Timing Bias?

[2006Cummins]

I was reading somewhere that the timing numbers that are displayed in the tune files are actually not correct. I think I remember something like up to 20* high? Can anyone straighten this out for me? Thank you.

[AdamRRT]

Yeah I'd like to know ahead of time what tables those are. Thanks.

[GMPX]

There is a bit of an unknown at the moment with regards to timing values. There is a calibration in the ECM that appears to put a -20 degrees bias on the final timing figures.
But at this stage I haven't been able to figure out if that timing bias is always applied to the final figure the engine sees or if it's just what is reported by the scantool data. There was going to be an experiment done on a truck with a scope but it's never happened.

[AdamRRT]

Yeah that's not uncommon as I'm sure you know. Many vehicle tables do this, so that they're always dealing with positive integers. I just would like to know what tables have it and what ones don't. It'd be nice to have a list rather than have to determine it each time I open a table.

Thanks for the assistance. I'm loving this!

[GMPX]

It's not quite the same as what you are suggesting (or I misread your post). The Cummins timing values are all signed numbers so it can go positive or negative anyway, the bias is almost like the system has a known delay of 20degrees before the fuel actually begins the injection so therefore they put that in so the numbers we see in the scantool or maps reflect the reality of what happens at the engine (this is just a wild guess).
So to answer what tables does it affect, the answer is all of them because it's more like Final Timing = Timing from all maps - Bias.

[AdamRRT]

Oh I misunderstood then. You understood me well, despite me being mistaken. Thanks for the response. So if we say I'm putting in a value of 5* BTDC, is that really going to be 25* BTDC in the engine?

I just can't imagine like say on the values that have us using a timing of -50*... so we're really beginning injection event at 70* BTDC? HOLY MOLY!

Or am I going the wrong way with it? Just seems like we'd be getting some serious timing rattle. Plus there's a point at which we're spraying outside the bowl if I am correct in my ideas as to why most burned pistons seem to occur (especially Smarty + TNT).

[GMPX]

Oh I misunderstood then. You understood me well, despite me being mistaken. Thanks for the response. So if we say I'm putting in a value of 5* BTDC, is that really going to be 25* BTDC in the engine?

Yes.

As an interesting comparison, if you look at the tables below. Both stock, one being an LMM the other a Cummins, however the Cummins has had 20 degrees removed from the table ('the' bias), interestingly that puts the part throttle timing values in a similar range as the Duramax runs. Unfortunately the X,Y axis's between Cummins and Duramax are opposite so a direct overlay is hard.

[AdamRRT]

Ok, so just to be SURE I get it, the -3 in the upper left corner of the Cummins table...

That is 3* BTDC?
That will give an actual timing of 23* BTDC? (provided the -20 is correct)

I know this is basic, but I've looked at so many tables for the past 48hrs that it's starting to all run together. And I want to be SURE I'm speaking the proper language before I go any further. As I've said, I've used HPT a bit, but every software is just a little different. As is the fact that it was gasoline stuff.

Thanks, Ross!
- Adam

[2007 5.9]

No..-3 is actually 3*ATDC.
0* is 0BTDC..20 is 20*BTDC.

The (-) values indicate timing ATDC

Kinda confusing at first.

~Les

[AdamRRT]

Yeah, only because it's backward. But not. LOL.
So the -3 value means it is asking to begin the event at 17* BTDC?
Sorry just wanna be sure I'm getting this 20* bias thing properly. Basically, I'm asking do we tell it 20* more than we want? Or it RESULTS in 20* more than what we ask for?