Thank you for taking the time to post that. Very helpful as my truck is a daily driver so I wanted to make sure I was running what was needed.


Best,


John