On the VE OS the max idle TP% is 18.8% and on L76 it is 21%. Which the later is more ideal for a cam swap.
There is no desired airflow which is a commanded TP% and then the pcm trims the idle from the desired position.
The E38 has a low limit and upper hard limit. Any min idle airflow figures above b1651 are ignored so it will command 18.8%(factory L98 settings). I have my idle perfect now. The trick is to use B1651 to find a desired airflow whilst zeroing the airflow correction tables. Run the min idle airflow tables to around 25g/s so they are ignored. And then manipulate b1651 until a steady idle is reached with timing in the correct region(say 20 degrees). Once you have found a desired value for b1651 "eg 2.20 area which is 21%" then turn on the aircon and see how it reacts. At this stage leave timing correction on. 2nd stage is to enable correction tables. Then set max idle area so it is slightly higher then the desired amount which may raise TP% upper limit to 21.4%. Also reenable to standard min idle airflow tables. Start the engine and watch the commanded TP% on coast down (which will always be max. idle area). Take notice of how rpms dip, also taking into consideration of timing correction and settings. Check it a/c on and off as it will dip slower a/c off.
Once you are satisfied with the upper limit find the lower limit. Which can be done driving a/c off and with stock correction tables enabled. Start the engine then log after the idle has settled, Go for a drive and then experiment to find what the lower limit is. You dont want it to low as if it overshoots on correction TP% will only drop so far. You only need a narrow gap in TP% limits. An example would be to have a lower limit of 20.0% and upper limit of 21.5%. Which is plenty to correct small changes in idle speed and eliminate hunting.
Last is to setup airflow correction. I take about 50% away from the stock proportion tables. And then as simon suggested, use a linear arrangement in the integral tables. The factory integral table will cause overshoot..... a linear arrangement is more effective. The upper parts of the integral table can be .05-.06 which is roughly .5% of correction then tapering to zero with a perfect linear graph. If you take notice with the stock table it isnt linear at all so slight errors in idle will cause larger fluctuations.
You can have excellent control with the e38. Using the above techniques it is possible to have a very choppy idle or very smooth idle both well mannered. Manipulating the airflow limits and timing can give you just that. More airflow-less timing is a choppy idle and vice versa.
It is very much trial and error. But as ringram has suggested small changes daily will reap rewards and once familiar with the tables it is a simple process.
It is hard to put it in steps but generally first off...
1. Find desired TP% using b1651 max idle area and zeroing correction tables.
2. Once you have found this by testing with a/c on and off reenable correction tables and stock airflow tables and then commanded a slightly higher max idle area then desired and test rpm dipping revving the car in neutral again a/c on and off. Find a happy medium.
3. Next we try to narrow the gap between min idle airflow and max idle area so there is a difference of approx 1.5-2% between the upper and lower limit.Set min idle airflow to 10g/s as a starting point at desired rpm. The low setting is easiest found driving with a/c off, higher then usual timing taking notice of the lowest idle limit achieved in the data tables. (please start logging once idle is settled as it will record a lower limit.) We end up with for eg a bottom limit off 19.5% and top limit of 21.5%.
4. Everything between these limits is learn't and idle steadied via the PI controller on the electronic throttle body and also timing correction. Timing correction will be minimal, and then adjust proportion and integral tables. Proportion is reduced and a linear integral table is built.
5. Automatic cars only. B1652 is max idle area for D/R. Select drive and watch what TP% is commanded adjusting the value in B1652 to a desired airflow is reached. Please note that B1651 max idle percentage can only be tested whilst free revving in neutral or park.
6. Once tables are set up effectively, the timing correction tables can be dulled down. At 16 deg over idle it could be 0 or -0.5 instead of -1.5(around stock). This way we are relying on the airflow correction to trim the idle. Timing correction is fast and if it pulls timing to quickly to drop idle rpms to commanded idle speed, airflow will stay high at the top limit until it settles for a few seconds which then it will fall midway(eg 20.5%). After the timing tables are redone and dulled somewhat the airflow and timing correction will both share an even amount of correction in obtaining a desired idle speed with consistent TP% and ignition timing. It is easy to notice if the timing is doing to much as the idle will be a little choppy till it settles. You want to see the idle settle midway between the upper and lower limit. If timing correction is to aggressive it won't do this, the whole purpose is to let the ETC do its job without over compensating with timing correction which is largely a common mistake.
All in all the purpose is to maximise control over the e38s settings(via the limits) whilst still being able to retain a certain amount of the factory learning capabilities for minor airflow corrections. I hope I have shed some light on the topic. If I have wrote anything confusing(from my poor grammar) please just pm me.
cheers
Joel
Reply With Quote
