Question about calculated airflow vs. measured airflow

[JR_VETTE]

What I understand is this :

CYLAIR_DMA is the actual measured airflow (via MAF)
DYNCYLAIR_DMA is the calculated airflow

I know if these values don't match up, more than likely the VE table still needs work, or the MAF needs recalibration (which I doubt mine is off much if any).

When I am running SD, these two values match up perfectly (MAF still connected, but MAF Fail Frequency set to 1), however when I enable the MAF, these values are off. For example, I'm looking at a value right now in a recent log where CYLAIR = .54 Grams/Cyl and DYNCYLAIR = .60 Grams/Cyl.

I know my VE table needs some additional work, which I'm going to work on today.

I guess my question is this. Is it normal for these values to be equal when running SD?

And before anybody asks, no I don't have a wideband yet. :D

[Redline Motorsports]

I'll take a stab at this.....:D

The calulated value is based upon other factors as its a "calculated" value. When you disconnect the MAF all that can be done to determine load is a calculation as you lost your direct measuring device.

It would make sense that when the MAF is enabled that there could be a slight variance between what is reported and what is calculated. I just recently have been using the DYNCYLAIR_DMA pid for load as its way more stable then the CYLAIR pid.

I think the VE does play a role in the calculation but others can confirm that.

Howard

[dfe1]

Actual VE would have to play a role because obviously an engine consumes more air if operating at a higher VE. If you look at the VE PID calculation--
% = {SAE.MAF.gps}*({SAE.IAT.C}+273.15)/((displacement()*61.024) *{SAE.RPM}*{SAE.MAP.kPa})*212544

it's apparent that what's reported is, in essence measured air flow divided by calculated air flow. If you're not measuring air flow (MAF sensor disabled or trashed) then the reported value IS the calculated value. That makes sense, because in a true speed density system, the only variables available for determining VE are engine displacement, MAP and RPM. According to the PID equation, the actual values in the VE table have nothing to do with either calculated or measured VE-- they're strictly numbers that are used by the PCM to determine fuel flow requirements. I don't know all of this to be actual fact, because I haven't been into the code. Ross should be able to verify or correct. I can tell you that in my experience, the VE table for SD operation is considerably different from the one required when the MAF is functioning. That's to be expected because when the MAF is being used, the system is really a hybrid that uses measured or calculated values, or a combination of both, depending on engine operating conditions.

[JR_VETTE]

Thanks guys! For once, it's actually all starting to make sense!

:Eyecrazy:

[dc_justin]

Actual VE would have to play a role because obviously an engine consumes more air if operating at a higher VE. If you look at the VE PID calculation--
% = {SAE.MAF.gps}*({SAE.IAT.C}+273.15)/((displacement()*61.024) *{SAE.RPM}*{SAE.MAP.kPa})*212544

it's apparent that what's reported is, in essence measured air flow divided by calculated air flow. If you're not measuring air flow (MAF sensor disabled or trashed) then the reported value IS the calculated value. That makes sense, because in a true speed density system, the only variables available for determining VE are engine displacement, MAP and RPM. According to the PID equation, the actual values in the VE table have nothing to do with either calculated or measured VE-- they're strictly numbers that are used by the PCM to determine fuel flow requirements. I don't know all of this to be actual fact, because I haven't been into the code. Ross should be able to verify or correct. I can tell you that in my experience, the VE table for SD operation is considerably different from the one required when the MAF is functioning. That's to be expected because when the MAF is being used, the system is really a hybrid that uses measured or calculated values, or a combination of both, depending on engine operating conditions.

I think you may be thinking of that PID incorrectly. That Calc.VE PID is a front-end calculation that serves to display a VE % based on the MAF input, not the actual VE% that the PCM is using for fueling. Just like Calc.Cylair is the EFILive front-end calculated airflow based on MAF sensor input, not the PCM calculated airflow based on MAF input. One possible use of this would be to actually map out the VE table based solely on airflow inputs from the MAF.