Originally Posted by
LaTechGTO
Thanks! Yeah, it is definitely much easier than I thought; As you have probably noticed, I have a nasty habit of overcomplicating things haha. I think the biggest things for me was keeping the nomenclature and roles for each item - realizing a lot of different values are treated as adders to make up the Base Idle Airflow, and that the logged Desired Idle Airflow accounts for all of these and should be pasted directly into the Base Running Airflow table in order to account for the adjustments being made by the STIT and LTIT, ultimately resulting in close-to-zero STIT and LTIT. From what I am seeing, the Desired Airflow only correlates to the desired airflow through the IAC, not for the engine - this was another point of confusion to me early on. -- Just to clarify, the Base Running Airflow is not just the air going through the IAC, its the air being injested by the throttle blade opening as well as the IAC based on the IAC count. Thats why as you move you IAC counts up and down, so must you adjust you BRAF to get the LTIT and STIT closest to 0. I tend to want them to be slightly negative like .2/.5.
Is there a target STIT value? I see most people like to run negative STITs as long as it doesn't cause any forms of "cruise control" becuase it ensures the engine is getting plenty of air between throttle transitions instead of stumbling/hunting for idle. Then again, I would think it would be best to have this as close to 0 as possible (negative or positive, no difference) since there are other tables that can be modified to aid in the previous idle issue scenario. -- See above
I am still not sure how to adjust for the IAC effective area, and the reading I have done surrounding this topic basically suggests a few things 1) the IAC hole in the TB is much smaller on aftermarket TBs, necessitating either a change in the effective area vs IAC steps or by modifying the hole to allow easier airflow to the IAC 2) getting the IAC counts as low as possible since it seems to be much more accurate in the lower flow scenarios 3.) the changes being made are more of guess work than science. I think the ultimate reason for this is to make the dynamic flow match the anticipated flow, correct? I don't think I said that right, but basically for the IAC steps (anticipated area by IAC position = certain amount of airflow) to correlate to the actual flow going through it, which alows easy, mythodical adjustment later for idle tuning while moving. -- This is how I was taught, if you look at a stock IAC effective table, you will see it is all zero's to 12 mm2 IIRC. That means that no steps are required and the base throttle blade opening (with the blade hole) is equal to about 12mm2 for the 78mm stock throttle body. Now that changes of course as you readjust the setscrew or go with a larger TB (i.e a 90MM would be closer to 24-26mm2). So the way it works is the PCM generates a mm2 airflow requirement based on the sum total or Idle Desired air and adjusts the IAC IAW the mm2 calculation. The desired IAC count at idle is like 60-80 for stock and 40-60 for a cam (depending on its specs). While it isnt a scientific theorem, there is some intelligence to the tuning, more intuitive I would say.
I am still hung up wondering if the NBs are actually adjusting for the anticipated stoich for the E10 fuel. I was always under the impression they targeted the stoich saved in the tune, not lambda = 1 for the fuel being used. In my mind, this would remove the need for a flex fuel sensor unless in OL. Maybe they do target Lamba = 1 for all different types of fuel while in CL (within the window for the NBs), hence the seemingly rich state I was noticing on the WB readings. -- All O2 sensors (WB or NB) all operate in Lambda and actual monitor Oxygen Molecules. The PCM and Wideband meters actually "convert" it to an AFR, which as you can see, you must define. Lambda 1.0 (stoich) is determined to be the most efficient burn. .9 is richer and 1.1 is leaner. Remember the desired Lambda for pure gas (AFR 14.7) is 1.0 and for E85 (AFR 9.3) is 1.0 as well. That is why Lambda is so much easier to tune by. The NB is only accurate around a very small window aroundstoich so it can't be used for enrichments in the .7/.8 area so widebands are used.
Also, for those that are reading, I stated something earlier about "In Gear STIT" - There isn't such a thing; STIT doesn't care if you are In Gear or in P/N; only LTITs care since the values for each could be very different. -- True
Thought I was done, didn't you? LOL