Idle spark correction, doesnt seem to be working correctly

[WS6FirebirdTA00]

Here is an example, commanded idle of 1050 rpm and at 1040 rpm actual the idle spark correction is -0.5. If anything, it should be going positive, adding timing to help the rpm. I dont really understand why at this point it hasnt already started to add timing?

Look at frame 1819 (AFR isnt hooked up in this log, so dont look at it haha)

[Frost]

Car isn't moving is it? I don't have the latest on this laptop so I cant see the log.

[WS6FirebirdTA00]

no it's not

[Chris@HPTuners]

Before the PCM can adjust the spark it first needs to measure the RPM error which takes time so its not going to instantaneously have the right spark value for the RPM error you see on the scanner. There is also the sampling and delay from the PCM for the actual class 2 bus scanner data you see which is small but nonetheless exists.

[WS6FirebirdTA00]

ok thanks Chris

[jimmyblue]

I think this correction lag can actually make the idle spark
correction more of a trouble than a help, especially if it has
too much authority. It seems like there is an overcorrection
sometimes and no spark dithering can be smoother than a
spark-correction setup. Spark dithering adds a "choppiness"
it seems like, it keeps the surging down but seems like it is
never smooth.

[WS6FirebirdTA00]

I may try to move the cells over, so its not pulling timing when it needs to be adding it.

[12secSS]

I think this correction lag can actually make the idle spark
correction more of a trouble than a help, especially if it has
too much authority. It seems like there is an overcorrection
sometimes and no spark dithering can be smoother than a
spark-correction setup. Spark dithering adds a "choppiness"
it seems like, it keeps the surging down but seems like it is
never smooth.

Absolutely! This is why on mild to large cam cars I usually allow for 100 (or more) rpm deviation for the spark adder tables. If set to stock, the idle is very lumpy (choppy as Jimmy said), which I do not like. I like smooth idles.

[WS6FirebirdTA00]

on my car, I can get in a surge if I don't allow enough spark correction.

[12secSS]

Right, so let's say you have the correction at 100 and it still surges, but gently and after a bit of time. Then I would shift the tables over to 75 rpms deviation and try it again. This is assuming the RAF is dailed in and that the car could almost idle on it's own (less the surging you feel after idle for a few).

[Chris@HPTuners]

you need to understand that 2 things control the idle

1. the IAC is the "slow" adjust for idle RPM. The IAC can't move fast enough to control idle quickly, also airflow or throttle changes do not have an immediate effect on RPM.

2. spark. spark is an instant way to control RPM within a couple hundred RPM or so. The spark can change very fast and also the impact is immediate.

IAC is used as the coarse trim and spark is used as the fine trim. Is you disable the spark part then you are more likely to get surging as the IAC cannot control idle as tight as spark can.

[WS6FirebirdTA00]

you need to understand that 2 things control the idle

1. the IAC is the "slow" adjust for idle RPM. The IAC can't move fast enough to control idle quickly, also airflow or throttle changes do not have an immediate effect on RPM.

2. spark. spark is an instant way to control RPM within a couple hundred RPM or so. The spark can change very fast and also the impact is immediate.

IAC is used as the coarse trim and spark is used as the fine trim. Is you disable the spark part then you are more likely to get surging as the IAC cannot control idle as tight as spark can.

Exactly. What I did on my car was hold idle spark constant and then hold IAC constant. I did this to see which played the largest, or any, factor in holding steady idle and returning back to idle. The IAC had no effect. The only thing I saw it was good for is being a base to allow the airflow in for an RPM. Other than that, there was really no control benefit.