Idle Tuning (spark igntion timing )

I started a new thread although I have posted in another as this is not really related to EGT&CHT tuning.

https://forum.efilive.com/showthread...nd-EGT-and-CHT


So I had set about seeing the effects on EGT by advancing idle spark, however I became more interested in how far I was able to advance timing, conventional wisdom on forums say advance timing until to lowest MAP is observed, retard back a few degrees to allow for idle spark corrections.


I was able to advance timing from 10* to 50* and observe MAP decreasing from average 50kPa to 41kPa, plotting a trend line suggested MAP would level out at 55*.

Fuel consumption at stock 13* is calculated at around 3 litres per hour, reducing to around 2.1 litres at 50*. Air flow reduced proportionally from around 9g/sec to 6g/sec.


I've never seen anyone suggest such high idle timing, in another article which kind of prompted me to do this analysis http://www.carprogrammer.com/Z28/PCM...k%20tuning.htm MAP was observed to drop to 45kPa and rise again at around 34* timing, I have a similar cam 216/229 with .610" lift in an LS2 with LS3 heads.

Has anyone else increased and logged their idle timing, what were the results?

I saw that in your charts, but the MAP chart did not click in my head until now (that you mentioned it)...

What is the slowest idle rpm that your cam likes...?

what are you seeing for egt's at idle? Mines at 800f right now

Quote:

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Originally Posted by joecar

I saw that in your charts, but the MAP chart did not click in my head until now (that you mentioned it)...

What is the slowest idle rpm that your cam likes...?

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When I was calibrating the MAF and VE using 40* idle timing, it would idle around 750 fairly consistent with idle spark correction tables zero'd.

Interestingly, and I forgot about this table in the LS1, B5919 optimal timing "This table is used as a reference for what would be best timing, which is then used for various torque calculations"

Which would indicate 50* at 0.08gcyl @ 800rpm and 38* at 0.16gcyl @ 800rpm

Interesting... so that (lowest MAP) is the basis of B5919.

I think there's a correlation, optimum timing should yield the lowest MAP, for later PCM's I can not locate B5919, I'm sure the table must be in there somewhere as torque calculations are still made. Noted that as bore and ci increase, required timing follows suit.

I understand retarded timings purpose is to keep EGT high and cats hot to meet emissions, id rather save a 1/3 of fuel at idle. I 'd question the logic of burning 1/3 more fuel to meet emissions? CO reduction vs CO produced from the extra fuel?

There is a way to find the GM optimal timing value, just need to additionally log the E40.TORQUECALC.DMA, TRQENG_C

The only consideration I can see for not going too close to lowest MAP is for idle control with AC, power steering load, alternator load etc. If you are too close to MBT and LBT at idle you have no room to take on load.

I log the torque pids and you can make a very accurate looking bhp and torque run with them. So they should tell you about best spark for MBT for sure. Once you get MBT then vary fueling in open loop to get LBT as well.
But as above, this is not what I would do as you then have zero room to vary spark for engine load. You are limited to throttle action which is slow and cumbersome and likely to cause erratic idle.

That being said you are likely to have the lowest airflow at LBT and MBT. Therefore lowest fuel consumption :)

Quote:

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Originally Posted by ringram

The only consideration I can see for not going too close to lowest MAP is for idle control with AC, power steering load, alternator load etc. If you are too close to MBT and LBT at idle you have no room to take on load.

I log the torque pids and you can make a very accurate looking bhp and torque run with them. So they should tell you about best spark for MBT for sure. Once you get MBT then vary fueling in open loop to get LBT as well.
But as above, this is not what I would do as you then have zero room to vary spark for engine load. You are limited to throttle action which is slow and cumbersome and likely to cause erratic idle.

That being said you are likely to have the lowest airflow at LBT and MBT. Therefore lowest fuel consumption :)

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I just looked at the AC engage /disengage in the calibration, it allows for 20NM, I need to plot the calculated torque to find out how much timing that would require and reduce back from MBT, bummer if its 13* lol

After studying the available E40 idle calibrations, I conclude that the torque reserve needs to be at least 13* {B1530} below MBT with -11* over speed reduction {B5131}.

The calculated torque is based on throttle angle vs RPM {B2513}, there isn't an optimal timing calibration like the LS1.

The extra load from AC auxiliary is compensated by adding airflow to the minimum idle airflow calibration {B1829}, opening the throttle further, there is a delay timer between switching on and AC on to allow rpm to increase accordingly. There is no viewable calibration for power steering load.

{B1829} can be considered to be the same as a stop screw on a carburettor, the minimum TB angle.

An idle tune strategy might be if your have changed cam and heads (or improve fuel consumption over stock),

Determine idle MBT and airflow, log airflow closed loop and trims to correct the airflow, MAP, fuel IPW and or Litres per hour and electronic throttle position, in efilive you can create a custom calc pid to increase the resolution for the TB (8/10ths per 1%), while increasing idle spark * incrementally until minimum airflow/MAP/IPW/TB% / diminishing returns.

Create a table to display airflow per 0.5* timing advance

Decide on the optimal idle timing value at least 13* back from MBT, set the minimum idle airflow to the idle timing value +13* in {B1829}. If more torque is required to maintain idle rpm, idle spark increases, the TB angle needs to decrease a little (more torque from less airflow), if minimum idle airflow is too high, TB can not close further so rpm will increase until it finds equilibrium state.

Test these settings with AC on /off, turn steering wheel to full lock and check for stall.

If it stalls, greater torque reserve is required, decrease idle timing repeat above.

So GM.AIRPERSEC is whichever of the other two (MAF, VE) the PCM is using at that moment...?

Quote:

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Originally Posted by joecar

So GM.AIRPERSEC is whichever of the other two (MAF, VE) the PCM is using at that moment...?

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Correct


Update: This is not completely correct, although the attached graph is correct AIRPERSEC follows either DYNAIR of MAF when not in dynamic air mode, when in dynamic air mode its a little confusing, AIRPESEC (and E40.APCYL, not the same as AIRPERSEC) follows which ever is reading higher, but not exactly, however the trims are applicable to DYNAIR.

Ooops:music_whistling_1:

GM.DYNAIR_LS2 of course is the dynamic air flow, in the attached graph over 5 logs of corrections, you can observe that the trims correction to DYNAIR (blue bar) result (blue wavy bar) in the first log was spot on, as all airflow pids arrived at that value in logs 4 and 5. I was correcting to AIRPERSEC.

A diagram that might help understand idle air and spark