Conflicting info on idle air flow

[johnsZ06]

I was reading on LS1tech that GM.IACDES_B and SAE.MAF do not have to be the same at idle yet in the sticky on the same subject here, it is mentioned that they should.

You would think that since they both measure the same thing they would be identical at idle. In my case they are not. SAE.MAF measures much more at idle than GM.IACDES_B and my car idles and transitions fine. BTW, I have a hole drilled in my TB blade.

So what is the general consensus? Would anyone care to explain it in more detail.

[VetPet]

Hey John,

Is it possible that one value is the raw data and the other is after it's been filtered by internal calculations that change the output? Your car is running fine as you said so it doesn't appear that the different readings matter. Sounds like you've got it running like you wanted it to.

[johnsZ06]

Since the MAF data is measured and the IAC is calculated I would say yes, that is true. I just want to know how IAC is calculated. I think I remember Blacky explaining it in a thread but I can't seem to find it.
.

[jfpilla]

JohnsZ06,
I'm not too familiar with f-body tuning. I do know that the C5 tune is more stable when the values are close. You can get a decent running tune without the tables close but I've found that it means some tables are not set quite right. ie: VE's, IFR and MAF freq. For a while my car had the same readings as you describe and from time to time transition would change some or after a hot start a quick release of the gas pedal would cause a surge. Those issues disappeared with well set tables.
Perhaps someone familiar with the f-body IAC setup will chime in. Might be the hole changes things.
The link has some interesting reading. It won't answer your questions, though.
Joe
http://www.diy-efi.org/gmecm/papers/747iac.txt

[Black02SS]

I honestly have never looked at mine to see if they match. On a fbody, I have never moved the effective area to tune for idle. I will adjust the set screw to get me in a ballpark area of where I want the car, adjust VE to get the fueling dialed in, and then adjust the Desired Airflow. I have used this process with great success on fbody's. I have not done a vette.

[jfpilla]

Look at a stock log. DesIac, MAF gms and Desired Airflow always match. I believe that IFRs and VE's are the keys to a good tune. When they are correct then you set your Ltrims to be negative using the MAF freq. The freq. table will be relatively higher than the stock table since we want negative trims(well neg.) with the MAF. Stock trims are not always negative. That means MAF gms will be a little higher than DesIac.
I've been told recently that the effective area table does not work for f-bodies but the ETC table does for C5's. Also, to leave those tables alone for stock TB's. If the T.Blade on a 90mm is set, right on, the the ETC table is not needed.
Black02SS, I think I'm doing the same as you and have not had to use any more tables other than the start up friction tables.

[Black02SS]

Black02SS, I think I'm doing the same as you and have not had to use any more tables other than the start up friction tables.

Yup, seems to work well for me.

[johnsZ06]

Did some more tuning yesterday! First, I replaced my Accel 36 lb injectors with Delphi 32, which at 58 PSI fuel pressure works out to apprx 36.5 lb. A much better injector IMO than the Accel. Adjusted the IFR table, went into SD mode a logged some data. LTrims were all rich so I remapped the VE table and went out again. All trims are within -2 to +1. Good enough. I reset my RAF and startup air back to stock and the car starts and idles well at 900 RPM. The car has no cruise control effect or hung idle anymore which I was experiencing with my modified RAF table. All is well in SD now.

Next weekend if time permits, I'll recalibrate the MAF so I can use it. I don't know, but I just think the car runs much better, cleaner and crisper with the MAF connected, just my opinion.

Looking at the logs for yesterday, the desIAC and dyncylair and MAF airflow at idle are very close but still no cigar.