Trying to walk back how spark advance is calculated based on VCM Scanner data.

[fumanchu182]

So I'm new to this, and have been trying to piece it together on my own. Here is the data, that I logged on my 2015 Challenger Hellcat.

RPM: 4382
Aircharge: 1.20(g)
Spark ADV: 17.0
Spark ACT: 17.0
IAT Advance: -1.5
Accel Pedal: 3.83v
Timing Advance (SAE): 17.0
ECT: 207F
ACT: 138F
IAT: 129F

So looking at the beginning I started with the WOT pedal threshold, 3.83 > 3.19 so it is definitely using the WOT Spark table (44361). So looking at the aircharge, I go to 1.20 and cross reference it with 4320 column as that is the closest value (is this correct?) and pull 18.5 as a value for degrees of advance. From there I look at the temperatures. ECT is below 244 so I don't think that the ECT Spark Hot (32094) table comes into play (even with the deadband of 244-32=212, this should still not come into play). Then I look at the IAT and see it is at 129F, so the IAT Spark Hot table (32124) should not come into play. Where is the IAT Advance of -1.5 coming from? What table am I missing that would have this value?

Also am I thinking about walking this back correctly, didn't see a formula posted about this.

logging_spark.jpg

Sorry if this has been covered before but I didn't find anything in search.

[B00STJUNKY]

So I'm new to this, and have been trying to piece it together on my own. Here is the data, that I logged on my 2015 Challenger Hellcat.

RPM: 4382
Aircharge: 1.20(g)
Spark ADV: 17.0
Spark ACT: 17.0
IAT Advance: -1.5
Accel Pedal: 3.83v
Timing Advance (SAE): 17.0
ECT: 207F
ACT: 138F
IAT: 129F

So looking at the beginning I started with the WOT pedal threshold, 3.83 > 3.19 so it is definitely using the WOT Spark table (44361). So looking at the aircharge, I go to 1.20 and cross reference it with 4320 column as that is the closest value (is this correct?) and pull 18.5 as a value for degrees of advance. From there I look at the temperatures. ECT is below 244 so I don't think that the ECT Spark Hot (32094) table comes into play (even with the deadband of 244-32=212, this should still not come into play). Then I look at the IAT and see it is at 129F, so the IAT Spark Hot table (32124) should not come into play. Where is the IAT Advance of -1.5 coming from? What table am I missing that would have this value?

Also am I thinking about walking this back correctly, didn't see a formula posted about this.

logging_spark.jpg

Sorry if this has been covered before but I didn't find anything in search.

It's possible (and likely) that the IAT timing modifier is more dynamic than you're thinking, in that it doesn't just wait until the Hot Reference Temperature is reached before it pulls 8.26* of timing at your referenced RPM vs aircharge. But, instead, it interpolates between the Part Throttle Reference temperature of 113* (where the base table was calibrated, and it may be safe to assume the WOT base timing table was calibrated as well) and the Hot Spark Reference temperature.

So, your Hot threshold is 212*, and your Base table reference is 113*, and your actual temperature at that point of your log was 129*. 129 - 113 = 16. 212-113 = 99. 16/99 = .1616161616...

I interpolated for the exact RPM and Aircharge that your log shows within your IAT Hot Spark table, and the results show that it would have been referencing a value of -8.26875*.

Multiply the interplation factor we determined above by that timing value, and it should be pretty close to what you're seeing for IAT timing modification.

.16161616 * -8.26875 = -1.33636* of IAT correction.

Consider that just before you went WOT, your IAT temps were ever so slightly higher than the exact moment you logged, and it's reasonable to believe there's enough lag in the scanner polling interval that the values you see have some small margin of error, the PCM could have been correcting for 131* IAT temps at the moment of the log capture.

131-113 = 18 18 / 99 = .18181818

.18181818 * -8.26875 = -1.5034* of IAT correction.

[fumanchu182]

Okay so I'm going to try your calcs myself on a different set of values, hopefully my maths don't fail me now!

Data:
RPM 5450.00
Airgcharge (g) 1.31
Spark ADV 17.00
Spark ACT 17.00
ACT 141.80
ECT 207.00
Air Temp 95.00
IAT 100.00
Speed 104.00
IAT Advance -1.50
Timing Advance 16.50
Hot Spark Ref IAT 212.00
Cold Spark Ref IAT 14.00
Hot Spark Ref ECT 244.00
Cold Spark REF ECT 176.00

IAT - PTRIAT
100 - 113 = -13 (should this be an absolute value?)
212 - 100 = 112

-13/112 = -0.11607142 (absolute value again?)

Interpolating the exact RPM and Aircharge from IAT Hot Spark Table (bilinear or trilinear?, i'm lazy bilinear also don't know how/have a tool to do a 3d lookup table)
-8.625 = ((1.20 - 1.26)(-6 - -7.50)/(1.34-1.26)) + -7.50

0.11607142 * 8.625 = 1.0011* of IAT correction.

This seems way off from 1.5 in the log.

[B00STJUNKY]

Okay so I'm going to try your calcs myself on a different set of values, hopefully my maths don't fail me now!

Data:
RPM 5450.00
Airgcharge (g) 1.31
Spark ADV 17.00
Spark ACT 17.00
ACT 141.80
ECT 207.00
Air Temp 95.00
IAT 100.00
Speed 104.00
IAT Advance -1.50
Timing Advance 16.50
Hot Spark Ref IAT 212.00
Cold Spark Ref IAT 14.00
Hot Spark Ref ECT 244.00
Cold Spark REF ECT 176.00

IAT - PTRIAT
100 - 113 = -13 (should this be an absolute value?)
212 - 100 = 112

-13/112 = -0.11607142 (absolute value again?)

Interpolating the exact RPM and Aircharge from IAT Hot Spark Table (bilinear or trilinear?, i'm lazy bilinear also don't know how/have a tool to do a 3d lookup table)
-8.625 = ((1.20 - 1.26)(-6 - -7.50)/(1.34-1.26)) + -7.50

0.11607142 * 8.625 = 1.0011* of IAT correction.

This seems way off from 1.5 in the log.

Indeed. In this scenario, I would have expected the IAT correction to be 0. Since your IAT temps are below the PT Reference temp, it would be subtracting actual from reference, then dividing by the range from reference to cold to get your factor.

IE. 113 refIAT 100 actualIAT

113 - 100 = 13

Divided by range of Ref to cold

113 - 14 = 99

13 / 99 = .13131313 (this would be the correction factor applied to the Cold Spark IAT table, but since it's zero'd out, it would equate to a zero correction).

Does your IAT Spark value every change from -1.5?

Also, there's the fact that MAT is selected as source, which I believe they average IAT with ECT with some sort of bias/blend factor to come up with a calculated MAT temperature.

I wonder if ACT is equal to MAT? That might follow...

[B00STJUNKY]

Indeed. In this scenario, I would have expected the IAT correction to be 0. Since your IAT temps are below the PT Reference temp, it would be subtracting actual from reference, then dividing by the range from reference to cold to get your factor.

IE. 113 refIAT 100 actualIAT

113 - 100 = 13

Divided by range of Ref to cold

113 - 14 = 99

13 / 99 = .13131313 (this would be the correction factor applied to the Cold Spark IAT table, but since it's zero'd out, it would equate to a zero correction).

Does your IAT Spark value every change from -1.5?

Also, there's the fact that MAT is selected as source, which I believe they average IAT with ECT with some sort of bias/blend factor to come up with a calculated MAT temperature.

I wonder if ACT is equal to MAT? That might follow...

Nope, that's not it either. Hot IAT spark table, interpolated for aircharge and RPM would equate to -0.4375* @ full scale hot temp reference. Apply the scaling factor, and it would only be:

141.8 - 113 = 28.8

28.8 / 99 = .2909

.2909 * -.4375 = -.1273*

So, back to the other question, does IAT Spark ever move from -1.5?

Edit: Please hold, I just realized I referenced ECT hot spark table rather than IAT hot spark. Recalculating.

OK, referencing the correct table results in a full scale hot temp reference of -5.96484* at the exact RPM vs Aircharge axis.

-5.96484 * .2909 = -1.7352*

Still not what I would expect given the theory of how it's all calculated, though not nearly as far off when using ACT instead of IAT. Still, I don't know how much the deadband settings affect things, so this may still be plausible.

[B00STJUNKY]

fumanchu182, reference my updated post above for the thought process on this. Still don't know if it's solid, but it would be interesting to flip the toggle from MAT to IAT to see how IAT Spark correction reacts. I suspect ACT might be synonymous with MAT.

[fumanchu182]

Scanning the whole log I have seen the following IAT Advance values: -3.00, -3.50, -2.00, -.50, -1.50, -1.00.

I have attached the log for anyone looking to extract more data.

Data:
RPM 4731.00
Airgcharge (g) 1.22
Spark ADV 17.1
Spark ACT 17.2
ACT 138.2
ECT 207.00
Air Temp 97
IAT 127.59
Speed 104.00
IAT Advance -2.0
Timing Advance 17.0
Hot Spark Ref IAT 212.00
Cold Spark Ref IAT 14.00
Hot Spark Ref ECT 244.00
Cold Spark REF ECT 176.00

IAT - PRTIAT
127.59 - 113 = 14.59
212 - 113 = 99

IAT Hot spark interpolation:
4320 -> 4800 @ 1.17 = -8.14375
4320 -> 4800 @ 1.26 = -7.428125

Now across the row axis gives me: 1.22 @ -7.746180555555555

14.59/99 = 0.147373737373737

0.147373737373737 * -7.746180555555555 = -1.14158357884399

20-07-12 14-04-26.hpl

[fumanchu182]

fumanchu182, reference my updated post above for the thought process on this. Still don't know if it's solid, but it would be interesting to flip the toggle from MAT to IAT to see how IAT Spark correction reacts. I suspect ACT might be synonymous with MAT.

Will try when I get an unlocked PCM, right now I am just working it back for my own edification. Also for when and if I tune the Hellcat. Thank you for your time. I'm going to spend tomorrow creating an excel to plot values in that will help out with the calculations so we can see the data sets.

I am learning that the resolution of the logs is limited but that's okay, gotta start somewhere.

[fumanchu182]

Went and did some studying, so the way the IAT and ECT tables are supposed to work. So IAT Spark has a few tables and from what I've studied/seen this is how they work. You have a Part Throttle Ref IAT (44664). Mine is set to 113 degrees farenheight(F) from the factory. So this means that whent his car was tuned 113F is where no spark was needed to be added or removed. I'm assuming this is because compressed air has heat and superchargers are for all intents and purposes a heat sink... There is a modifier to this table called the Hot Deadband (32122), set to 32F. So from 113F to 145F no spark is removed using the IAT Spark Hot (32124) table. Enter, Hot Spark Ref IAT (44665), which is a temperature set at 212F. From 146F to 212F the computer will use a percentage of the IAT Spark Hot table values in the cells (probably interpolated across temperature). At 213F and greater it will use the whole value in the cell.

ECT follows a similar pattern but what confuses me is that the initial value is 212F and the deadband added is 244F which is exactly the value of the Hot Spark Ref ECT (44662) entry. So it's acting as if it wants to use the whole value in that cell in the ECT Hot Spark Table (32094) immediately (I guess overheating is a more extreme scenario than IAT being too hot) and I fully understand why.



Now lets look at the Baro Spark table (44667), this should modify spark based on the barometric pressure. On a Hellcat that is stock, which mine is, this table should be zeroes across the board and mine is so this mathematically can't add/subtract from the equation.

However walking it back in the example and yes we are WOT.

RPM: 2202
Aircharge (g): 0.79
Spark ADV: 12.5
Spark ACT: 11.9
ACT: 140.0F
ECT: 205F
Air Temp: 91F
IAT: 131F
IAT Advance: -3.0

The IAT and ECT are no where near the values they would need to be for these two tables to be kicking in. I'm going to do more digging but I don't think the IAT Spark Hot or the ECT Spark Hot tables are coming into play. However this is my first foray into this so who knows, I would love to be wrong (at least I'd learn something).